Full-duplex Intercom

Full-duplex Intercom

Circuit diagram:

Parts:

P1_____________22K  Log. Potentiometer
R1_____________22K  1/4W Resistor
R2,R3_________100K  1/4W Resistors
R4_____________47K  1/4W Resistor
R5______________2K2 1/4W Resistor (See Notes)
R6______________6K8 1/4W Resistor
R7_____________22K  1/2W Carbon or Cermet Trimmer
R8______________2K7 1/4W Resistor

C1,C6_________100nF  63V Polyester or Ceramic Capacitors
C2,C3__________10µF  63V Electrolytic Capacitors
C4_____________22µF  25V Electrolytic Capacitor
C5_____________22nF  63V Polyester or Ceramic Capacitor
C7____________470µF  25V Electrolytic Capacitor

Q1____________BC547  45V 100mA NPN Transistor
IC1_________TDA7052  Audio power amplifier IC
SW1____________SPST  miniature Switch
MIC____________Miniature electret microphone
SPKR___________8 Ohm Loudspeaker

Screened cable (See Text)

---

Comments:

This design allows to operate two intercom stations leaving the operator free of using his/her hands in some other occupation, thus avoiding the usual "push-to-talk" operation mode.

No complex changeover switching is required: the two units are connected together by means of a thin screened cable.

As both microphones and loudspeakers are always in operation, a special circuit is used to avoid that the loudspeaker output can be picked-up by the microphone enclosed in the same box, causing a very undesirable and loud "howl", i.e. the well known "Larsen effect".

A "Private" switch allows microphone muting, if required.

Circuit operation:

The circuit uses the TDA7052 audio power amplifier IC, capable of delivering about 1 Watt of output power at a supply voltage comprised in the 6 - 12V range.

The unusual feature of this design is the microphone amplifier Q1: its 180° phase-shifted audio output taken at the Collector and its in-phase output taken at the Emitter are mixed by the C3, C4, R7 and R8 network and R7 is trimmed until the two incoming signals almost cancel out. In this way, the loudspeaker will reproduce a very faint copy of the signals picked-up by the microphone.

At the same time, as both Collectors of the two intercom units are tied together, the 180° phase-shifted signal will pass to the audio amplifier of the second unit without attenuation, so it will be loudly reproduced by its loudspeaker.

The same operation will occur when speaking into the microphone of the second unit: if R7 will be correctly set, almost no output will be heard from its loudspeaker but a loud and clear reproduction will be heard at the first unit output.

Notes:

• The circuit is shown already doubled in the diagram. The two units can be built into two separate boxes and connected by a thin screened cable having the length desired.
• The cable screen is the negative ground path and the central wire is the signal path.
• The power supply can be a common wall-plug transformer-rectifier having a voltage output in the 6 - 12V dc range @ about 200mA.
• Enclosing the power supply in the box of one unit, the other unit can be easily fed by using a two-wire screened cable, its second wire becoming the positive dc path.
• To avoid a two-wire screened cable, each unit may have its own separate power supply.
• Please note that R5 is the only part of the circuit that must not be doubled.
• Closing SW1 prevents signal transmission only, not reception.
• To setup the circuit, rotate the volume control (P1) of the first unit near its maximum and speak into the microphone. Adjust Trimmer R7 until your voice becomes almost inaudible when reproduced by the loudspeaker of the same unit.
• Do the same as above with the second unit.
  

Amplifier Timer

Amplifier Timer

Circuit diagram:

Parts:

R1,R8___________1K   1/4W Resistors
R2,R3___________4K7  1/4W Resistors
R4_____________22K   1/4W Resistor
R5______________4M7  1/4W Resistor
R6,R9__________10K   1/4W Resistors
R7______________1M5  1/4W Resistor
R10___________100K   1/4W Resistor
R11____________15K   1/4W Resistor
R12____________10M   1/4W Resistor
R13_____________1M   1/4W Resistor
R14_____________8K2  1/4W Resistor
R15_____________1K8  1/4W Resistor

C1____________470µF   25V Electrolytic Capacitor
C2,C3,C6______100nF   63V Polyester Capacitors
C4,C5__________10µF   25V Electrolytic Capacitors

D1_____Diode bridge  100V 1A
D2,D7________1N4002  100V 1A Diodes
D3__________Red LED  5mm.
D4_______Yellow LED  5mm.
D5,D6________1N4148  75V 150mA Diodes

IC1___________78L12  12V 100mA Voltage regulator IC
IC2___________LM358  Low Power Dual Op-amp
IC3____________4060  14 stage ripple counter and oscillator IC

Q1____________BC557  45V 100mA PNP Transistor
Q2____________BC337  45V 800mA NPN Transistor

J1______________RCA audio input socket
P1_____________SPST Mains suited Pushbutton
P2_____________SPST Pushbutton
T1_____________220V Primary, 12V Secondary 3VA Mains transformer
RL1___________10.5V 270 Ohm Relay with SPST 5A 220V switch
PL1____________Male Mains plug
SK1__________Female Mains socket

---

Circuit operation:

This circuit turns-off an amplifier or any other device when a low level audio signal fed to its input is absent for 15 minutes at least.

Pushing P1 the device is switched-on feeding any appliance connected to SK1. Input audio signal is boosted and squared by IC2A & IC2B and monitored by LED D4. When D4 illuminates, albeit for a very short peak, IC3 is reset and restarts its counting. Pin 2 of IC3 remains in the low state, the two transistors are on and the relay operates. When, after a 15 minutes delay, no signal appeared at the input, IC3 ends its counting and pin 2 goes high. Q1 & Q2 stop conducting and the relay switches-off. The device is thus completely off as also are the appliances connected to SK1. C5 & R9 reset IC3 at power-on. P2 allows switch-off at any moment.

Notes:

• Simply connect left or right channel tape output of your amplifier to J1.
• You can employ two RCA input sockets wired in parallel to allow pick-up audio signals from both stereo channels.
• The delay time can be varied changing R13 and/or C6 values.

Needing to operate a device not supplied by power mains, use a double pole relay switch, connecting the second pole switch in series to the device supply.

