Basic Transistor Tester

About: Just a guy who likes to build stuff. Please consider subscribing to my YouTube Channel for more awesome projects. https://www.youtube.com/channel/UCDkYnToaY_G0-O_aQo3GrYA

In this instructable I will be showing you how to create a Simple Transistor Tester!

Step 1: Intro

In this project I will be using one of my favorite IC’s, the 555 timer, to build a simple transistor tester circuit with a custom 3D printed case that I can put in my pocket or toolbox. It is a very basic transistor tester circuit but is much faster than using a multimeter and going one terminal to another. I often buy transistors in large quantity’s and many of them I have found to not work so I am hoping this tester will help save some time.

Step 2: 555 Timer Background

The 555 Timer is a fantastic precision timer that can act as either an oscillator (astable mode) or as a timer (monostable mode). In monostable mode it resembles a one-shot timer where a trigger voltage is applied and the chips output goes from low to high based on a time set by an external RC circuit. I rarely use the 555 timer in monostable mode but have had many applications where I have used the IC in astable mode. In this mode the 555 acts as a square wave generator whose waveform can be adjusted by two external RC circuits.

If you look at the image above, you can begin to see just where the 555 timer gets its name from, the three 5k resistors in series. These resistors act a a three step voltage-divider between +Vcc and Ground. The outputs from each divider represent 2/3 Vcc and 1/3 Vcc which are then fed into two comparators. A comparator is pretty simple, it looks at its terminals + and – and if + is greater than the – input, it drives the output high or low. These are fed into the Set and Reset inputs on the flip-flop. The flip-flop looks at the S and R values and produces either a high or a low based on the voltage states at the inputs. Using external RC circuits we can control the frequency of the output pin.

Step 3: Components

1. 555 Timer IC

2. 100 and .01 uF capacitor

3. 10k potentiometer with nut and cover

4. 1K Resistor (2)

5. 2.5K Resistor

6. 100 Ohm Resistor

7. 9V Battery

8. LED

9. Soldering iron

10. 3D printer and filament

Step 4: Electrical Schematic

In this circuit I will be using the 555 timer in a very basic astable mode.

The 555 timer above works in the following way.


1. When power is first applied the capacitor C1 is initially uncharged. This means that 0V is on pin 2, forcing its comparator high. This in turn sets Q- low and since there is an inverter on the ouput, sets pin 3 high which turns on an NPN transistor. For PNP it will use the opposite cycle.

2. With Q- low, the NPN transistor internal to the 555 is turned off, which lets the capacitor C1 charge toward Vcc through R2 and R1.

3. As soon as the capacitor reaches 2/3 Vcc, the comparator goes high and resets the flip-flop. Q- goes high and output goes low turning on a PNP transistor.

4. The 555 timers NPN transistor turns on and discharges the capacitor through R2 and R1.

5. When the capacitor reaches 1/3 Vcc Q- goes low and the output is turned on, resetting the cycle.

I wanted to make the circuit work for both PNP and NPN transistors which this circuit does by using the opposite outputs from the 555 timer.

The time on/off is determined by the following:

Time Low = .693(R2+R1)

Time High= .693(R3+R2+R1)*(C1)

The duty cycle will be given by:

Duty Cycle = Time High/ Time High + Time Low

By adjusting the 10k potentiometer, I will be able to control the speed of the duty cycle. It is easy to see how such a simple and common ic can be used in a lot of different applications.

Step 5: Building the Circuit

I suggest that you build the circuit on a breadboard first to verify it works. After you have tested the circuit out on a breadboard, then begin to solder up all the components onto a perf board.

Step 6: 3D Design and Print

Since I wanted this simple tester to be durable enough to throw around in a toolbox, I designed a custom 3D printed enclosure.

I wanted the tester to be portable so I made a simple holder for a 9V battery. I also made holes for the On/Off pushbutton, potentiometer, LED, and for the transistor connections.

After measuring the perf board and the 9V battery, I decided to make the case 100 x 60 x 25 mm.

The files can be downloaded from thingiverse here.

Step 7: Assemble and Test It Out!

After you have soldered up your perf board and printed the enclosure, it is time to assemble everything together and test it out!

You will need to install/connect the on/off switch, potentiometer, transistor connections, and the LED.

Once everything is installed/connected, turn on power, insert a transistor, and if it functioning properly, the LED will blink. You can adjust the potentiometer to increase the speed of the 555 timer output. This circuit is by no means a comprehensive tester but it will work as a quick check to see if the transistor is totally broken.

Thanks for reading!

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