Newbie help, re triggering voltage please

Thread Starter

Chris Wilson

Joined Dec 2, 2011
Without boring you with the details I would like to remove the socketed microcontroller in this circuit and apply a voltage to where pin 2 of the socket and ground to see if the LED part of the circuit works.

Can anyone work out what voltage and current would be needed to test it please?

If I measure at pin 2 with the circuit intact the two stares I see at pin 2 are 97mV and 47mV and the LED does not illuminate. Thanks and a very happy and healthy New Year to all.



Joined Feb 24, 2006
Those resistor values look pretty big for what they are supposed to be doing.
What voltage(s) would normally be present on pin 2 of the microcontroller?
What is the voltage on top of the resistor (looks like R11) just above the LED?

Whatever the intention was, the transistor is not being turned on. If it was you would be seeing 0.6-0.7 volts on the base of the BC547, and nearly 5.0 volts out of pin 2. The current in the 33kΩ resistor would be:

\( 5\text{ Volts} / 33\text{ kΩ}\;\approx\;151.5\text{ μA} \)

That should certainly turn the transistor on, except the 10K pulldown consumes 500 μA (5V/10KΩ) for no real good reason I can think of.

Thread Starter

Chris Wilson

Joined Dec 2, 2011
Thanks again for the help. The issue was the generator needed to be running and making power with the contactor energised.

It is hard coded in the MCU and testing statically gave erroneous results. I rang the makers and they put me straight. Sorry to have wasted anyone's time, I assumed testing without it (the main 22kVA alternator) making power would be OK...

With it running pulling the WL control panel terminal low, (to ground), killed the fuel pump and flagged a charging fault as it should. Duh....