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I am working on a project to make my motor starter completely automatic (turns on/off once the water level goes below or above the desired level) and I have gotten to the part where it automatically turns off almost figured out using a SPDT relay and an SSR.

But for turning on the motor starter, I need to press a momentary switch (almost for 1 s) and there are no "momentary relays" in existence that I know of.

So what I need is a simple circuit that turns on the output signal for one second (lets the input signal of 12 V pass through which imitates the momentary switch being pressed for a short duration) and then turns off the output signal even after the input signal is continuously on.

This function should repeat, once every time the input signal is turned off and then turned on again.

Any ideas on how to do it will be appreciated.

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    \$\begingroup\$ a microcontroller would work ... very simple circuit \$\endgroup\$
    – jsotola
    May 28, 2023 at 6:51
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    \$\begingroup\$ There really is no need to shout. \$\endgroup\$ May 28, 2023 at 7:18

3 Answers 3

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A web search for "on-delay relay module" will return plenty of options. There are many very cheap modules available but beware of build quality and the lack of datasheets.

Your specification can be more easily described using a timing diagram.

                 a              b       c
                 _______        _       ________
Trigger     ____|       |______| |_____|
                    ____                   _____
Relay       _______|    |_________________|
                 __             _       __
Output      ____|  |___________| |_____|  |_____

Figure 1. Timing diagram.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Electrical schematic.

How it works:

  • (a) When the trigger signal goes high (+12 V) it starts the timer and, at the same time, goes through the normally closed (NC) contact to the output. After the preset time delay the relay will be energised and the output will disconnect.
  • (b) If the trigger turns off before the time-delay is complete then the output pulse will be the same length as the trigger signal.
  • (c) is the same as (a).
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    \$\begingroup\$ This is the answer - as it's just wires into screw terminals and a part. No soldering, no programmer. Sure, it's not the cheapest way but when you say "pump" and "water level" the absolute minimum cost might not be such a factor. \$\endgroup\$
    – D Duck
    May 28, 2023 at 10:29
  • \$\begingroup\$ Nice ascii art... \$\endgroup\$
    – 2e0byo
    May 28, 2023 at 16:01
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You could use a 555

schematic

simulate this circuit – Schematic created using CircuitLab

The simulation predicts no pulse at power-up, but that's not guaranteed, so make sure that can't cause any issues.

enter image description here

Or there are definitely things with names like interval timer module, used for industrial and hobbyist applications, but suggesting specific ones is verboten here.

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Here's how.

enter image description here

On application of voltage 'V', the relay coil is on till the capacitor charges for a period that is determined by time constant RC.

On removal of voltage 'V', the capacitor discharges through R1 and is ready for the next pulse.

The value of R1 could be ~ 5 times R.

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