Headphone Amplifier

Headphone Amplifier

Circuit diagram:

Amplifier parts:

P1_____________22K   Log.Potentiometer (Dual-gang for stereo)
R1____________560R   1/4W Resistor
R2,R3__________10K   1/4W Resistors
R4_____________12K   1/4W Resistor
R5,R6___________2R2  1/4W Resistor
R7_____________22R   1/2W Resistor 

C1______________1µF   63V Polyester Capacitor
C2,C3,C4______100µF   25V Electrolytic Capacitors
C5_____________22pF   63V Polystyrene or Ceramic Capacitor
C6_____________22µF   25V Electrolytic Capacitor 

IC1___________LM833 or NE5532 Low noise Dual Op-amp
Q1,Q3_________BC337   45V 800mA NPN Transistors
Q2,Q4_________BC327   45V 800mA PNP Transistors 

J1______________RCA  audio input socket

Power supply parts:

R8______________2K2  1/4W Resistor
C7,C8________2200µF   25V Electrolytic Capacitors
D1____________100V 1A Diode bridge
D2____________5mm. or 3mm. Red LED

IC2___________7815  15V 1A Positive voltage regulator IC
IC3___________7915  15V 1A Negative voltage regulator IC 

T1____________220V Primary, 15 + 15V Secondary 5VA Mains transformer
PL1___________Male Mains plug
SW1___________SPST Mains switch

---

Notes:

• Can be directly connected to CD players, tuners and tape recorders.
• Tested with several headphone models of different impedance: 32, 100, 245, 300, 600 & 2000 Ohm.
• Old 8 Ohm impedance headphones can be also driven, but these obsolete devices are not recommended.
• Schematic shows left channel and power supply (common to both channels).
• Numbers in parentheses show IC1 right channel pin connections.
• A correct grounding is very important to eliminate hum and ground loops. Connect to the same point the ground sides of J1, P1, C2, C3 & C4. Then connect separately the input and output grounds to the power supply ground.

---

Technical data:

Output voltage: Well above 5V RMS into all loads

Sensitivity: 250mV input for 5V RMS output

Frequency response: Flat from 30Hz to 20KHz

Total harmonic distortion @ 1KHz & 10KHz:
Below 0.005% into 32 Ohm loads and up to 4V RMS output (typical 0.003%)

Total harmonic distortion @ 1KHz & 10KHz:
Below 0.005% into 100 to 2000 Ohm loads and up to 5V RMS output (typical 0.003%)

Unconditionally stable on capacitive loads

---

Modular Audio Preamplifier

Modular Audio Preamplifier

Main Module circuit diagram:

Parts:

R1_______________1K5  1/4W Resistor
R2_____________220K   1/4W Resistor
R3______________18K   1/4W Resistor
R4_____________330R   1/4W Resistor
R5______________39K   1/4W Resistor
R6______________56R   1/4W Resistor
R7,R10__________10K   1/4W Resistors
R8______________33K   1/4W Resistor
R9_____________150R   1/4W Resistor
R11_____________ 6K8  1/4W Resistor
R12,R13________100R   1/4W Resistors
R14____________100K   1/4W Resistor

C1_____________220nF   63V Polyester Capacitor
C2_____________220pF   63V Polystyrene or ceramic Capacitor
C3_______________1nF   63V Polyester or ceramic Capacitor
C4,C7___________47µF   50V Electrolytic Capacitors
C5,C6__________100µF   50V Electrolytic Capacitors

Q1,Q2_________BC550C   45V 100mA Low noise High gain NPN Transistors
Q3____________BC556    65V 100mA PNP Transistor
Q4____________BC546    65V 100mA NPN Transistor

---

Tone Control Module circuit diagram:

Parts:

R1,R7___________47K   1/4W Resistors
R2_____________220K   1/4W Resistor
R3______________18K   1/4W Resistor
R4_____________330R   1/4W Resistor
R5______________39K   1/4W Resistor
R6______________56R   1/4W Resistor
R8_____________150R   1/4W Resistor
R9______________10K   1/4W Resistor
R10,R16__________6K8  1/4W Resistors
R11,R12________100R   1/4W Resistors
R13____________100K   1/4W Resistor
R14______________1K5  1/4W Resistor
R15,R21,R22______4K7  1/4W Resistors
R17,R24,R26______8K2  1/4W Resistors
R18______________3K3  1/4W Resistor
R19______________1K   1/4W Resistor
R20____________470R   1/4W Resistor
R23,R25_________12K   1/4W Resistors
R27,R28__________4K7  1/4W Resistors

C1_____________220nF   63V Polyester Capacitor
C2_______________1nF   63V Polyester or ceramic Capacitor
C3,C6___________47µF   50V Electrolytic Capacitors
C4,C5__________100µF   50V Electrolytic Capacitors
C7______________10nF   63V Polyester Capacitor
C8,C9__________100nF   63V Polyester Capacitors

Q1,Q2_________BC550C   45V 100mA Low noise High gain NPN Transistors
Q3____________BC556    65V 100mA PNP Transistor
Q4____________BC546    65V 100mA NPN Transistor

SW1,SW2_______2 poles 6 ways Rotary Switches

Simpler, alternative Tone Control parts:

P1______________22K   Linear Potentiometer
P2______________47K   Linear Potentiometer
R29,R30________470R   1/4W Resistors
R31,R32__________4K7  1/4W Resistors

C10_____________10nF   63V Polyester Capacitor
C11,C12________100nF   63V Polyester Capacitors

---

Comments:

To complement the 60 Watt MosFet Audio Amplifier a High Quality Preamplifier design was necessary. A discrete components topology, using + and - 24V supply rails was chosen, keeping the transistor count to the minimum, but still allowing low noise, very low distortion and high input overload margin. Obviously, the modules forming this preamplifier can be used in different combinations and drive different power amplifiers, provided the following stages present a reasonably high input impedance (i.e. higher than 10KOhm).

Main Module:

If a Tone Control facility is not needed, the Preamplifier will be formed by the Main Module only. Its input will be connected to some sort of changeover switch, in order to allow several audio reproduction devices to be connected, e.g. CD player, Tuner, Tape Recorder, iPod, MiniDisc etc. The total amount and type of inputs is left to the choice of the home constructor.

The output of the Main Module will be connected to a 22K Log. Potentiometer (dual gang if a stereo preamp was planned). The central and ground leads of this potentiometer must be connected to the power amplifier input.

Tone Control Module:

This Module employs an unusual topology, still maintaining the basic op-amp circuitry of the Main Module with a few changes in resistor values.

A special feature of this circuit is the use of six ways switches instead of the more common potentiometers: in this way, precise "tone flat" setting, or preset dB steps in bass and treble boost or cut can be obtained. Tone Control switches also allow a more precise channel matching when a stereo configuration is used, avoiding the frequent poor alignment accuracy presented by common ganged potentiometers.

Six ways (two poles for stereo) rotary switches were chosen for this purpose as easily available. This dictated the unusual "asymmetrical" configuration of three positions for boost, one for flat and two for cut. This choice was based on the fact that tone controls are used in practice more for frequency boosting than for cutting purposes. In any case, +5dB +10dB and +15dB of bass boost and -3dB and -10dB of bass cut were provided. Treble boost was also set to +5dB +10dB and +15dB and treble cut to -3.5dB and -9dB.

Those wishing to use common potentiometers in the usual way for Tone Controls may use the circuit shown enclosed in the dashed box (bottom-right of the Tone Control Module circuit diagram) to replace switched controls.

