Bypassing a momentary switch

Question

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.

Answer 1

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.

^{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).

Answer 2

You could use a 555

^{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.

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

Answer 3

Here's how.

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.