latching relays

I am using the HFD2/005-S-L2-D dual coil relays. They work for my purpose....

See:

Thanks for the reply. I was considering complete units (like eg url below (*)), which differ in operation. They take a pulse on a single logic-level input, and flip-flop between states: my query is how to reset from a random state to the power-up state.

The one you reference uses either reverse polarity or a 2nd coil to flip and flop.

(*)

(No, I'd not pay that sort of price :-) )

It has always been my understanding that they retain their current state indefinitely, until the next pulse. So there is no such thing as a reset state.

David

In that case, the only reliable way to proceed is to provide a way to sense the current state of the relay so that the desired state can be selected.

Those are not conventional latching relays but standard one with an external latching circuit. The semantics oare defiferent. latching relays use two coildand don't need power to maintain their state. The page tells you all you need to know, namely the relay is not energised when power is first applied, so conductivity is between NCx and COMx, switching to NOx and COMx on the first pulse.

Lack of proper data sheets is one of the reasons I'm not generally a fan of commercial building block modules like this.

Such as an extra pole that you could connect to a sense input to poll and check the relay state.

OK, I get it. I'm after something that'll retain the same setting for extended periods, hence the desire for a latching relay to keep power dissipation down. Indeed, it does look as though what I've been looking at is a normal relay plus a flip-flop - not what I want, and I think you've saved me from a bad mistake.

Not wanting to mess with relay driver circuitry, I've found something that's definitely latching at PiHut (I hope!!). Pricey, but complete modules don't seem available otherwise.

Thanks to all for commenting.

Look at the Hongfa HFD2 data sheet. The range includes single side stable (i.e. an "ordinary" relay), 1 coil latching, and 2 coils laching, and shows how to connect and how to energise to get latching relays of either type into the state you want.

The latching relays are the real thing; they retain state indefinitely, without power, after being set to either state.

David

The one thing: You HAVE to prevent your control circuit (like Arduino) from sending spurious signals to the relay on power-up to prevent unwanted/uncontrolled switching. Known issue and solutions are on the internet i.e.

Yes they remain their state BUT sometimes Arduino likes (from experience) & probably Raspi's give of spurious pulses on startup which can be/needs to be prevented:

Known issue and solutions are on the internet i.e.

Yes. The important thing is that, although they may be regaded as spurious, they are /not/ /random/; they are completely predictable and completely curable.

David