The Tone Control Module should usually be placed after the Main Input Module, and the volume control inserted between the Tone Control Module output and the power amplifier input. Alternatively, the volume control can also be placed between Main Input Module and Tone Control Module, at will. Furthermore, the position of these two modules can be also interchanged.

Power supply:

The preamplifier must be feed by a dual-rail, +24 and -24V 50mA dc power supply. This is easily achieved by using a 48V 3VA center-tapped mains transformer, a 100V 1A bridge rectifier and a couple of 2200µF 50V smoothing capacitors. To these components two 24V IC regulators must be added: a 7824 (or 78L24) for the positive rail and a 7924 (or 79L24) for the negative one.

The diagram of such a power supply is the same of that used in the Headphone Amplifier, but the voltages of the secondary winding of the transformer, smoothing capacitors and IC regulators must be updated. Alternatively, the dc voltage can be directly derived from the dc supply rails of the power amplifier, provided that both 24V regulators are added.

Note:

• If this preamplifier is used as a separate, stand-alone device, thus requiring a cable connection to the power amplifier, some kind of output short-circuit protection is needed, due to possible shorts caused by incorrect plugging. The simplest solution is to wire a 3K3 1/4W resistor in series to the output capacitor of the last module (i.e. the module having its output connected to the preamp main output socket).

---

Technical data:

Main Module Input sensitivity:
250mV RMS for 1V RMS output

Tone Control Module Input sensitivity:
1V RMS for 1V RMS output

Maximum output voltage:
13.4V RMS into 100K load, 11.3V RMS into 22K load, 8.8V RMS into 10K load

Frequency response:
flat from 20Hz to 20KHz

Total harmonic distortion @ 1KHz:
1V RMS 0.002% 5V RMS 0.003% 7V RMS 0.003%

Total harmonic distortion @10KHz:
1V RMS 0.003% 5V RMS 0.008% 7V RMS 0.01%

3 - 5 Watt Class-A Audio Amplifier

3 - 5 Watt Class-A Audio Amplifier

Circuit diagram:Parts:

P1_____________47K  Log. Potentiometer (Dual-gang for stereo)
R1____________100K  1/4W Resistor
R2_____________12K  1/4W Resistor (See Notes)
R3_____________47K  1/4W Resistor
R4______________8K2 1/4W Resistor
R5______________1K5 1/4W Resistor (Optional, see Notes)
R6______________2K7 1/4W Resistor
R7,R9_________100R  1/4W Resistors
R8____________560R  1/2W Resistor (See Notes)
R10_____________1R  1/2W Resistor 

C1,C2__________10µF  63V Electrolytic Capacitors
C3_____________47µF  25V Electrolytic Capacitor
C4____________100µF  35V Electrolytic Capacitor
C5____________150nF  63V Polyester Capacitor (Optional, see Notes)
C6,C7_________220µF  25V Electrolytic Capacitors
C8___________1000µF  25V Electrolytic Capacitor 

Q1___________BC560C  45V 100mA Low noise High gain PNP Transistor
Q2,Q3________BD439   60V 4A NPN Transistors 

SPKR___________One or more speakers wired in series or in parallel
Total resulting impedance: 8 Ohm
Minimum power handling: 5W

---

Comments:

In the old valve days, most commercial audio amplifiers suited for compact integrated mono or stereo record players used a one-valve amplifier topology. The circuit was usually implemented by means of a multiple type valve, e.g. a triode pentode ECL86.

Common features for those amplifiers were: Class A operation, output power in the 3 - 5W range, input sensitivity of about 600mV for full output power, THD of about 3% @ 3W and 1KHz.

Best types showed THD figures of 1.8% @ 3W and 0.8% @ 2W.

This solid-state push-pull single-ended Class A circuit is capable of providing a sound comparable to those valve amplifiers, delivering more output power (6.9W measured across a 8 Ohm loudspeaker cabinet load), less THD, higher input sensitivity and better linearity.

Voltage and current required for this circuit are 24V and 700mA respectively, compared to 250V HT rail and 1A @ 6.3V filament heating for valve-operated amplifiers.

The only penalty for the transistor operated circuit is the necessity of using a rather large heatsink for Q2 and Q3 (compared to the maximum power delivered).

In any case, the amount of heat generated by this circuit can be comparable to that of a one-valve amplifier.

An optional bass-boost facility can be added, by means of R5 and C5.

This circuit was built and compared with a one-valve box gramophone circuit of the late 1950s by , a Dutch biochemist working in the field of medical imaging (PET) with a strong interest in audio and valve amplifiers.

A thorough description of both circuits and the results of subjective test comparisons made by this distinguished Author appeared on AudioExpress magazine: February, March and April 2005 issues.

Technical data:

(measured on 8 Ohm resistive load unless otherwise specified)

Sensitivity:

230mV input for 1.5W output
380mV input for 3.5W output
560mV input for 5.6W output

Sensitivity with bass-boost:

400mV input for 1.5W output
630mV input for 3.5W output
850mV input for 5.6W output

Sensitivity with 8 Ohm nominal, loudspeaker cabinet load:

210mV input for 1.5W output
325mV input for 3.5W output
477mV input for 6.9W output

Frequency response:

100Hz to 20KHz 0dB; -3dB @ 40Hz

Frequency response with bass-boost:

+5dB @ 100Hz; +3.9dB @ 200Hz; +2.5dB @ 400Hz; -1dB @ 10KHz and 20KHz

Total harmonic distortion @ 1KHz:

0.3% @ 0.5W; 0.45% @ 1W; 1% @ 5.6W

Unconditionally stable on capacitive loads

Notes:

• If necessary, R2 can be adjusted to obtain 13V across C8 positive lead and negative ground.
• Total current drawing of the circuit, best measured by inserting the probes of an Avo-meter across the positive output of the power supply and the positive rail input of the amplifier, must be 700mA. Adjust R8 to obtain this value if necessary.
• Q2 and Q3 must be mounted on a finned heatsink of 120x50x25mm. minimum dimensions.
• Add R5 and C5 if the bass-boost facility is required.

Mini-box 2W Amplifier

Mini-box 2W Amplifier

Circuit diagram:

Parts:

P1_____________10K   Log.Potentiometer
R1,R2__________33K   1/4W Resistors
R3_____________33R   1/4W Resistor
R4_____________15K   1/4W Resistor
R5,R6___________1K   1/4W Resistors
R7____________680R   1/4W Resistor
R8____________120R   1/2W Resistor
R9____________100R   1/2W Trimmer Cermet 

C1,C2__________10µF   63V Electrolytic Capacitors
C3____________100µF   25V Electrolytic Capacitor
C4,C7_________470µF   25V Electrolytic Capacitors
C5_____________47pF   63V Ceramic Capacitor
C6____________220nF   63V Polyester Capacitor
C8___________1000µF   25V Electrolytic Capacitor

D1___________1N4148   75V 150mA Diode
Q1____________BC560C  45V 100mA PNP Low noise High gain Transistor
Q2____________BC337   45V 800mA NPN Transistor
Q3____________TIP31A  60V 4A    NPN Transistor
Q4 ___________TIP32A  60V 4A    PNP Transistor 

SW1___________SPST switch
SPKR__________3-5 Watt Loudspeaker, 8, 4 or 2 Ohm impedance

---

Device purpose:

This amplifier was designed to be self-contained in a small loudspeaker box. It can be feed by Walkman, Mini-Disc, iPod and CD players, computers and similar devices fitted with line or headphone output. Of course, in most cases you will have to make two boxes to obtain stereo.

