The circuit in post #1 and clarified in post #9 will do what you have described. However, I would add a resistor from the Q1 gate to GND. You are relying on a trickle of current to through LED1 to turn off Q1. 10 K from Q1 gate to GND will give more stable performance.

ak

Thanks, that's exactly what I was trying to get at. I thought I was going mad

 

Sorry, I don't know what you mean?

Something like this:

The problem I see is that your measuring thing can't turn on LEDs and measure the "resistance" at the same time. And it can't do it without a double throw switch.

 

Hey All,

I need to make a SPST momentary switch act like a SPDT switch so thought I could use a PNP transistor for the job. I'm just struggling to understand from my research the correct setup.

I've mocked up a circuit in the falstad circuit editor (link here) which seems to work:

The load I'll be using is different but I've just added an LED + Resistor as POC. The thing I'm unsure of though is if I should need any additional resistors or something around the PNP? All the examples I see have a resistor on the base pin, but I'm not sure if this tends to be more for microcontroller use? I'll be using a physical switch.

Any advice is much appreciated.

Cheers

Matt

That works, like this. You still have to deal with contact bounce though.