Radio Remote Control using DTMF

Here is a circuit of a remote control unit which makes use of the radio frequency signals to control various electrical appliances. This remote control unit has 4 channels which can be easily extended to 12. This circuit differs from similar circuits in view of its simplicity and a totally different concept of generating the control signals. Usually remote control circuits make use of infrared light to transmit control signals. Their use is thus limited to a very confined area and line-of-sight. However, this circuit makes use of radio frequency to transmit the control signals and hence it can be used for control from almost anywhere in the house. Here we make use of DTMF (dual-tone multi frequency) signals (used in telephones to dial the digits) as the control codes. The DTMF tones are used for frequency modulation of the carrier. At the receiver unit, these frequency modulated signals are intercepted to obtain DTMF tones at the speaker terminals. This DTMF signal is connected to a DTMF-to-BCD converter whose BCD output is used to switch-on and switch-off various electrical appliances (4 in this case).

Updated Circuit

The remote control transmitter consists of DTMF generator and an FM transmitter circuit. For generating the DTMF frequencies, a dedicated IC UM91214B (which is used as a dialer IC in telephone instruments) is used here. This IC requires 3 volts for its operation. This is provided by a simple zener diode voltage regulator which converts 9 volts into 3 volts for use by this IC. For its time base, it requires a quartz crystal of 3.58 MHz which is easily available from electronic component shops. Pins 1 and 2 are used as chip select and DTMF mode select pins respectively. When the row and column pins (12 and 15) are shorted to each other, DTMF tones corresponding to digit 1 are output from its pin 7. Similarly, pins 13, 16 and 17 are additionally required to dial digits 2, 4 and 8. Rest of the pins of this IC may be left as they are. The output of IC1 is given to the input of this transmitter circuit which effectively frequency modulates the carrier and transmits it in the air. The carrier frequency is determined by coil L1 and trimmer capacitor VC1 (which may be adjusted for around 100MHz operation). An antenna of 10 to 15 cms (4 to 6 inches) length will be sufficient to provide adequate range. The antenna is also necessary because the transmitter unit has to be housed in a metallic cabinet to protect the frequency drift caused due to stray EM fields. Four key switches (DPST push-to-on spring loaded) are required to transmit the desired DTMF tones. The switches when pressed generate the specific tone pairs as well as provide power to the transmitter circuit simultaneously. This way when the transmitter unit is not in use it consumes no power at all and the battery lasts much longer. Updated Circuit -(Bug Free)

The receiver unit consists of an FM receiver (these days simple and inexpensive FM kits are readily available in the market which work exceptionally well), a DTMF-to-BCD converter and a flip-flop toggling latch section. The frequency modulated DTMF signals are received by the FM receiver and the output (DTMF tones) are fed to the dedicated IC KT3170 which is a DTMF-to-BCD converter. This IC when fed with the DTMF tones gives corresponding BCD output; for example, when digit 1 is pressed, the output is 0001 and when digit 4 is pressed the output is 0100. This IC also requires a 3.58MHz crystal for its operation. The tone input is connected to its pin 2 and the BCD outputs are taken from pins 11 to 14 respectively. These outputs are fed to 4 individual ?D? flip-flop latches which have been converted into toggle flip-flops built around two CD4013B ICs. Whenever a digit is pressed, the receiver decodes it and gives a clock pulse which is used to toggle the corresponding flip-flop to the alternate state. The flip-flop output is used to drive a relay which in turn can latch or unlatch any electrical appliance. We can upgrade the circuit to control as many as 12 channels since IC UM91214B can generates 12 DTMF tones. For this purpose some modification has to be done in receiver unit and also in between IC2 and toggle flip-flop section in the receiver. A 4-to-16 lines demultiplexer (IC 74154) has to be used and the number of toggle flip-flops have also to be increased to 12 from the existing 4

Download PCB (EXPRESS PCB FORMAT)

1) Transmitter Section - download-it
2) Receiver Section - download-it
3) Front panel (for cabin) - download-it

4) Data flow Diagram (PPT) - download-it

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MAINS operate Christmas Star

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Here is a low-cost circuit of Christmas star that can be easily constructed even by a novice. The main advantage of this circuit is that it doesn’t require any step-down transformer or ICs. Components like resistors R1 and R2.capacitors C1, C2, and C3, diodes D1 and D2, and zener ZD1 are used to develop a fairly steady 5V DC supply voltage that provides the required current to operate the multivibrator circuit and trigger triac BT136 via LED1.
The multivibrator circuit is constructed using two BC548 transistors (T1 and T2) and some passive components. The frequency of the multivibrator circuit is controlled by capacitors C4 and C5 and resistors R3 through R7. The output of the multivibrator circuit is connected to transistor T3, which, in turn, drives the triac via LED1. During positive half cycles of the multivibrator’s output, transistor T3 energizes triac BT136 and the lamp glows.

LED Lighting For Christmas

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Using light effects for decoration on festive occasions is a normal practice. Designers are coming up with varieties of electronic circuits to fill the imagination of users. Here is an easy-to-assemble circuit for christmas decoration as shown in Fig.1. It comprises four transistors, eighteen LEDs, a few resistors and two capacitors. Transistors T1 and T2 are configured as an astable multivibrator, which means one of the two transistors is always conducting. Thus the combination produces clock pulses. The values of time-constants formed with R6-C2 and R8-C1 pairs have been selected to produce a lowfrequency clock that is visible to human eye. The collectors of transistors T1 and T2 are connected to driver transistors T3 and T4. These are used to light up two rows of LEDs connected in parallel with alternate clock pulses. The frequency at which LED1 through LED9, and LED10 through LED18, alternately light up is about 2 Hz. You can easily change this frequency by changing the values of capacitors C2 and C1.
Resistors R2 and R4 are used to set the current through the LEDs. Red (LED1 through LED9) and green LEDs (LED10 through LED18) are used for simulating christmas decoration effects. For the brightness variation, you can change the values of resitors R2 and R4. Take any general-purpose PCB and cut it into a star shape. Thereafter, assemble the circuit and solder the colour LEDs onto it such that it looks like a christmas star.
Alternatively, you can design the PCB in circular shape with a festive white lacquer finish on component side and conductor tracks on the other. Place the control circuit at the centre of the PCB board, with LEDs mounted along the outer edge as shown in Fig. 2. Along this edge, there are three circular tracks:
The middle one is the positive supply, which goes to the anodes of all LEDs. The outer track is connected to the cathodes of the red LEDs and the inner tracks are connected to the cathodes of the green LEDs.

To obtain the best effect with the combination of red and green LEDs, mount them alternately on the PCB board. Exercise care so that you do not accidentally connect the red and green LEDs in parallel. The forward voltage drops of red and green LEDs are different. The circuit works off a 3V-9V battery. It consumes little current, so two/ four AA cells or a 9V battery can easily power the electronic star. You can also use a stabilised 3V-9V DC mains adap-
Fig.1: LED lighting circuit for Christmas tor in place of the battery.

Reference : EFY
www.electronicsforu.com/electronicsforu/lab/ad.asp?url=/EFYLinux/circuit/December2007/CI-01_Dec07.pdf&title=LED%20Lighting%20for%20Christmas

Long Range FM Transmitter Circuit

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This circuit is a circuit diagram fm transmitter. This circuit is somewhat different from the previous fm transmitter circuit. Transmitter circuit described here has the additional RF power amplifier stage, after the oscillator stage, to increase the power output of 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element yagi antenna, this transmitter can provide a good enough signal strength to a distance of about 2 kilometers. The circuit built around transistor T1 (BF494) is the basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts.
Transistor T2 (2N3866) forms a VHF-class power amplifier. This increases the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power produced at the collector of transistor T2. For better results, assemble the circuit on a good quality glass epoxy board and house the transmitter in the case of aluminum. Shield the oscillator stage using aluminum sheets. Transistor T2 must be mounted on the heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.

Coil winding details are given below:
L1 – 4 changes of 20 SWG wire close wound over 8mm diameter plastic former.
L2 – 2 changes of 24 SWG wire near top end of L1.
(Note: There is no core (ie air core) is used to coil on top)
L3 – 7 changed from 24 SWG wire close wound with 4mm diameter air core.
L4 – 7 changed from 24 SWG wire-wound on ferrite beads (choking)
Potentiometer VR1 is used to change the fundamental frequency whereas potentiometer VR2 is used as power control.

Simple DC to AC Inverter

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This DC to AC inverter circuit work based on unstable multi vibrator does. In this circuit, IC CD4047 is chosen as a heart of unstable multi vibrator, because this IC type gives a complementary output that has opposite phase to another ( pin 10 and 11 as seen in Figure 1), and has 50 % duty cycle that satisfy to generate a pulse for inverter.

In order to increase the current out of multi

vibrator so enough to generate a higher AC power too, then we must use MOSFET IRFZ44. IRFZ44 gives out high current to drive a step-up transformer, so AC power is available at the high voltage side of transformer.

This circuit is called as simple DC to AC inverter because of the output haven't a sinusoidal signal

yet, so there are many harmonic signal at the output. To suppress this signal we must use a filter such as a capacitor C. Because of this simplicity this circuit is suitable only for lighting demand. To build a sinusoidal DC to AC inverter, we can use a PWM signal for driving a step-up transformer, such as at page of DC to AC inverter using AT89C2051.