This evening I have been developing the circuit so that the relay turns on when the circuit is first turned on. Before when the circuit was first energised, there was an approximately on RC delay between first turn on and first time the relay is energised.

I really would like the relay to come on for about 1 second immediately upon turn on. To do this I decided to use an RC circuit on the reset pin of the 555 timer. The reset pin when pulled low, sets the output low. If this pins goes above 0.7V then the reset pin will release the output pin and astable operation will continue. I constructed the circuit below.

R1 and C1 for the RC network. To be honest, I didn't calculate the values, I picked some rough values. C2 is 2x 100uF in parallel. R2 is a bleed resistor, so that when the circuit is turned off, C2 discharge quick enough, should I change my mind.

I tested the charge time:

Took about 1 second to reach 0.7V.

I then tested it fully with 555 and it worked, the relay turned on as soon as the power was applied for about 1 second.

However there was a slight side affect: it seems the first interval is longer than the proceeding intervals, is is cos the reset pin doe not only pull the output low but also pulls the threshold pin low and thus it takes longer to charge to the upper threshold (about 40% longer), which can be shown from this scope grab of monitoring the threshold pin:

as it can be seen; the time taken to charge to the upper threshold on the first period is much longer than the second, third, forth etc.

I was testing this with the resistor value for a 5 second interval. Measured the difference between the first and second interval (N.B. the interval is the time between the relay being de-energised and then re-energised). First was 8.5 sec and second was 5.2 sec. I then change to the highest interval resistor value and repeated: first was 53 sec and second was 33 sec.

Given that at the greatest interval I would hardly notice whilst driving, I could live with this side affect, so long as I get my first immediate wipe!