I'm trying figure out how to close a circuit once a specific resistance is reached. I have a photocell and want to close the contacts in a 12V system once the resistance in the photocell hits about 1K Ω. I'm sure I'll have to stick a variable resistor in between the photocell and this unknown switch to easily adjust when the switch closes. Ideas? thanks!

Rick

 

Just about any NPN transistor will work. Set the lighting where you what the trigger, than adjust the POT until the transistor closes. Anyhting brighter will open the switch and when it hits that low level, it will close.

 

In addition to retched's good suggestion, you could also consider using an LM339 chip -- 4 comparators so you could use the spare comparators for e.g. sensing when things went out of limits.

 

Thanks for the tips! Transisitor, duh. Been a while since I got my "D for done" in EE class.

So I found this circuit diagram when I tried figuring out how I'd wire up the transistor. The site suggested a 10K Ω resistor in addition to the POT to protect the transistor if the POT is reduced to 0 Ω. It also has a diode off the collector...think either or both are necessary? Would this be how you'd wire it?

If I was better at this stuff, I'd try to figure out the LM339 too...

Thanks!

 

Yup, that looks good.