TRANSISTOR TESTER

The 555 operates at 2Hz. Output pin 3 drives the circuit with a positive then zero voltage. The other end of the circuit is connected to a voltage divider with the mid-point at approx 4.5v. This allows the red and green LEDs to alternately flash when no transistor is connected to the tester.

If a good transistor is connected, it will produce a short across the LED pair when the voltage is in one direction and only one LED will flash. If the transistor is open, both LED’s will flash and if the transistor is shorted, neither LED will flash.

SUPER EAR

This circuit is a very sensitive 3-transistor amplifier using a speaker transformer. This can be wound on a short length of ferrite rod as show above or 150 turns on a 10mH choke.  The biasing of the middle transistor is set for 3v supply. The second and third transistors are not turned on during idle conditions and the quiescent current is just 5mA. The project is ideal for listening to conversations or TV etc in another room with long leads connecting the microphone to the amplifier.

Simple Circuit tests Zener Diodes

 

This circuit will test zener diodes up to 56v. See Talking Electronics website, left index, 200 Transistor Circuits (circuits 1-100) and go to Zener Diode (making) to see how to make a zener diode and how to create a zener voltage from a combination of zeners. Place the zener across the terminals in the circuit below and read the value across it with a multimeter set to 50v range.