Electronics > Beginners
A simple circuit to trigger relay for 30 seconds
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vk6zgo:
A big, fat electrolytic capacitor in parallel with the relay coil may do this job, although 30 seconds could be a bit long to achieve with readily available electrolytics.
Of course, this also has a very short operate delay as well, which probably wouldn't be much of
a problem in your application.

This was a widely used method of obtaining a delay back in the day.
It is always a good idea to look at the K.I.S.S method before getting too deep into complex alternatives.
Richard Crowley:
Consider:     CD4541B

You may find this video helpful:

https://youtu.be/DeMuNXZ5jLc
iMo:
As a child I built the above 555 timer (set to around 5 secs) switching on a melody door-bell, with an interesting option - the whole stuff did not draw any current when idle.

The 555 was not started by the "trigger Push Button" (in the above schematics) but a it was powered ON by a Push Button wired in parallel to the relay contacts (normally off).

When the PB was pushed down momentarily, it shorted the relay contacts (normally off) and the 555 got powered, 555 switched the relay on, the relay's contacts shorted the PB, it started to count (555 wired as a monostable with T=5secs) and after 5 secs the 555 switched the relay off. That removed the power off the 555 and the whole system went idle and did not draw any current.
drussell:

--- Quote from: imo on September 15, 2018, 09:59:33 am ---When the PB was pushed down momentarily, it shorted the relay contacts (normally off) and the 555 got powered, 555 switched the relay on, the relay's contacts shorted the PB, it started to count (555 wired as a monostable with T=5secs) and after 5 secs the 555 switched the relay off. That removed the power off the 555 and the whole system went idle and did not draw any current.
--- End quote ---

That is a perfectly cromulent way of doing a circuit for that particular application.  :)
Zero999:

--- Quote from: drussell on September 14, 2018, 11:00:39 pm ---
--- Quote from: Hero999 on September 14, 2018, 10:18:26 pm ---EDIT: I've just realised something: 4700μF is far too larger capacitor for the 555 timer. The energy stored in it can easily blow up the discharge pin, when the transistor turns on!
--- End quote ---

If the discharge current would be too high on a very-long duration set-up with a large capacitor, couldn't you put a low-value resistor between the capacitor and the discharge pin?

I suppose I should look up the internal diagram of the 555 and re-acquaint myself with the circuitry, perhaps that won't work.  :)

--- End quote ---
It's only a problem for the monostable circuit. In the astable circuit, the capacitor is discharged via a resistor anyway. Post edited.

Yes, a series resistor will help to reduce the power dissipation in the 555, but it's easier just to use a smaller capacitor and larger timing resistor.
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