converting circuit from relay to something else ?

I would suggest an OPTOISOLATOR chip. Dave

Been looking for a PNP version, will have another look, its the inverting thing which is the problem...

Chris

No the tricky bit is getting a low impedence output when there is no power on the circuit to turn on a regular transistor. Try a depletion mode FET.

Yeah low output impedence with no power is the fundimental problem, not heard of a fet like that, just googled for them and doing a bit of reading now :-)

Cheers, Chris

several pages latter I quote..

"A depletion-mode MOSFET conducts current without a gate bias. In an n-channel device, a thin n-type region exists under the oxide in the absence of an applied bias. It connects the source and drain allowing current flow. In fact, a negative voltage is required to drive the electrons out of (deplete) the region to increase channel resistance and reduce current flow."

so basically its normally on though I found they actually have a on resistance of 100ohms going by the ones ive found so far. All sounds good until you read that it needs a negative to switch it, this reminds me of thermal valves also, maybe if I fake the ground level by a volt or 2 then in effect the gate voltage will be 2 volts under the source voltage... even so the actual supply rail goes high so not sure how I can get negative switch from a possitive rail.. oh the joy.....

Chris

I'm a bit rusty but I think you are referring to an N-Type device. Perhaps look at at P-Type instead.

good

See also P-Type JFETs

will have a look for some p devices,

cheers, chris