Home > mini projects > TRANSISTOR TESTER USING IC 555 TIMER



In electronics most commonly required component is transistor. Therefore the testing of the transistor is very important to avoid the future inconvenience and the loss to the manufacturer. Transistor tester is an instrument which is used to test the electrical behavior of a transistor. Commonly used devises to test the transistor are costly.So, in order to rectify this problem, we will go for transistor tester using IC 555 Timer.This report will explains the circuit based on the above. In this system, we will use green and red colour LEDs to identify the condition of the given transistor to be tested. Based on the glowing pattern of the LEDs, we can detect whether the transistor is connected, good or shorted.


Bipolar junction transistors are often used in physical laboratory and in different electrical and

electronic circuits for a variety of purposes. Sometimes during experiments or projects, they are required to test functioning of transistors. In general, a transistor tester is done

using microprocessor based expensive

apparatus.The following circuit explains the easy and comparatively less expensive to test the transistor using IC 555 timer


By Transistor switching property. It switches an LED ON and OFF continuously. The timer here generates clock and is fed to transistor which is to be tested to drive an LED.A NPN transistor respond to positive voltage only, so to turn on an NPN transistor we should give a voltage greater than 1V. Once the positive voltage is provided at the base, the NPN transistor move from cutoff mode to saturation mode.


The working of transistor tester circuit is such that it operates at 2Hz of frequency. The output pins 3 makes the transistor tester circuit with a positive voltage, and then with a non-zero voltage. To the other end of this circuit, a voltage divider is connected with the midpoint at approximate 4.5V. When no transistor is connected orwhen a faulty transistor is connected, the output pulse of 555 timer at pin no.3 has two paths- a) Through resistorsb) Through LEDsDuring the positive half cycle of the square wave output, the green LED is in forward bias and red LED is in reverse bias whereas during the negative half cycle, the red LED is in forward bias and green LED is in reverse bias.Hence, for half time period of the output square wave, green LED is ON and for the next half time period, red LED is ON. When a properly working transistor is connected, during the positive half cycle, it is similar to the previous case as the transistor will be OFF. In this case, the green LED is ON.During the negative half cycle, the diode D2 is in conduction and the current flows in the transistor from collector to emitter through base but not through the LED branch. Hence no LED is ON. When there is a short circuit, the output pulse of 555 timer at pin no.3 chooses the short circuit path since there is no resistance in that path and hence the LEDs will not glow in any half cycle. In the nutshell the result will be like this:When no transistor is connected to the tester, the green and red LEDs flash alternatively. If only green LED blinks, the condition of the transistor will be OK. If none of the LED flashes, the transistor will be shorted .



In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and another resistor (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor. In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C



WHEN NO TRANSISTOR IS CONNECTED The green and red LEDs flash alternatively.

(fig:Green LED glowing for first half cycle)

(fig:Red LED glowing for second half cycle)




IF the transistor is ok, then green led will blink continuously.

FIG:green led is blinking.



The circuit is simple as compared to other transistor testers, and therefore, is useful for technicians as well as students. It can be easily build up on a general purpose PCB. To develop this circuit, basic electronic components like resistors, diodes, LEDs and NE555 are used. By using this circuit, different faults can be checked -like to know whether the condition of a transistor is good or not, and opened or shorted