This is a hack I did to control a 12 vdc electric door strike. Since I was doing this on the cheap parts on hand where used. I used a surplus power supply and remote control (Key Chain Remote). The only problem using this remote control is that it’s output once turned on will stay on until it is turned off. I rather have the output on only for a short time and then turn off. So I added some circuitry to accomplished this. I also had to modify the relay in the remote module so that it connects the output wire to ground instead of 12 volts.I did this by isolating the relay contact connection from the 12 volts and then ran a wire from the relay contact connection to the black wires of the remote.
I am not worried about the mosfet operating in it’s analog region since the current through the relay is very low so the Mosfet will not get warm enough to be a problem. (The device I used was a 10 Amp Mosfet. It’s what I had on hand.) Also it should be noted that the coil for the latch has to be connected so that the circuit will function correctly.
Operation: On button is pressed door strike retracts to release the door. If the Off button is not pressed the circuit times out after 12 seconds and the door release latches back and the circuit locks out until the off button is pressed. The preferred method was to press the off button on the remote so that it would reset and be ready for the next activation. In practice this worked fine, in fact this unit was used for over a year until we relocated the office.
Conclusion: If I did it over I would use a lm555 as a on shot so that the turn off time for the mosfet would be faster. Below is what the relay K1 voltage looks like when it is allowed to time out.
For over 2 seconds the mosfet is operating in it’s linear region. If the load was large enough it would actually cause the part to heat up noticeably, but being over-sized by around 800% it’s not a problem powering that small pc board relay.