Description.
This circuit can be primarily used for charging 12V Ni-Cd battery packs. Any way 6V and 9V battery packs can be also charged by using this circuit a little compromise on power efficiency. The built in automatic current regulator regulates the charging current to somewhat 4 amperes. When the charging current reaches 4A, the voltage across resistor R1 becomes 0.7V and switches the transistor Q1 ON. The transistor Q2 which is now in ON state will shorts the base of Q3 to ground and inhibits the biasing of Q4 through which the charging is done. That is how the current regulation is achieved. When charging low voltage battery packs, the excess voltage will be dropped across Q4.It is advised to use a heat sink with transistor Q4.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a Vero board.
• T1 can be a 230V primary, 12V secondary,4A step down transformer.
• Bridge D1 can be made by using 1N5400 diodes.
• Fuse F1 can be a 500mA type.

Description.
Here is a simple circuit that can be used as a telephone status indicator. When the telephone is in use (off hook) the transistor Q1 switches ON making the red LED D2 glow. When the telephone is not in use (on hook) the Q1 turns OFF and Q2 turns ON. This makes the red LED D2 off and green LED D3 ON. The circuit is powered from the phone line itself and no external power supply is required.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a general purpose PCB.
• IF 2A ampere bridge is not available, make one using diodes like 1N4007.
• Note that some countries prohibit people from connecting other devices to the phone line.

Description.

This 40V dual power supply circuit was designed in response to a request made by Michael from Philippines. His application is to power the 150 Watt amplifier circuit published here. I think this power supply design is adequate for the purpose. The transformer T1 steps down the mains voltage, bridge D1 performs the rectification, C1 and C2 does the job of filtering.C3 and C4 are decoupling capacitors.

Circuit diagram with Parts list.

Notes.

• The T1 must be a 230V primary, 30-0-30V secondary, 200VA transformer.
• All capacitors must be rated at least 63V.
• C3 and C4 are polyester capacitors.
• If 8A bridge is not available, make one using diodes like P600A.

Description.
Here is a very simple wind meter (anemometer) circuit. I can’t guarantee much on the accuracy of this circuit but it circuit works quite fine. You can measure wind speeds up to 75m/s using this circuit.
The transistors Q1 and Q2 are used for sensing the wind. The relationship between thermal impedance of the transistor and the surrounding wind speed is utilized here. Transistors Q1 and Q2 are wired so that the Vce of Q1 is higher than Q2 and therefore there will be a higher power dissipation. The wind causes cooling and so the Vce of Q1 changes. The ends in different power dissipations and different voltages across R10.This variation is detected by the opamp and amplified to produce the Vout which is proportional to the wind speed. For still air Vout will be 0V and at 75m/s wind speed the Vout will be 2.5V. A 3V FSD voltmeter connected across the Vout terminal and ground can be used as the display.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a Vero board.
• The circuit can be powered from 5V DC.
• For proper working, the air must pass over both the transistors (Q1 and Q2).
• Most of the resistors used here are not standard values. So you need to use the combination (series or parallel) of resistors to attain the specified values. Please note that the resistor values are very critical in this circuit.
• IC1 is an LT1013 dual opamp.

Description.
Here is a very simple circuit that can be used for testing Zener diodes. The IC1 NE555 is wired as an astable multivibrator and the output of the IC is stepped up to a high voltage AC using the transformer T1.The unloaded AC voltage will be around 120V. The AC voltage is rectified by the diode D1 and filtered by the capacitor C3.This voltage is applied across the Zener diode through the current limiting resistors R2 or R3 which can be selected by switch S2 for testing the diode at 1mA or 2mA respectively. The digital multimeter in DC voltage mode is connected across the Zener diode under test. The Zener diode is a fine one; the meter will show the correct Zener voltage specified by the manufacturer.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a Vero board.
• Use a 9V PP3 battery for powering the circuit.
• The capacitor C3 must be rated at least 150V.
• Use a holder for mounting IC1.
• T1 can be a 110V primary , 9V secondary step down transformer connected in reverse.