Inverter ( NOT Gate ) not working

Thread Starter


Joined Nov 12, 2010

Newbie question.....

I am testing a simple inverter I made as per

At the output I have connected a LED via a resistor.

Problem: The output ( LED ) is always ON whether the base current is switched on or switched off.

I attached a LED after the emitter. And this is OK , ie it comes on only when the base current is on.

Could somebody kindly clarify ?
Thanks in advance.



Joined Jul 17, 2007
The circuit you're trying to use:

The circuit you should be using instead:

See how similar they are?
Basically the same thing. Just put your LED in series with Rc.

Thread Starter


Joined Nov 12, 2010
Thank you SgtWookie.

As I had mentioned, I am testing this circuit and had put the LED only for testing. Actually I need the inverted signal to control another part of the circuit.

How do I do it then ? ie get the inverted waveform....

Searching the net for such NOT-gate-type inverter turns up so many circuits but they are so similar to this....

I dont know what is it that I am doing wrong.

I have been testing the circuit with 4.5 volts.


Joined Oct 26, 2011
your circuit may be picking up noise if the base is floating and the transistor is sensitve enough.....both these circuits work(with an additional pull down/pull up resistor)