The circuit was deliberately designed using no ICs and in a rather old-fashioned manner in order to obtain good harmonic distortion behavior and to avoid hard to find components. The amplifier(s) can be conveniently supplied by a 12V wall plug-in transformer.

Closing SW1 a bass-boost is provided but, at the same time, volume control must be increased to compensate for power loss at higher frequencies.

In use, R9 should be carefully adjusted to provide minimal audible signal cross-over distortion consistent with minimal measured quiescent current consumption; a good compromise is to set the quiescent current at about 10-15 mA.

To measure this current, wire a DC current meter temporarily in series with the collector of Q3.

---

Technical data:

Output power:
1.5 Watt RMS into 8 Ohm, 2.5 Watt into 4 Ohm, 3.5 Watt into 2 Ohm (1KHz sinewave)

Sensitivity:100mV input for 1.5W output @ 8 Ohm
Frequency response:30Hz to 20KHz -1dB
Total harmonic distortion @ 1KHz & 10KHz:<0.2% @ 8 Ohm 1W, <0.3% @ 4 Ohm 2W, <0.5% @ 2 Ohm 2W

60W MosFet Audio Amplifier

60W MosFet Audio Amplifier

Circuit diagram:

Parts:

R1______________47K   1/4W Resistor
R2_______________4K7  1/4W Resistor
R3______________22K   1/4W Resistor
R4_______________1K   1/4W Resistor
R5,R12,R13_____330R   1/4W Resistors
R6_______________1K5  1/4W Resistor
R7______________15K   1/4W Resistor
R8______________33K   1/4W Resistor
R9_____________150K   1/4W Resistor
R10____________500R   1/2W Trimmer Cermet
R11_____________39R   1/4W Resistor
R14,R15___________R33 2.5W Resistors
R16_____________10R   2.5W Resistor
R17_______________R22   5W Resistor (wirewound) 

C1_____________470nF   63V Polyester Capacitor
C2_____________470pF   63V Polystyrene or ceramic Capacitor
C3______________47µF   63V Electrolytic Capacitor
C4,C8,C9,C11___100nF   63V Polyester Capacitors
C5______________10pF   63V Polystyrene or ceramic Capacitor
C6_______________1µF   63V Polyester Capacitor
C7,C10_________100µF   63V Electrolytic Capacitors 

D1___________1N4002   100V 1A Diode
D2_____________5mm. Red LED 

Q1,Q2,Q4_____MPSA43   200V 500mA NPN Transistors
Q3,Q5________BC546     65V 100mA NPN Transistors
Q6___________MJE340   200V 500mA NPN Transistor
Q7___________MJE350   200V 500mA PNP Transistor
Q8___________IRFP240  200V 20A N-Channel Hexfet Transistor
Q9___________IRFP9240 200V 12A P-Channel Hexfet Transistor

---

Power supply circuit diagram:

Parts:

R1_______________3K9   1W Resistor
C1,C2_________4700µF  63V Electrolytic Capacitors (See Notes)
C3,C4__________100nF  63V Polyester Capacitors 

D1_____________400V 8A Diode bridge
D2_____________5mm. Red LED
F1,F2__________4A Fuses with sockets
T1_____________230V or 115V Primary, 30+30V Secondary 160VA Mains transformer 

PL1____________Male Mains plug
SW1____________SPST Mains switch

---

Comments:

To celebrate the hundredth design posted to this website, and to fulfil the requests of many correspondents wanting an amplifier more powerful than the 25W MosFet, a 60 - 90W High Quality power amplifier design is presented here.

Circuit topology is about the same of the above mentioned amplifier, but the extremely rugged IRFP240 and IRFP9240 MosFet devices are used as the output pair, and well renowned high voltage Motorola's transistors are employed in the preceding stages.

The supply rails voltage was kept prudentially at the rather low value of + and - 40V. For those wishing to experiment, the supply rails voltage could be raised to + and - 50V maximum, allowing the amplifier to approach the 100W into 8 Ohm target: enjoy!

Matching, discrete components, Modular Preamplifier design are available here: Modular Audio Preamplifier.

Notes:

• In the original circuit, a three-diode string was wired in series to R10. Two of these diodes are now replaced by a red LED in order to achieve improved quiescent current stability over a larger temperature range. Thanks to David Edwards of LedeAudio for this suggestion.
• A small, U-shaped heat sink must be fitted to Q6 & Q7.
• Q8 & Q9 must be mounted on large heat sinks.
• Quiescent current can be measured by means of an Avo-meter wired in series to the positive supply rail and no input signal.
• Set the Trimmer R10 to its minimum resistance.
• Power-on the amplifier and adjust R10 to read a current drawing of about 120 - 130mA.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.
• The value suggested for C1 and C2 in the Power Supply Parts List is the minimum required for a mono amplifier. For optimum performance and in stereo configurations, this value should be increased: 10000µF is a good compromise.
• A correct grounding is very important to eliminate hum and ground loops. Connect to the same point the ground sides of R1, R3, C2, C3 and C4 and the ground input wire. Connect R7 and C7 to C11 to output ground. Then connect separately the input and output grounds to the power supply ground.

---

Technical data:

Output power:
60 Watt RMS @ 8 Ohm (1KHz sinewave) - 90W RMS @ 4 Ohm
Sensitivity:
1V RMS input for 58W output
Frequency response:
30Hz to 20KHz -1dB
Total harmonic distortion @ 1KHz:
1W 0.003% 10W 0.006% 20W 0.01% 40W 0.013% 60W 0.018%
Total harmonic distortion @10KHz:
1W 0.005% 10W 0.02% 20W 0.03% 40W 0.06% 60W 0.09%

Unconditionally stable on capacitive loads

Mini-MosFet Audio Amplifier

Mini-MosFet Audio Amplifier

Power Amplifier Circuit diagram:Power Amplifier Parts:

R1_______________2K2 1/4W Resistor
R2______________27K  1/4W Resistor
R3,R4____________2K2 1/2W Trimmers Cermet or Carbon (or 2K)
R5_____________100R  1/4W Resistor
R6_______________1K  1/4W Resistor
R7,R8__________330R  1/4W Resistors 

C1______________22µF  25V Electrolytic Capacitor
C2______________47pF  63V Polystyrene or Ceramic Capacitor
C3,C4__________100µF  50V Electrolytic Capacitors
C5____________2200µF  50V Electrolytic Capacitor 

Q1____________BC550C  45V 100mA Low noise High gain NPN Transistor
Q2___________IRF530  100V 14A N-Channel Hexfet Transistor (or MTP12N10)
Q3__________IRF9530  100V 12A P-Channel Hexfet Transistor (or MTP12P10)

---

Comments:

This project was a sort of challenge: designing an audio amplifier capable of delivering a decent output power with a minimum parts count, without sacrificing quality.

