Voltage-controlled switch

I have a requirement to put a solid-state switch of some kind in a non-conducting state when its input control voltage is zero - i.e. when a capacitor is fully depleted. A relay may suffice but I would prefer a transistor, mostly for the switching speed.

I know that such devices exist but so far I have been unable to find one that operates at a threshold of exactly zero volts; most seem to require 0.7v or thereabouts.

So does anyone know of such a device please, or alternatively can recommend a circuit design that would turn a switch off once a capacitor is fully discharged? Also the switch itself would need to handle large voltage drops, around 20v or so.

Please bear in mind that my electronics knowledge is very "rusty". I haven't touched the subject since I was at school, over forty years ago!


Thanks ...

Richard
PS I also live in the UK, so American terminology may differ from that here.
PPS I searched these forums beforehand but found little that was directly relevant.

 

Sorry but this may be commercially-sensitive information and I tried to provide sufficient details.

All I can add I believe is that the power source will be a battery, probably 9V but it could be less perhaps; that will charge up the capacitor initially but will then be disconnected (in the prototype) and instead the capacitor will be connected to the rest of the circuit, to be discharged.

So I would be grateful if anyone could point me towards an electronic switch that will be set 'off' with zero input volts (across the capacitor).

If I have still not provided enough information then I may have to try another tack.


Thanks ...

Richard