Time delayed switch

[Billybaxter]

Hi,
I struggle to even use the correct subject title to capture what I mean - hopefully I am better at explaining it:

I am trying to design a circuit which can be described as follows:

1. A 12v input gets applied to the circuit and remains applied.
2. A 12v output is connected to a lamp, which glows briefly (say 2-5 sec) when the 12v input is applied.
3. After that short period, the lamp stops glowing even though the 12v input remains.
4. If the 12v input is removed we are back at step 1.

I have considered a monostable 555 IC, but that appears to work on the basis of there being a trigger button, rather than a switch where the state changes and stays changed beyond the timer period.

I can imagine a simple circuit with a very large capacitor in series with the lamp, where current flows through the lamp until the capacitor becomes charged, but my guess is the required size of the capacitor would not be practical, even if the lamp were modest.

Any guidance will be most appreciated,
Billy

[pebe]

Use the 555 and change your steady trigger voltage into a pulse by feeding Pin6 of the 555 through a 10nF capacitor and use a 10K resistor between Pin6 and Pin8.

Edited Sat 15th @ 16:25
Sorry. It should be Pin2 - not pin6

[Billybaxter]

Thank you for your suggestion. I think I understand what you mean, and I'll look at it in the context of a complete diagram to help understand the flow of electricity through the circuit.

Thank you again,
Billy

[Billybaxter]

For anyone still following this quiet thread, I investigated the solution kindly offered by one reader (thank you), and although it works, the problem I was faced with was the continuous current drain of the 555 when in the off position, which it would be in most of the time. My solar powered resource is not sufficient to support that.

I did eventually come up with a solution using a second conventional relay. I have attached a rough sketch of it for anyone who's interested. Essentially, in the off position the relays draw no current. When the main double-poled relay is switched, the secondary relay switches on for 1-2 seconds until the capacitor charges up, then the relay switches off again, thus drawing no further current. When the main relay is switched off, the capacitor discharges through the secondary relay. The capacitor setup I found to work best after a bit of trial and error was 3 x 1000microF in parallel.

I hope this intrigues someone if nothing else

Billy