The Power Amplifier section employs only three transistors and a handful of resistors and capacitors in a shunt feedback configuration but can deliver more than 18W into 8 Ohm with <0.08%>

Setting up the Power Amplifier:

The setup of this amplifier must be done carefully and with no haste:

1. Connect the Power Supply Unit (previously tested separately) to the Power Amplifier but not the Preamp: the input of the Power Amplifier must be left open.
2. Rotate the cursor of R4 fully towards Q1 Collector.
3. Set the cursor of R3 to about the middle of its travel.
4. Connect a suitable loudspeaker or a 8 Ohm 20W resistor to the amplifier output.
5. Connect a Multimeter, set to measure about 50V fsd, across the positive end of C5 and the negative ground.
6. Switch on the supply and rotate R3 very slowly in order to read about 23V on the Multimeter display.
7. Switch off the supply, disconnect the Multimeter and reconnect it, set to measure at least 1Amp fsd, in series to the positive supply (the possible use of a second Multimeter in this place will be very welcomed).
8. Switch on the supply and rotate R4 very slowly until a reading of about 120mA is displayed.
9. Check again the voltage at the positive end of C5 and readjust R3 if necessary.
10. If R3 was readjusted, R4 will surely require some readjustment.
11. Wait about 15 minutes, watch if the current is varying and readjust if necessary.
12. Please note that R3 and R4 are very sensitive: very small movements will cause rather high voltage or current variations, so be careful.
13. Those lucky enough to reach an oscilloscope and a 1KHz sine wave generator, can drive the amplifier to the maximum output power and adjust R3 in order to obtain a symmetrical clipping of the sine wave displayed.

---

Preamp Circuit diagram:

Preamp Parts:

P1______________50K  Log. Potentiometer (or 47K)
 (twin concentric-spindle dual gang for stereo)
P2,P3__________100K  Linear Potentiometers
 (twin concentric-spindle dual gang for stereo) 

R1_____________220K  1/4W Resistor
R2_____________100K  1/4W Resistor
R3_______________2K7 1/4W Resistor
R4,R5____________8K2 1/4W Resistors
R6_______________4K7 1/4W Resistor
R7,R8,R13________2K2 1/4W Resistors
R9_______________2M2 1/4W Resistor
R10,R11_________47K  1/4W Resistor
R12_____________33K  1/4W Resistor
R14____________470R  1/4W Resistor
R15_____________10K  1/4W Resistor
R16______________3K3 1/4W Resistor (See Notes) 

C1,C2,C9_______470nF  63V Polyester Capacitors
C3,C4___________47nF  63V Polyester Capacitors
C5,C6____________6n8  63V Polyester Capacitors
C7______________10µF  63V Electrolytic Capacitor
C8,C10__________22µF  25V Electrolytic Capacitors
C11____________470µF  25V Electrolytic Capacitor (See Notes) 

Q1,Q3_________BC550C  45V 100mA Low noise High gain NPN Transistors
Q2___________2N3819   General-purpose N-Channel FET

---

Comments:

The Preamp sensitivity and overload margin were designed to cope with most modern music programme sources like CD players, Tape recorders, iPods, Computer audio outputs, Tuners etc. The source selecting switches and input connectors are not shown and their number and arrangement are left to the constructor's choice.

To obtain a very high input overload margin, the volume control was placed at the preamp input. After a unity gain, impedance converter stage (Q1) a negative-feedback Baxandall-type Bass and Treble tone control stage was added. As this stage must provide some gain (about 5.6 times) a very low noise, "bootstrapped" two-transistors circuitry with FET-input was implemented. This stage features also excellent THD figures up to 4V RMS output and a low output impedance, necessary to drive properly the Mini-MosFet Power Amplifier, but can also be used for other purposes.

---

Regulated Power Supply Circuit diagram:

Regulated Power Supply Parts:

R1_______________3R9 1 or 2W Resistor
R2______________22R  1/4W Resistor
R3_______________6K8 1/4W Resistor
R4_____________220R  1/4W Resistor
R5_______________4K7 1/2W Resistor

C1____________3300µF  50V Electrolytic Capacitor (or 4700µF 50V)
C2,C5__________100nF  63V Polyester Capacitors
C3______________10µF  63V Electrolytic Capacitor
C4_____________220µF  50V Electrolytic Capacitor 

D1_____Diode bridge  100V 4A
D2___________1N4002  200V 1A Diode
D3______________LED  Any type and color 

IC1___________LM317T 3-Terminal Adjustable Regulator
Q1____________TIP42A  60V 6A PNP Transistor
SW2_____________SPST  Mains switch
T1_____________230V Primary, 35-36V (Center-tapped) Secondary,
 50-75VA Mains transformer (See Notes) 

PL1____________Male Mains plug with cord

---

Comments:

A very good and powerful Regulated Power Supply section was implemented by simply adding a PNP power transistor to the excellent LM317T adjustable regulator chip. In this way this circuit was able to deliver much more than the power required to drive two Mini-MosFet amplifiers to full output (at least 2Amp @ 40V into 4 Ohm load) without any appreciable effort.

Notes:

• Q2 and Q3 in the Power Amplifier must be mounted each on a finned heatsink of at least 80x40x25mm.
• Q1 and IC1 in the Regulated Power Supply must be mounted on a finned heatsink of at least 45x40x17mm.
• A power Transformer having a secondary winding rated at 35 - 36V and 50VA (i.e. about 1.4Amp) is required if you intend to use Loudspeaker cabinets of 8 Ohm nominal impedance. To drive 4 Ohm loads at high power levels, a 70 - 75VA Transformer (2Amp at least) will be a better choice. These transformers are usually center tapped: the central lead will be obviously left open.
• For the stereo version of this project, R16 and C11 in the Preamp will be in common to both channels: therefore, only one item each is necessary. In this case, R11 must be a 1K5 1/2W resistor. The value of C11 will remain unchanged.

