Where to find hardware interlock relais

In the Esphome docs it is said:


These are software interlocks. As such, a software bug (which can always happen) can still activate both switches at the same time. Similarly, at reset time (before any of ESPHome’s code runs) the relay GPIO pins may have pull-ups active, so the relay may be active before ESPHome can manually deactivate them.

So it is highly recommended to use hardware interlocks (like SPDT-type relays) that ensure that two GPIOs are never active at the same time.

I was searching in the web but could not find these types of relais anywhere. Can anybody give me a hint where to find these types of relais?

How many switches are you interlocking?

The commonly available relay modules are SPDT (they have C, NC and NO contacts), which are good for interlocking two switches.

The C (common) terminal connects to the power for your loads.
The NC (normally closed) contact connects to one of your loads. This one will be on when your relay is off.
The NO (normally open) contact connects to your other load. This one will be on when your relay is on.

This way only one of the loads can be on at a time. You can not turn off both loads. One will always be on.

If you have a need for more than 2 interlocked switches, have a look at the Sonoff CH4 Pro R2 which you can also flash with Tasmota.

That is still relying on a software interlock isn’t it?

You now have me wondering, Nick…

My understanding is that the R2 (and R3) specifically has a hardware interlock but the other models don’t (there’s a table on the page I shared). I’ve tried to find a good schematic now to double check but can’t find one. I have an R2 but it’s in use (irrigation controller) so I can’t open it up to check.

R2 is using hardware interlock. For R3, they changed it to software interlock.

See here-

@tom_l I need to interlock 2 switches but I also need to be able to turn off both loads to stop the blind motor. Due to this I am using a 2 way relais. So are there any 2 way relais where only one load can be turned on or is there any option to solve this issue?

While thinking a little bit about it I found the solution. Just connect NO1 with COM2 instead of powering COM2 directly.

You beat me to it. One relay for power on/off, one for directing the power to where it has to go.

I rather thought about this concept:

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That will also work.

This is what I was thinking

In your setup, you have to switch both relays off for all off (UP off and DOWN off), and one on and one off for movement (UP on and DOWN off, or UP off and DOWN on).

If you switch both on by accidental software fault your blinds go up (as currently wired, you can change this to down).

In mine you have to switch one relay off for both off (POWER), but two for movement (POWER on, and DIRECTION on or off for DOWN or UP respectively).

If POWER accidentally gets switched on by software fault the direction the blinds go depends on the DIRECTION relay (off = UP, on = DOWN).

Both methods fail to no movement on power fail.

Do whatever you find most logical.