Electronic – arduino – Latching switch ideas

The function you're looking for is very common in, for example, a sump-pump controller. If the sump starts to overflow (because the pump has failed), an alarm contact closes, and it usually sounds a very loud buzzer. There's a momentary-action switch called "Alarm Silence" that will shut off the buzzer (so that you can fix the problem in peace), but then it will re-arm itself once the original alarm contact opens.

It is easily implemented with a single SPDT relay, as shown below. S1 is the alarm contact, S2 is the "silence" pushbutton, and the relay K1 keeps the load (buzzer) switched off until S1 opens again and the relay drops out. "Power Source" and "Load" can be pretty much anything, as long as the coil of K1 is compatible with the power source.

I'm going to assume that the AVR controller on the Arduino board is similar to the Microchip PIC family in that the pins default to INPUT upon reset.

If that assumption is correct, all you need is a really simple sample-and-hold circuit for the relay driver. It also requires you to think about how you have your software structured.

First, I'll show the circuit. Then I'll talk about the software implications.

^{simulate this circuit – Schematic created using CircuitLab}

Software is really easy.

1) Controller is just coming out of reset. Port pins are all currently set as Input.

2) Set port pin to desired level (Hi or Lo)

3) Change port pin(s) DDR to Output

Note that the above order is important if you want to avoid glitches. If you set the pin to Output first, then set the level, there can be a glitch.

Also note that the circuit introduces a delay of about 1 ms. I assume that is completely inconsequential if you are driving a relay.

You must choose a low-leakage capacitor for C1 but that's easy. Tantalum works pretty good.