12V Car Battery Charger

The usual chargers of battery automotive, are simple and cheap appliances that charge continuously the battery, with a rythm of few amperes, for the time where the appliance is ON. If the holder do not close in time the charger, the battery will overcharge and her electrolytic faculty are lost with evaporation or likely exists destruction of her elements. The charger of circuit exceeds these faults. It checks electronic the situation of charge of battery and it has circuit of control with retroaction, that forces the battery charge with biggest rythm until charge completely. When charge completely, it turns on one RED led (LD2). The charger has been drawn in order to charge batteries of 12V, ONLY. What should watch it from what it manufactures the circuit, they are the cables that connect the transformer with the circuit and in the continuity the battery, should they are big cross-section, so that heat when it passes from in them the current of charge and also they do not cause fall of voltage at the way of current through them.

Adjustment

After assembling of the circuit, adjust TR1 to null value, power-up and make the following adjustments :-

[1] Without connecting the battery check that the 2 LED?s are turned on.

[2] Connect a car battery to the circuit and check that LD2 is OFF and a current (normally 2A to 4A) is flowing to the battery.

[3] Adjust TR1 until LD2 turns ON and the charge current is cut.

[4] Adjust TR1 to null value and charge the battery using the hydrometer technique (if you do not have or do not know how to use a hydrometer, then use a good condition battery and charge).

Carefully adjust TR1 so that LD2 begins to turn ON and the charge current falls to a few hundred milliamps (mA). If TR1 is set correctly then in the next round of charging you will noticed LD2 begin to flicker as the battery is being charged. When battery is completely charged, LD2 turns ON completely.TR1 does not need further adjustment anymore. Q1 is connected in line with the battery and is fired by R3, R4 and LD2. The R2, C1, TR1 and D2 sense the voltage of the battery terminal and activate Q2 when the voltage of the battery terminal exceeds the value predetermined by TR1. When an uncharged battery is connected, the terminal voltage is low. Under this circumstance, Q2 is turned OFF and Q1 is fired in each half cycle by R3, R4 and LD2. The Q1 functions as a simple rectifier and charges the battery. If the battery terminal voltage is increased above the level that had been fixed by TR1, then Q2 shifts the control of Q1 gate. This deactivates Q1 and cuts off the current supply to the battery and turns LD2 ON indicating that the charge has been completed. Q1 and bridge rectifier GR1 should be mounted on heatsinks to prevent overheating. M1 is a 5A DC ammeter to measure the charge current.

12V to 16V DC/DC Converter with LM2577

The circuit is a boost step-up regulator based around an LM2577-ADJ voltage regulator chip and a few other discrete components. Resistors R1 and R2 set the regulated output voltage.

A switch inside the voltage regulator closes between pins 4 and 3, causing current to flow through the inductor to ground. When the switch is released a few microseconds later, a back-EMF 'kick' is produced by the inductor, resulting in a positive pulse with respect to the input voltage. This pulse charges the output capacitor via the schottky diode, which tends towards an equilibrium voltage.

The switch continues to oscillate, the diode preventing the switch from shorting the output capacitor during the 'on' phase. The output voltage is monitored via the voltage divider R1/R2, causing the duty cycle of the switch oscillator to be continuously regulated in order to maintain a constant output voltage under varying loads.

DTMF Proximity Detector

A DTMF-based IR transmitter and receiver pair can be used to realize a proximity detector. The circuit presented here enables you to detect any object capable of reflecting the IR beam and moving in front of the IR LED photo detector pair up to a distance of about 12 cm from it. The circuit uses the commonly available telephony ICs such as dial-tone generator 91214B/91215B (IC1) and DTMF decoder CM8870 (IC2) in conjunction with infrared LED (IR LED1), photodiode D1, and other components as shown in the figure. A properly regulated 5V DC power supply is required for operation of the circuit.

The transmitter part is configured around dialer IC1. Its row 1 (pin 15) and column 1 (pin 12) get connected together via transistor T2 after a power-on delay (determined by capacitor C1 and resistors R1 and R16 in the base circuit of the transistor) to generate DTMF tone (combination
of 697 Hz and 1209 Hz) corresponding to keypad digit “1” continuously.LED 2 is used to indicate the tone output from IC3. This tone output is amplified by Darlington transistor pair of T3 and T4 to drive IR LED1 via variable resistor VR1 in series with fixed 10-ohm resistor R14. Thus IR LED1 produces tone-modulated IR light. Variable resistor VR1 controls the emission level to vary the transmission range. LED 3 indicates that transmission is taking place. A part of modulated IR light signal transmitted by IR LED1, after reflection from an object, falls on photodetector diode D1. (The photodetector is to be shielded from direct IR light transmission path of IR LED1 by using any opaque partition so that it receives only the reflected IR light.) On detection of the signal by photodetector, it is coupled to DTMF decoder IC2 through emitter-follower transistor T1. When the valid tone pair is detected by the decoder, its StD pin 15 (shorted to TOE pin 10) goes ‘high’. The detection of the object in proximity of IR transmitter receiver combination is indicated by LED1. The active-high logic output pulse (terminated at connector CON1, in the figure) can be used to switch on/off any device (such as a siren via a latch and relay driver) or it can be used to clock a counter, etc.

This DTMF proximity detector finds applications in burglar alarms, object counter and tachometers, etc.

20M, 4W QRP TRANSMITTER

Using this small yet powerful 4W transmitter Hams (licenced amateur radio operators) can transmit Morse code signals to long distances in 20-metre band. Morse code communication with European and neighbouring countries is possible. The transmitter (refer Fig. 1) comprises an oscillator, driver, and power amplifier. The oscillator is crystal-controlled. For this, an inexpensive and readily available crystal with fundamental frequency of 14.314 MHz is used with the horizontal dipole antenna oscillator. The oscillator delivers power of about 200 milliwatts. The next stage is a class-C driver that delivers power of nearly 1 watt. The final stage is a class-C power amplifier wired around transistor BD139, which delivers power of 4 to 5 watts.

After assembling the circuit, connect a 12V, 5W bulb across the antenna’s terminals. Apply regulated 12-15V DC to the circuit. Adjust gang condenser’s knob until the bulb glows, which indicates that the

Fig. 2: Connection of the transmitter to a horizontal dipole antenna

transmitter is okay. (Caution. Don’t switch on the transmitter without an output load.) After checking with bulb as load, the transmitter can be connected to a horizontal dipole antenna via 75-ohm coaxial cable as shown in Fig. 2. Each arm of the dipole antenna is about 5 metre long. The correct length L (in metres) of single pole of the antenna can be calculated using the following relationship: L = 71.6/ frequency (MHz) Winding details of coils are as follows: L1: 9 turns of 24 SWG insulated copper wire over 8mm dia. oscillator coil former with ferrite bead L2: 3 turns of 24 SWG insulated copper wire over coil L1 L3: 9 turns of 24 SWG insulated copper wire over 8mm dia. former with movable ferrite bead L4: 14 turns of 20 SWG insulated copper wire over 1.5cm dia. PVC former. (The coil is to be tapped from 9th to 10th turn from the top end.) RFC: 12 turns of 36 SWG insulated copper wire using TV balun core Warning. Use of this transmitter without Ham licence is illegal, hence only Ham licence holders should assemble this project.

FM Booster

Here is a low-cost circuit of an FM booster that can be used to listen to programmers from distant FM stations clearly. The circuit comprises a common-emitter tuned RF preamplifier wired around VHF/UHF transistor 2SC2570. (Only C2570 is annotated on the transistor body.)

Assemble the circuit on a good-quality PCB (preferably, glass-epoxy). Adjust input/ output trimmers (VC1/VC2) for maximum gain.

Input coil L1 consists of four turns of 20SWG enameled copper wire (slightly space wound) over 5mm diameter former.It is tapped at the first turn from ground lead side. Coil L2 is similar to L1, but hasonly three turns. Pin configuration of transistor2SC2570 is shown in the figure