Simple Transistor Tester Circuit Using 555 Timer IC

Most electronics hobbyists face such difficulties. Faulty components drive us crazy and make us hate electronics hobby circuits. It’s quite a common scenerio for electronics DIYers to run into some faults while setting up a circuit. In a good number of cases the problem lies regarding troubles in the transistor. But How to test a transistor in circuit? 

You might be knowing how to test transistors using digital multimeters. But this conventional method is always time-consuming and only more skilled persons can do this with multimeter. By considering these matters, I planned to give a Simple transistor tester circuit using IC 555 for hobbyists.

This Transistor Tester circuit is built around two 555 timers. And the kit will help you to identify faulty transistors easily with a transistor tester circuit led flasher. So let’s start building the in-circuit transistor tester kit.

Circuit Diagram of Transistor Tester

Components Required

• 555 Timer IC x 2
• Resistors (1kΩx3; 10kΩx2; 120Ω; 330Ωx2)
• Potentiometers (10k; 47k; 1M)
• Capacitors (0.01µF)
• Electrolytic Capacitors (22µF,16V; 10µF,16V)
• Button Switch x 2
• DPDT Switch x 1
• LED x 2

NB: Instead of RV3 (1MΩ Pot) you can use a 1MΩ resistor

Transistor Tester Circuit Analysis and Working

• The circuit includes two NE555 timer ICs, one (U1) is configured as Astable Multivibrator Mode and the other (IC2) wired as Monostable Multivibrator mode.
• The combined operation of Astable and Monostable provides this awesome transistor tester circuit.

Transistor tester circuit logic is given below as a block diagram.

• The time period of the Astable multivibrator is calculated by using our Astable calculator tool and it is found to be approx. 0.5 seconds.
• Output of Astable drives the base of NPN/PNP transistor under test with the help of DPDT switch. (A typical DPDT switch is shown below).

• This DPDT switch chooses the transistor to be tested, we cannot test both NPN and PNP simultaneously. Only one, either NPN or PNP at a time.
• The push button switch SW triggers Monostable multivibrator (IC2).
• The time period of this Monostable can be calculated using our Monostable calculator tool, and it is set to 10 seconds approx.

For quick reference, the output LED status for each transistor is given in the following table.

Transistor Tester Circuit NPN Operation

Now let’s see how to test an NPN transistor using this circuit.

Place NPN transistor at the specified area in the circuit, then slide the DPDT switch towards the NPN section to activate NPN testing.

To start the test, please trigger the Monostable by pushing the button SW.
Case 1: Good NPN transistor (Keep pressing trigger button SW)

• If the LED 1 and LED 2 start flickering, the transistor is OK and working fine. Because the collector of NPN transistor is connected to Reset pin (Pin 4) of IC2.
• Here our transistor works as a switch, if the transistor is OK, it will allow inverted output of Astable to pass to Monostable.
• Since we keep triggering the output of IC2 blinks with period of 0.5 seconds.
• A good transistor working animation according to base voltage is shown below.

Case 2: Collector-Emitter junction is short (Just apply single trigger)

• If LED 1 remains ON and LED 2 remains OFF, the transistor is having shorted collector-Emitter junction.
• If the transistor is with Collector-Emitter junction shorted, the Reset pin of Monostable (Pin 4) always get 0V (Ground Potential). So it remains OFF.
• Thus even if you trigger the Monostable, LED 1 remains glowing and LED 2 remains OFF.
• An NPN transistor with Shorted collector-Emitter junction is shown below.

Case 3: Collector-Emitter junction is open (Just apply single trigger)

• Once you have triggered, if LED 1 is OFF for 10 sec and LED 2 ON for 10 sec, the transistor is having opened collector-Emitter junction.
• If the transistor is with Collector-Emitter junction opened, the Reset pin of Monostable (Pin 4) always get 5V (Vcc Potential). So Monostable works as normal.
• Thus for the trigger, LED 1 OFF and LED 2 ON for the duration of 10 seconds.
• An NPN transistor with Opened collector-Emitter junction is as shown in below figure.

Transistor Tester Circuit PNP Operation

Now let’s see how to test a PNP transistor using this circuit.
Place PNP transistor at the stated position in the circuit, then slide the DPDT switch towards the PNP section to activate PNP testing.
To start the test, please trigger the Monostable by pushing the button SW.
Case 1: Good PNP transistor (Keep pressing trigger button SW)

• If LED 1 and LED 2 starts flickering, the transistor is OK and working fine. Because the collector of PNP transistor is connected to Reset pin (Pin 4) of IC2.
• Here also PNP transistor works as a switch, if the transistor is OK, it will allow inverted output of Astable to pass to Monostable.
• Since we keep triggering the output blinks with period of 0.5 seconds each.
• Typical working of PNP transistor with emitter voltage is as shown. (Please note base voltage is 0V)

Case 2: Collector-Emitter junction is short (Just apply single trigger)

• Once you triggered, if LED 1 remains OFF and LED 2 remains ON for a duration of 10 seconds, the transistor is having shorted collector-Emitter junction.
• If the transistor is with Collector-Emitter junction shorted, the Reset pin of Monostable (Pin 4) always get 5V (Vcc Potential). So Monostable works as normal.
• Thus for the trigger LED 1 OFF and LED 2 ON for the duration of 10 seconds.
• A shorted Collector-Emitter PNP transistor is just like this.

Case 3: Collector-Emitter junction is open (Just apply single trigger)

• If LED 1 remains ON and LED 2 remains OFF, the transistor is having opened collector-Emitter junction.
• If the transistor is with Collector-Emitter junction opened, the Reset pin of Monostable (Pin 4) always get 0V (Ground Potential). So Monostable remains OFF.
• Thus even if you trigger LED1 remains glowing and LED 2 remain OFF.
• An opened Collector-Emitter junction transistor just like,

Set up instructions

This circuit is one of the basic requirements of an Electronics hobbyist. Hence please build this by considering future use.

• Assemble the circuit on PCB with transistor socket, so that you can easily insert transistors any time for testing.

• In the simulation I have used Green LED and Red LED, no matter you can use same colour or up to your own choice.
• Provide a perfect housing for the circuit by only the transistor socket comes out.

Conclusion

The transistor tester circuit using 555 timer ic is capable of testing both NPN as well as PNP transistors. And it shows whether the transistor is good, open or shorted with the help of two LEDs. So, the LEDs are quite vital.