Transistor Leakage PLEASE HELP!

Thread Starter


Joined Sep 30, 2010

I have a module which has a ground-switching input (meaning it goes high when you ground the input((it draws about 40mA at -24V)))

I am trying to control a ground signal to connect to this module using a microcontroller and 2 transistors (1 NPN one, and 1 PNP)

an I/O pin on my Microcontroller is connected to the base of a NPN transistor. The Emiter on that transistor is connected to GND. The Collector is connected to the base of a PNP transistor. On the PNP transistor the Collector is connected to the input on the "ground switching" module, and the emmitter is connected to a 5 V source.

There are various resistors in between each stage so nothing is directly connected without a resistor in between.

For some reason no matter what the module is always being triggered high, which means it's always being grounded or connected to a more positive source.

I checked the transistors and they are working. I can't seem to think of a reason why i am getting this leakage. I think it may have something to do with the fact that when i send a low signal from the MC, it leaves the base line on the second transistor (the PNP) floating, which may pick up noise and turn on, but i have a hard time finding documentation to support that.

Can anyone help please



Joined Nov 28, 2008
A circuit diagram is best in this situation.

Basically, you have an input point which is normally below 0V (at -24V) and you want to connect this to 0V(or +5V) using transistors via MCU control. The current via the transistor is about 30mA.

Does this correctly describe your case? If so, the following will do. Just make sure the voltage rating of the PNP transistor is 100V or higher.

If you want the switching of input to happen at about 0V instead of +5V, you can add the zener diode in series as shown.

However, nothing beats the simplicity of using just an optocoupler and one series resistor as shown below.



Last edited: