Inverter Circuitry

[Bulldozer2003]

Hi, thanks to all the people who have been answering my questions on this forum, I've been doing a few electronics projects and this is where I come for help when things don't behave as I expect them to .

Right now I've got an odd problem, the horn switch on my car is a normally on momentary. I want to hook up something that turns on when I press the horn button. When I hooked up a single n-channel mosfet to behave as a relay to provide power to the device when there is voltage at the gate, I hooked the gate up to the horn switch line (+12 all the time). As expected the mosfet was on all the time and only turned off when I pressed the horn: the opposite of what I want. Using my Intro to ECE knowlege I ordered a p-channel mosfet ($3) from mouser and built a CMOS inverter. I tested the inverter and it worked just fine, until I hooked it up to the car and it fried my p-channel (I assume this because upon testing the output was always high).

Is there any other commercial device, or IC, or something I can buy to do this inverted relay for me?

 

[mAdD INDIAN]

You can create an inverting circuit out of an op-amp and two resistors. I don't know if it will fit your application though.

 

[damonpip]

Is there a reason why you can't use a SPDT relay?

 

[Bulldozer2003]

No, the device I want to turn on has a switch that closes the circuit, I will fry the electronics of the device if I allow current from the horn switch, or the +12 from the car, to enter the circuit I am trying to close.

 

[blahblah99]

Originally posted by: Bulldozer2003
Hi, thanks to all the people who have been answering my questions on this forum, I've been doing a few electronics projects and this is where I come for help when things don't behave as I expect them to .

Right now I've got an odd problem, the horn switch on my car is a normally on momentary. I want to hook up something that turns on when I press the horn button. When I hooked up a single n-channel mosfet to behave as a relay to provide power to the device when there is voltage at the gate, I hooked the gate up to the horn switch line (+12 all the time). As expected the mosfet was on all the time and only turned off when I pressed the horn: the opposite of what I want. Using my Intro to ECE knowlege I ordered a p-channel mosfet ($3) from mouser and built a CMOS inverter. I tested the inverter and it worked just fine, until I hooked it up to the car and it fried my p-channel (I assume this because upon testing the output was always high).

Is there any other commercial device, or IC, or something I can buy to do this inverted relay for me?

The p-channel fried because you didn't pull the gate voltage up (or down) completely. If left floating, it could cause the FET to operate in its linear region where it would burn a LOT of power and ultimately overheat and fail.

Try doing what you did with the p-channel fet, except this time put a 20k - 100k resistor from gate to ground.