I need to switch on a NE555 timer relay from a 12 VDC supply when the DC supply is switched on. Also, when I switch off the 12 VDC supply, the relay should switch on again as it is a timer relay.
What I am doing currently:
I am connecting 12 VDC to an NPN transistor. The 12 V supply gives power to the collector and the emitter emits 12 V and gives it to a 4700 μF, 25 V capacitor and also to the timer relay and the relay switches on and off (because it is a timer relay.) Now when I switch off the power supply the capacitor discharges immediately and I guess it is not allowing the timer relay to de-energize there by not switching on the relay. I have tried adding a resistor and it still doesn't work, but when I try to discharge the capacitor directly to the relay, the relay operates.
My use case:
I need to switch on and off a CPU (NUC) from the ignition of a car battery. That is, when I switch on 12 V I need an on pulse and when I turn off the 12 V supply I need an another pulse. Just a pulse is enough (like pressing the switch to on or off the CPU).
Even if my capacitor starts discharging after a 2 second delay when I switch off the 12 VDC in the circuit I have provided, it would be great.
Is there any device to delay capacitor discharge start timing or any modification needed in this circuit?