Originally Posted by SirBob

could I put a capacitor across S2 to get the relay to reset everytime the circuit is powered up?

You could, but that could be a little hard on the Reset switch. A better solution is below. Note that I also added catch diodes across the relay coils. You can only eliminate those if they are built into the relay. The size of the capacitor depends on the current and pulse time requirements of the SR relay. It also depends on the power supply voltage rise time.

Depending how frequently the 12v in turned on, you may need a discharge path for the capacitor, so it will be discharged fully after the 12v power is turned off.

Ken

I see, Thanks

That would that need to be quite a high resistance I'd imagine so as not to drop the voltage across the relay coils?

Originally Posted by SirBob

What is the discharge resistor for would the capacitor not discharge through the relay? (please don't take the question the wrong way, I'm not argueing wether it should be there, I'm just trying to get an understanding of the circuit.

No problem. The capacitor wouldn't discharge through the relay because of the diode I added to prevent S2 from discharging the capacitor. Hence the need for a discharge resistor. You could also use a diode.

The relay coil is 800Ω 15mA. Min switching voltage 8.4V, Switching time (max) is 5ms.

If you use a 100uF 25V capacitor and a 4.7KΩ discharge resistor, then you'll get a nominal 19ms pulse for the relay. By 19ms pulse, I mean that the voltage across the coil will be higher than 8.4V for 19ms. The tolerance on most small electrolytics is around +-20% so your timing will vary with different capacitors.

The power supply rise time would be pretty quick (i'd think) as it's a 12v car battery.

I assume it'll be switched by the key in a car? That should be a pretty quick rise time. It wouldn't be cycled that quickly so a 4.7KΩ discharge resistor will be fine.