Simple BC547 Transistor Tester





Introduction: Simple BC547 Transistor Tester

In this instructable I'm going to show you how to make a transistor tester. Are you going to make a project, and find a transistor anywhere and you're wondering if it does still work? You can test it with this transistor tester.

First it's helpfull to know how a transistor works. Actually, a transistor is just nothing more than a tiny switch, which you don't controll with your finger, but with an electronic signal. There are 3 pins on a transistor. A collector, a Base and an Emitter. If you put an electronic signal on the Base, the switch is open and electricity can flow through the Emitter and the Collector. In this instructable I show you how to make a transistor tester which shows you if the 3 pins are still working, and if they don't have a deffect. 

We need a few components in for this.
1.  piece of prototype board
2.  battery clip
3.  330R resistor and a 22k resistor
4.  some pieces of wire
5.  a led (any color) 
6.  9 volt battery
7.  some old (or new) transistors

Also you need some soldering equipment. 

Make the schematic and solder everything together. Take 3 pieces of wire which stick out the circuit board, where you put the transistor leads to it.

If you made everything just like the schematic, you're ready to test your tester! Take the three transistor leads, bend them a little and put the Base to the middle pin and the Collector and Emitter to the other pins. 

If you want to make it more advanced, I put another schematic in the pictures selection. Here the NE555 timer switches the Base on and off, so you're sure that it's really working. This is a little bit heavier to make, because you also need a few more resistors and a capacitor, and a good prototype board. In my prototype board the holes are to far away from eachother, so a chip won't fit in it. You can also use a breadboard for this. Depending the resistors and capacitors you use, you can chanche the frequency from switching the Base.

Thanks for making my little, but verry effective project! 
Cheers from Holland

Electronics Tips and Tricks

Runner Up in the
Electronics Tips and Tricks



  • Planter Challenge

    Planter Challenge
  • Oil Contest

    Oil Contest
  • Clocks Contest

    Clocks Contest

We have a be nice policy.
Please be positive and constructive.




just made it! thanks for the great instructable :)

Hey just correct one line: the switch is open to the switch is closed.

All the best

What denominations in resistors R1 and R2, capacitor C in the circuit of NE555?


Thanks :D worked fine with npn transistors

Cool, I need to make one of these!

I use NPN. If you use PNP this is not the right solution. The led would normally stay off, but when it's broken it should also be off so you never know then if it's broken or not.

Great job! I'm going to be using this for my robotics group. Does it matter that you use two different types of resistors?

1 reply

Thank for your comment!
You need two different types of resistors because the base always needs lower voltages than the collector and emitter. Please check the datasheet of the transistor you use to calculate the resistor you need ;)

I mean pnp sorry

Nice work Marco but you need to look into Floxin advice more seriosly

Sorry to be negative, your instrument assumes that all transistors have the "legs" in the same position which is far from true . Also it is not true that all transistors are like the picture above showing a basic TO 92 package.
Look for "Mouser Electronics" on Google. (they are a large component distributor. Their phone number in Holland is +31 402 6476 57, E/mail is: Go now to Product Finder (left on the page) + Discrete Semiconductors +Transistors Bipolar + Show products (13 162 animals in the jungle). On Page 1, 3rd from top, click on "data sheet" in blue, open document and scroll to page 5. Look at the table on lower right. OOPS. That vendor puts 5 different leg arrangements on the market.
Stay on Mouser and google 2N2222 (a very basic transistor) in the search window. Look at 6th from top and click on "data sheet". OOOPS. that is a TO 18 or 39 metal package. What do we do?


What if the transistor is broken, but instead of not switching ON, it doesn't switch OFF. Since this only checks if it can close, not if it can open, I'd suggest an oscillator.

1 reply

I put a new picture with a NE555 timer in it. If you make this schematic, you are 100% sure if you're transistor is broken or not. If the LED is flashing, you know your transistor can be on and off, and works perfectly!