---

Technical data:

Output power:

18 Watt RMS into 8 Ohm (1KHz sine wave) - 30 Watt RMS into 4 Ohm
Input sensitivity of the complete Amplifier:
160mV RMS for full output
Power Amplifier Input sensitivity:
900mV RMS for full output
Power Amplifier Frequency response @ 1W RMS:
flat from 40Hz to 20KHz, -0.7dB @ 30Hz, -1.7dB @ 20Hz
Power Amplifier Total harmonic distortion @ 1KHz:
100mW 0.04% 1W 0.04% 10W 0.06% 18W 0.08%
Power Amplifier Total harmonic distortion @10KHz:
100mW 0.02% 1W 0.02% 10W 0.05% 18W 0.12%
Unconditionally stable on capacitive loads
Preamp Maximum output voltage:4V RMS
Preamp Frequency response:flat from 20Hz to 20KHz
Preamp Total harmonic distortion @ 1KHz:
1V RMS 0.007% 3V RMS 0.035%
Preamp Total harmonic distortion @10KHz:
1V RMS 0.007% 3V RMS 0.02%
Bass control frequency range referred to 1KHz:
±20dB @ 40Hz
Treble control frequency range referred to 1KHz:
+18dB/-20dB @ 20KHz

Guitar Amplifier

Guitar Amplifier

Circuit diagram:Parts:

P1______________4K7  Linear Potentiometer
P2_____________10K   Log. Potentiometer
R1,R2__________68K   1/4W Resistors
R3____________220K   1/4W Resistor
R4,R6,R11_______4K7  1/4W Resistors
R5_____________27K   1/4W Resistor
R7______________1K   1/4W Resistor
R8______________3K3  1/2W Resistor
R9______________2K   1/2W Trimmer Cermet
R10___________470R   1/4W Resistor
R12_____________1K5  1/4W Resistor
R13___________470K   1/4W Resistor
R14____________33K   1/4W Resistor 

C1____________100pF   63V Ceramic Capacitor
C2____________100nF   63V Polyester Capacitor
C3____________470µF   35V Electrolytic Capacitor
C4____________220nF   63V Polyester Capacitor (Optional, see Notes)
C5_____________47µF   25V Electrolytic Capacitor (Optional, see Notes)
C6______________1µF   63V Polyester Capacitor
C7,C8,C9,C10___47µF   25V Electrolytic Capacitors
C11____________47pF   63V Ceramic Capacitor
C12__________1000µF   35V Electrolytic Capacitor
C13__________2200µF   35V Electrolytic Capacitor 

D1_____________5mm. Red LED
D2,D3________1N4004  400V 1A Diodes 

Q1,Q2________2N3819   General-purpose N-Channel FETs
Q3____________BC182   50V 200mA NPN Transistor
Q4____________BD135   45V 1.5A NPN Transistor (See Notes)
Q5____________BDX53A  60V 8A NPN Darlington Transistor
Q6____________BDX54A  60V 8A PNP Darlington Transistor 

J1,J2________6.3mm. Mono Jack sockets
SW1____________1 pole 3 ways rotary switch (Optional, see Notes)
SW2____________SPST Mains switch
F1_____________1.6A Fuse with socket 

T1_____________220V Primary, 48V Center-tapped Secondary
 20 to 30VA Mains transformer
PL1____________Male Mains plug

SPKR___________One or more speakers wired in series or in parallel
 Total resulting impedance: 8 or 4 Ohm
 Minimum power handling: 20W

---

Circuit description:

The aim of this design was to reproduce a Combo amplifier of the type very common in the 'sixties and the 'seventies of the past century. It is well suited as a guitar amplifier but it will do a good job with any kind of electronic musical instrument or microphone.

5W power output was a common feature of these widespread devices due to the general adoption of a class A single-tube output stage (see the Vox AC-4 model).

Furthermore, nowadays we can do without the old-fashioned Vib-Trem feature frequently included in those designs.

The present circuit can deliver 10W of output power when driving an 8 Ohm load, or about 18W @ 4 Ohm.

It also features a two-FET preamplifier, two inputs with different sensitivity, a treble-cut control and an optional switch allowing overdrive or powerful treble-enhancement.

Technical data are quite impressive for so simple a design:

Sensitivity: 30mV input for 10W output
Frequency response: 40 to 20KHz -1dB
Total harmonic distortion @ 1KHz and 10KHz, 8 Ohm load: below 0.05% @ 1W, 0.08% @ 3.5W, 0.15% at the onset of clipping (about 10W).

Notes:

• SW1 and related capacitors C4 & C5 are optional.
• When SW1 slider is connected to C5 the overdrive feature is enabled.
• When SW1 slider is connected to C4 the treble-enhancer is enabled.
• C4 value can be varied from 100nF to 470nF to suit your treble-enhancement preferences.
• In all cases where Darlington transistors are used as the output devices it is essential that the sensing transistor (Q4) should be in as close thermal contact with the output transistors as possible. Therefore a TO126-case transistor type was chosen for easy bolting on the heatsink, very close to the output pair.
• To set quiescent current, remove temporarily the Fuse F1 and insert the probes of an Avo-meter in the two leads of the fuse holder.
• Set the volume control to the minimum and Trimmer R9 to its minimum resistance.
• Power-on the circuit and adjust R9 to read a current drawing of about 25 to 30mA.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.

60W Bass Amplifier

60W Bass Amplifier

Amplifier circuit diagram:
Amplifier parts:

R1__________________6K8    1W Resistor
R2,R4_____________470R   1/4W Resistors
R3__________________2K   1/2W Trimmer Cermet
R5,R6_______________4K7  1/2W Resistors
R7________________220R   1/2W Resistor
R8__________________2K2  1/2W Resistor
R9_________________50K   1/2W Trimmer Cermet
R10________________68K   1/4W Resistor
R11,R12______________R47   4W Wirewound Resistors

C1,C2,C4,C5________47µF   63V Electrolytic Capacitors
C3________________100µF   25V Electrolytic Capacitor
C6_________________33pF   63V Ceramic Capacitor
C7_______________1000µF   50V Electrolytic Capacitor
C8_______________2200µF   63V Electrolytic Capacitor (See Notes) 

D1_________________LED    Any type and color
D2________Diode bridge   200V 6A  

Q1,Q2____________BD139    80V 1.5A NPN Transistors
Q3_____________MJ11016   120V 30A NPN Darlington Transistor (See Notes)
Q4_____________MJ11015   120V 30A PNP Darlington Transistor (See Notes) 

SW1_______________SPST Mains switch
F1__________________4A Fuse with socket 

T1________________220V Primary, 48-50V Secondary 75 to 150VA
 Mains transformer (See Notes) 

PL1_______________Male Mains plug 

SPKR______________One or more speakers wired in series or in parallel
 Total resulting impedance: 8 or 4 Ohm
Minimum power handling: 75W

---

Preamplifier circuit diagram:

Preamplifier parts:

P1_________________10K   Linear Potentiometer
P2_________________10K   Log. Potentiometer
R1,R2______________68K   1/4W Resistors
R3________________680K   1/4W Resistor
R4________________220K   1/4W Resistor
R5_________________33K   1/4W Resistor
R6__________________2K2  1/4W Resistor
R7__________________5K6  1/4W Resistor
R8,R18____________330R   1/4W Resistors
R9_________________47K   1/4W Resistor
R10________________18K   1/4W Resistor
R11_________________4K7  1/4W Resistor
R12_________________1K   1/4W Resistor
R13_________________1K5  1/4W Resistor
R14,R15,R16_______100K   1/4W Resistors
R17________________10K   1/4W Resistor 

C1,C4,C8,C9,C10____10µF   63V Electrolytic Capacitors
C2_________________47µF   63V Electrolytic Capacitor
C3_________________47pF   63V Ceramic Capacitor
C5________________220nF   63V Polyester Capacitor
C6________________470nF   63V Polyester Capacitor
C7________________100nF   63V Polyester Capacitor
C11_______________220µF   63V Electrolytic Capacitor 

Q1,Q3____________BC546    65V 100mA NPN Transistors
Q2_______________BC556    65V 100mA PNP Transistor 

J1,J2___________6.3mm. Mono Jack sockets 

SW1_______________SPST Switch

---

Circuit description:

This design adopts a well established circuit topology for the power amplifier, using a single-rail supply of about 60V and capacitor-coupling for the speaker(s). The advantages for a guitar amplifier are the very simple circuitry, even for comparatively high power outputs, and a certain built-in degree of loudspeaker protection, due to capacitor C8, preventing the voltage supply to be conveyed into loudspeakers in case of output transistors' failure.

The preamp is powered by the same 60V rails as the power amplifier, allowing to implement a two-transistors gain-block capable of delivering about 20V RMS output. This provides a very high input overload capability.

Technical data:

Sensitivity:

70mV input for 40W 8 Ohm output
63mV input for 60W 4 Ohm output

Frequency response:

50Hz to 20KHz -0.5dB; -1.5dB @ 40Hz; -3.5dB @ 30Hz
Total harmonic distortion @ 1KHz and 8 Ohm load:
Below 0.1% up to 10W; 0.2% @ 30W
Total harmonic distortion @ 10KHz and 8 Ohm load:
Below 0.15% up to 10W; 0.3% @ 30W
Total harmonic distortion @ 1KHz and 4 Ohm load:
Below 0.18% up to 10W; 0.4% @ 60W
Total harmonic distortion @ 10KHz and 4 Ohm load:
Below 0.3% up to 10W; 0.6% @ 60W

Bass control:

Fully clockwise = +13.7dB @ 100Hz; -23dB @ 10KHz
Center position = -4.5dB @ 100Hz
Fully counterclockwise = -12.5dB @ 100Hz; +0.7dB @ 1KHz and 10KHz

Low-cut switch:

-1.5dB @ 300Hz; -2.5dB @ 200Hz; -4.4dB @ 100Hz; -10dB @ 50Hz

Notes:

• The value listed for C8 is the minimum suggested value. A 3300µF capacitor or two 2200µF capacitors wired in parallel would be a better choice.
• The Darlington transistor types listed could be too oversized for such a design. You can substitute them with MJ11014 (Q3) and MJ11013 (Q4) or TIP142 (Q3) and TIP147 (Q4).
• T1 transformer can be also a 24 + 24V or 25 + 25V type (i.e. 48V or 50V center tapped). Obviously, the center-tap must be left unconnected.
• SW1 switch inserts the Low-cut feature when open.
• In all cases where Darlington transistors are used as the output devices it is essential that the sensing transistor (Q2) should be in as close thermal contact with the output transistors as possible. Therefore a TO126-case transistor type was chosen for easy bolting on the heatsink, very close to the output pair.
• R9 must be trimmed in order to measure about half the voltage supply across the positive lead of C7 and ground. A better setting can be done using an oscilloscope, in order to obtain a symmetrical clipping of the output wave form at maximum output power.
• To set quiescent current, remove temporarily the Fuse F1 and insert the probes of an Avo-meter in the two leads of the fuse holder.
• Set the volume control to the minimum and Trimmer R3 to its minimum resistance.
• Power-on the circuit and adjust R3 to read a current drawing of about 30 to 35mA.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.

---

Mini Portable Guitar Amplifier

Mini Portable Guitar Amplifier

Circuit diagram:
Parts:

R1______________22K  1/4W Resistor
C1______________10µF  25V Electrolytic Capacitor
C2_____________100nF  63V Polyester or Ceramic Capacitor
C3_____________220µF  25V Electrolytic Capacitor
IC1__________TDA7052  Audio power amplifier IC
J1,J2__________6.3mm  Stereo Jack sockets (switched)
SPKR___________8 Ohm  Loudspeaker (See Notes)
B1________________9V  PP3 Battery or

3V  Battery (2 x 1.5V AA, AAA Cells in series etc.) 

Clip for PP3 Battery or socket for 2 x 1.5V AA or AAA Cells

---

Comments:

This small amplifier was intended to be used in conjunction with an electric guitar to do some low power monitoring, mainly for practice, either via an incorporated small loudspeaker or headphones.

The complete circuit, loudspeaker, batteries, input and output jacks can be encased in a small box having the dimensions of a packet of cigarettes, or it could be fitted also into a real packet of cigarettes like some ready-made units available on the market.

This design can be used in three different ways:

• Loudspeaker amplifier: when powered by a 9V alkaline battery it can deliver about 1.5W peak output power to the incorporated loudspeaker.
• Headphone amplifier or low power loudspeaker amplifier: when powered by a 3V battery (2x1.5V cells) it can drive any headphone set type at a satisfactory output power level or deliver to the incorporated loudspeaker about 60mW of output power. This configuration is useful for saving battery costs.
• Fuzz-box: when powered by a 3V battery (2x1.5V cells) and having its output connected to a guitar amplifier input the circuit will behave as a good Fuzz-box, showing an output square wave with marked rounded corners, typical of valve-circuits output when driven into saturation.

Notes:

• For the sake of simplicity and compactness, this unit employs a dual bridge IC amplifier and a few other parts. For the same reason no volume or tone controls are provided as it is supposed that the controls already existing on the electric guitar will serve satisfactorily to the purpose.
• No power switch is used: the battery voltage will be applied to the circuit when the input plug will be inserted in the input jack socket J1. For this purpose be sure that the input plug is a common 1/4 inch guitar mono jack plug and J1 is a 1/4 inch stereo jack socket.
• The output jack socket J2 must be a switched stereo type. The changeover switching is arranged in such a way that, when a common headphones stereo jack plug is inserted into the socket, the loudspeaker will be disabled and the mono output signal will drive both the headsets in series, allowing full headphone reproduction. When used as a Fuzz-box output, a mono jack plug must be inserted into J2.
• If the amplifier is intended to be encased in a packet of cigarettes, standard loudspeaker diameter should be 57 or 50mm.

