Off-delay 555 circuit (negative trigger)
I needed a way to have a relay “ride” through a starting sequence on a vehicle. Annoyingly there was no steady ignition sources so it was to the market on off-delay timers so the circuit can ride through the start while still turning off when the circuit is truly off. There is one constant source and a source that goes negative during starting.
With the vehicle off the circuit is as above. The 555 is timed out, the seal in and timer mosfet is off and the trigger mosfet is sealed.
When the ignition is turned on the seal in mosfet is active holding the relay on. ( this can be done with bjt’s as that is what was originally in the circuit)
During starting the trigger circuit goes negative activating the 555 to hold the relay through the start cycle through the timer mosfet.
Then the vehicle is running and the trigger circuit holds the relay high till the vehicle is turned off. Then the circuit returns to a off state after the set time.
Note: The trigger mosfet can be removed and the 100k ohm resistor moved onto the trigger circuit.