I am trying to switch on and off a circuit that draws about 0.8 amp at 12 V. The obvious choice is to set a MOSFET. The problem is that the control signal (the one that goes on the gate of the MOSFET) is changing very slowly (1 volt per hour). So when this slow signal reaches MOSFET threshold voltage it is not instantly turned off or on, it is operating in bad non saturated mod for some time.

How do I make a switch that turn on and off instantly?

I tried to use a relay that is controlled by MOSFET with zener diode as voltage sense. That will not work because when relay is switched off the current stops and power supply increases the voltage a little (because no load), and sense circuit thinks that voltage is ok again and turns relay back on. So relay open and closes like crazy near that threshold region!

![Diagram](https://i.stack.imgur.com/NnQvq.png)