---

Technical data:

Max output power: 1.5W @ 9V supply - 8 Ohm load; 60mW @ 3V supply - 8 Ohm load
Frequency response: Flat from 20Hz to 20kHz
Total harmonic distortion @ 100mW output: 0.2%
Max input voltage @ 3V supply: 8mV RMS
Minimum input voltage for Fuzz-box operation: 18mV RMS @ 3V supply
Current consumption @ 400mW and 9V supply: 200mA
Current consumption @ 250mW and 9V supply: 150mA
Current consumption @ 60mW and 3V supply: 80mA
Quiescent current consumption: 6mA @ 9V, 4mA @ 3V supply
Fuzz-box current consumption: 3mA @ 3V supply

IR Remote-Control Checker

---

Suitable for any Infra-red emitting device -- 3V battery supply

---

Circuit diagram:

Parts:

R1_____________470K  1/4W Resistor
R2______________47R  1/4W Resistor
D1______________LED  (Any dimension, shape and color)
Q1____________Infra-red Photo Transistor (Any cheap type)
Q2____________BC327   45V 800mA PNP Transistor
SW1____________SPST  Toggle or Slide Switch (Optional, see Note)
B1_______________3V Battery (2 x 1.5V AA, AAA or smaller type Cells in series)

---

Comments:

A very simple device allowing a quick check of common Infra-red Remote-Controls can be useful to the electronics amateur, frequently asked to repair or test these ubiquitous devices.

A reliable circuit was designed with a handful of components: the LED will flash when any of the Remote-Control pushbuttons will be pressed. The side of the Remote-Control bearing the IR emitting diode(s) must be directed towards the Photo Transistor (Q1) of the checker circuit: maximum distance should not exceed about 20 - 25cm.

Note:

• Current drawing of the circuit is less than 1mA when the LED illuminates and 0mA when no signal is picked-up by the Photo Transistor: therefore, SW1 can be omitted.

Dual-rail Variable DC Power Supply

Dual-rail Variable DC Power Supply

---

Simple add-on for a single-rail supply ±2.5V to ±15V output

---

Circuit diagram:

Parts:

R1,R2____________4K7 1/2W 1% or 2% Metal Oxide Resistors
C1,C4,C5_______100nF  63V Polyester Capacitors
C2,C3__________220µF  25V Electrolytic Capacitors
Q1____________BD437   45V 4A NPN Transistor
Q2____________BD438   45V 4A PNP Transistor
IC1___________LM358   Low Power Dual Op-amp

Input and output connecting terminals etc.

---

Comments:

This design was conceived as an add-on for the Variable DC Power Supply, a very successful circuit posted to this website in the year 2000.
This simple unit provides a dual-rail variable output ranging from ±2.5V to ±15Vdc with precise tracking of the positive and negative output voltages, still retaining the current limiting and short-proof capabilities of the "master" circuit.
As the purpose of such a dual-rail design is to supply experimental or under-repair circuits, the maximum current output delivered was deliberately kept to about 500 - 600mA per rail, thus avoiding the use of expensive power transistors and complex circuitry.

Notes:

• The circuit can be placed into the existing Variable DC Power Supply metal cabinet.
• Q1 and Q2 must be mounted on heatsinks. Usually, bolting them to the metal case (through insulating washers etc.) proved effective.
• The full ±15V output can be achieved only if the secondary winding of the supply Transformer used in the Variable DC Power Supply is rated at 48V minimum (center tapped).
• When using this circuit, please set the Current-limit control (P1) of the Variable DC Power Supply to any value comprised in the 50mA - 1A range but not higher.
• The second Op-amp (IC1B) contained in the LM358 chip was not used, but its input pins were tied to the negative supply and the output was left open.

Programmable LED Flashers

Programmable LED Flashers

---

LED changing to steady state after a preset number of flashes

Two simple, wide supply range operating circuits

---

Circuit diagram #1:

Parts:

R1______________10K  1/4W Resistor
R2_______________1M  1/4W Resistor
R3_______________1K  1/4W Resistor (See Notes)
C1_______________4µ7  25V Electrolytic Capacitor
C2______________10nF  63V Polyester Capacitor
D1___________1N4148   75V 150mA Diode
D2______________LED (Any dimension, shape or color)
IC1____________4060  14 stage ripple counter and oscillator IC

P1_____________SPST  Pushbutton
SW1____________SPST  Toggle or Slider Switch
B1______________3V to 15V Battery or dc power source (See Notes)

---

Circuit diagram #2:

Parts:

R1_____________100K  1/4W Resistor
R2_______________1K  1/4W Resistor (See Notes)
R3______________10K  1/4W Resistor
C1,C2____________4µ7  25V Electrolytic Capacitors
D1___________1N4148   75V 150mA Diode
D2______________LED (Any dimension, shape or color)
IC1____________7555 or TS555CN CMos Timer IC
IC3____________4017  Decade counter with 10 decoded outputs IC
SW1____________1 pole 9 ways Rotary Switch (Optional)
SW2____________SPST  Toggle or Slider Switch
B1______________3V to 15V Battery or dc power source (See Notes)

---

Comments:

These circuits were designed on request. Both feature a flashing LED that, after a preset number of flashes will illuminate steadily until P1 (Reset) will be pressed.

Circuit #1 uses only one chip and can be useful if a not very precise number of flashes of the LED is needed before reverting to the steady-on state. In fact, connecting D1 Anode to different output pins of the IC, the steady-on state of the LED will be obtained after 2, 4, 8, 16 flashes and so on.
Connecting D1 Anode as shown, the LED will start flashing at about two times per second after power-on and will revert to the steady state after 8 flashes. P1 resets the circuit and C1 automatically resets IC at power-on.
Connecting D1 Anode to pin #13 of IC1 the flashes will be 4; to pin #1 will be 16 etc.
The flashing frequency of the LED can be varied by changing the values of R2 and/or C2.

Circuit #2 is more precise and uses about the same parts count of Circuit #1, though requiring two ICs. By choosing the appropriate output pin of IC2, the steady-on state of the LED will be obtained after 1 to 9 flashes, as shown in the drawing at SW1 pins. This switch is optional, as D1 Anode can be hard wired directly to the required output pin of IC2. P1 will work as in Circuit #1 but with some difference: after a momentarily press the LED will restart to flash, but the total number of flashes will be one less than obtained after power-on. Furthermore, if P1 is closed permanently, the circuit will flash permanently.
The flashing frequency of the LED can be varied by changing R1 and/or C1 values.

Notes:

• Circuits were tested at 9V supply, but they might work in the 3 - 15V dc supply range.

The LED current limiting resistor value was calculated for 9 - 12V supply and should be changed to suit different supply voltages