The traditional relay looks like this (thank you Wikipedia):

A spring pulls on the vertical lever on the right (above its pivot), keeping the contact pushed agains the “NC” contact – hence the name: normally closed.

An electromagnet can pull the (iron) lever towards the left, against the “NO” contact – i.e. normally open, but closed once the electromagnet is powered.

Great invention. Perhaps the first example of electric amplification: using a small amount of electricity to switch a potentially much larger voltage or current.

For ultra-low power devices, ordinary relays have a drawback: you have to keep them energized as long as you want to keep the “NO” contact closed. With the Relay Plug, things are no different – the latest relays used on it have a coil resistance of about 125 Ω, and each of the two requires 40 mA @ 5V to stay “on”:

That amount of current consumption is not so convenient with batteries – when turned on, they wouldn’t last more than a day or two on a bunch of AA batteries.

Fortunately there’s an alternative, called a “bi-stable” or latching relay. It uses two coils to move that lever back and forth, with weak magnets in the relay set up in such a way that they’ll stay in place without using a spring as counter-force.

The benefit is that latching relays don’t need any power to stay in their current state (be it open or closed), you only need to give them a pulse to change their state from ON to OFF or from OFF to ON.

There are actually two types of latching relays:

• dual coil, usually with a common pin which should be tied to ground
• single coil, where changing the state is done by applying reverse voltages

Most dual coil latching relays can also be used in single-coil mode, by simply leaving that common pin floating.

In principle, the circuitry for a dual-coil latching relay is simple: you just need two relay drivers and then turn one or the other on briefly to make the relay change its state. The point being that you only need to pulse them very briefly, 10..100 msec should be enough.

Unfortunately, the Relay Plug isn’t usable for bi-stable relays, because it assumes a common PWR pin, not GND. It turns out that these relays are polarized. Thinking about this a bit more, this is actually quite logical: the force of the little latching magnet(s) need to be overcome with a magnetic field with a specific opposite orientation.

But there’s a surprising way out…

With single-coil plugs, the trick is to let current flow in different directions to set or reset the relay. IOW, either connect one pin to PWR and the other to GND (briefly), or vice versa. Hm, that sounds awfully like running an electric motor forwards or backward…

Now here’s the trick: instead of a Relay Plug, use the DC Motor Plug!

It contains two H-Bridges which are intended to control two small DC motors (or one stepper motor), allowing them to run in either direction.

You even get 4 additional general-purpose I/O pins thrown in…

Now, instead of hooking up a motor, just hook up a relay, and only pulse the power briefly (by making both sides GND or PWR the rest of the time). With as added benefit that the DC Motor Plug will support two latching relays, and being an I2C device, it’ll also allow daisy-chaining with other I2C plugs.

Note that the chips used on the DC Motor Plug require at least 4.5V to operate, according to the data sheet. Maybe a slightly lower voltage will work – I haven’t tried it.

Update – DC Motor Plug is confirmed to work. The 5V relays I was testing this with appear to switch reliably with pulses down to 4 ms, using this test code (modified from the dcmotor_demo.pde) sketch: