Electronics > Beginners

how do I make a time delay that turns on a relay upon power on and turns it off.

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I wanted a rude username:
If you want a ready-made solution, there are $2 modules that do this ... just wire and go.

https://www.aliexpress.com/wholesale?SearchText=delay+relay

I use one to suppress the aftermarket alarm lock/unlock chirp in my motorbike.

Zero999:

--- Quote from: TerminalJack505 on February 14, 2020, 11:35:37 pm ---Are there two delays?  That circuit would turn the relay on immediately upon power-up, so if there are two delays then it wouldn't work.

--- End quote ---
Yes. I think most people here appear to have misread the original poster!

There are two delays!


--- Quote from: ELS122 on February 14, 2020, 07:32:44 pm ---basically I want ~10s delay and when I power on the dalay circuit it turns on a relay but after 10s it turns it off.

--- End quote ---

I'll post a schematic later.

brucehoult:

--- Quote from: Zero999 on February 14, 2020, 11:21:35 pm ---If you're after simplicity, then forget about discrete components. It will be much more complicated. :palm:

--- End quote ---

Yup.

You can get ATtiny85;s for $1.20 each right off Amazon.

https://www.amazon.com/MICROCHIP-TECHNOLOGY-ATTINY85-20PU-ATtiny-Microcontroller/dp/B07348SMC3

Or around $2 each if you only want five.

They're dead easy to use, have 8 KB of flash for the program (4096 instructions), 512 bytes of RAM, 512 bytes of non-volatile memory, have digital and analogue (10 bit ADC, 15k samples per second) inputs, and digital and analogue (PWM) output. They run at up to 20 MHz (which is 20 MIPS), run off 1.8 V to 5.5 V, and need no external components.

The ADC even does differential mode, so you could program it to mimic a 555 exactly (though with a lower analogue input frequency response)

A little bit more expensive than a 555, but not much, and once you start adding external components to the 555 the total price can easily become more.

It's pocket change in both cases, of course.

Zero999:
Yes a microcontroller is a good idea but:

--- Quote from: brucehoult on February 15, 2020, 02:45:44 pm --- run off 1.8 V to 5.5 V, and need no external components.
--- End quote ---
Well that's not true. The original poster's application is for 12V, so an LM78L05 or similar and a transistor to switch the relay will be required.

Here's the simplest discrete deign I could think of, which doesn't use monster capacitors. The values of R1, C1, R2 and C2 will need to be tweaked, as it's dependant on the threshold voltages of the MOSFETs. The precision will be poor, but if all that's required is an approximate delay, it will do.

ELS122:
well I meant 1 delay, one delay that when powers on IS on, but after 10s turns of. so is on. then turns off.
why I want this?
because I don't want the relay being on for all the time when the amp is on.
although it shouldn't be a problem if the voltage is right I just don't like the idea of something uselessly drawing current all the time.

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