# Would this circuit work? NPN & PNP transistors ~in parallel:

Want to make a thing to test for both NPN and PNP transistors (using RPi GPIO power) and have sketched a diagram. Can you use both NPN and PNP resistors in parallel like this? Could this circuit fulfill my dream? Just need someone to tell me I know nothing, and this won't work :/ Would appreciate any advice, thanks.

RANTING FOOTNOTE (Bonus Round): I want something like this that gives a positive output for whether the transistor is NPN or PNP rather than a simple test that just tests for one and doesn't light up if it's an alternate transistor because what if the LED doesn't light up because it's a faulty circuit? and I don't want to have to make two of these. Though, come to think of it you could add an extra LED that always lights if the circuit works possibly, and only test for NPN for example?

This doesn't satisfy everything that Dwayne speaks of, but for a simple tester it should serve your purposes.

simulate this circuit – Schematic created using CircuitLab
Q1 can be either an NPN or a PNP. This is really just testing the polarity of the base terminal by assuming that current will flow out of the base in a pnp and into the base for an npn.

This will dimly light the LEDs depending on the base current the transistor can handle. If you need more brightness, I would go ahead and use an op-amp or a comparator to detect current flow between the voltage divider and Q1. That might look like this:

simulate this circuit
Note that this circuit doesn't detect if there's a short in your transistor or not so it's still a rudimentary transistor checker.

• Thank you very much for these circuit diagrams, very pretty. Would up vote but can't because I'm a noob. Commented Feb 26, 2015 at 15:05

What you have drawn will work but it may not give you the actual result that you are looking for.

Yes: one or other of the LEDs will light if the correct transistor that is working properly is installed into the correct socket.

However, the LED will also light up if a transistor is inserted not correctly. For example, if you swap the B & C lead on the transistor, the LED will light up even if the B C lead wasn't connected.

A LED would also light if you inserted a transistor that has either a B-C or C-E short.

Successful testers of this sort usually use a controller to dynamically swap polarity of all of the leads as well as supply various amounts of base current to the device under test.

You can do the same: create a table showing all of the possible lead configurations and having at least two different base currents available. Then test all of those possibilities and use the table to decide what the device under test is: transistor, FET, diode, LED, whatever.

• Sorry, my edit just clobbered your edit. Although it'd be best to keep the E and C correct, it wouldn't really matter in a basic checker like that because although E & C are fabricated differently, you can still run a transistor in reverse active. My circuit isn't really utilizing gain of the transistor and the resistors should protect the transistor. More than anything it's a diode checker with 3 terminals. Commented Feb 26, 2015 at 2:01
• Thank you very much. Would up vote but can't because I'm a noob. Commented Feb 26, 2015 at 15:01