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555 monostable timer Answered

I've no idea what I'm talking about here but can only explain what im looking for which is help. I'm basically looking for a relay or multifunction relay not really sure but here's the plan. When my smart plug is turned on (at certain programmed times through the smart plug app) to provide power to a 12v 6amp pump (I have 240v ac to 12v DC transformer pluged into smart plug I'm in uk) I would like for this pump to turn itself back off after 10 or 15 seconds. I cannot for the life of me find an analogue or digital timer that can do seconds. The smart plug can work down to minutes and not seconds. So basically smart plug on = pump run for 15 seconds cuts itself off and then the smart plug turns off once the minute is up and when plug is on again at next programmed interval it does same thing. Any help would be great as I've no idea what I can buy or try to make thanks When trying to research I hear about 555 monostable circuit but I don't know how to wire something up or what to buy to make this work.



4 weeks ago

Thanks for responses IL have a look into both options much appreciated

Jack A Lopez

4 weeks ago

I think the Wikipedia articles for "Monostable" and "555_timer_IC",



do a pretty good job of explaining what these things are.

There are a number of 'ibles here at Instructables,


that can show you what the 555 is. Although, most of them show how to wire the 555 as an oscillator, aka astable mode.

Wiring it as a monostable (sometimes called "one shot") pulse generator, is not too hard, although there are a couple of caveats I can think of.

One thing is the trigger pulse has to be shorter (i.e. less time) than the output pulse. So you'll have to come up with something like a high-pass, like a capacitor and a resistor, to turn the minute long pulse from your existing "smart" relay, into just a short blip that happens when it turns on (and maybe a complementary negative shaped blip when it turns off).

Another thing is, I think, the 555 monostable wanted that pulse inverted; i.e. goes low for a brief time, is high the rest of the time.

Another thing is, 15 seconds is kind of a long time for the usual R and C combo the 555 uses to measure time. As a result, actual R and C used might be somewhat large. Like, R in mega (10^6) ohm, and C in 10s or 100s of micro(10^-6) farad.

By the way, there are other ways to implement a time delay.

Like, if you have a circuit that can count, and if you have a source of regularly timed pulses, e.g. a circuit that watches the AC mains (50 or 60 cycles per second, depending on where you live) and converts this to pulses. How many in 15 seconds? Like 750 or 900, or so, right? Or 1500 or 1800 if we are counting half cycles.

Also the well-known Arduino is capable of measuring time, and I think it does that, at the low level, by counting its own clock cycles. At the programming level it is just a call to one of the functions from the "time" library.


4 weeks ago

Go to a supply store for electricians, not your normal hardware store.
Ask for a 240V non latching timer relay that has a OFF Delay independent from the switching signal.
Works like this:
Power to the relay and the contact closes, timer starts.
Contact opens again once the timer is run our or the power to the relay is cut.

This sort of relay was often used to start big motors, the timer relay energises a relay that lets the motor run in a start configuration, once at speed it switches another relay so the motor runs in the normal delta configuration.
Please specify that you need a relay for only a few seconds, as they comes in seconds, minutes an hours as different types!
To make it work for your pump you then just give power to the relay and also jumper the power over a contct of the relay, the other side of the contact then connects to your pump.