Securing a doorbell system with a tamper switch and a latching relay

Question

I have a doorbell that has a relay to open the door. The issue I'm having is that my doorbell is outside the house and thus people would be able to come up to the doorbell, open it, and connect it themselves. I don't want that to be possible.

Thankfully, my doorbell also has a tamper switch. This is a contact that is closed when the doorbell is normal, but when its housing is opened, the contact becomes open. That way I can securely connect my door.

Now, I don't just want to connect these in series, because the tamper switch is also outside the house and a clever burglar could see the cables and trigger both at once.

I tried to check out latching relays, and I bought the CK21 switch (like this). Turns out, I can't figure this thing out. I want to connect it so that this button is inside the house and unlatches if someone triggers the tamper switch. Then, when the tamper switch is reconnected, I would need to re-set this button to reactivate the front door.

Is there a way to do this with this button? Or did I buy the wrong thing? I should also mention that all this runs on 24 VDC.

Answer 1

Your tamper switch is normally closed (closed when no tampering is happening) so you're going to need a current running through it all the time, and then the tamper is detected when there's suddenly no current. I think this is where you're running into a problem with latching relays and latching buttons, because they need a current to latch, and then no current makes them hold the same state.

You can use the trick of wiring a relay's coil in series with its own contact to make it latch. When you bypass the contact with a separate switch, the relay turns on and then keeps itself on. When the coil loses power, it turns off and stays off. This arrangement uses power all the time, not just when there's tampering, but I think that's unavoidable because the tamper switch is normally-closed.

By using a relay with 2 contacts you can put the other one in series with the doorbell. Actually, you even do this with just 1 contact - it's left as an exercise to the reader. Using a separate contact makes for less confusing and more flexible wiring.

^{simulate this circuit – Schematic created using CircuitLab}

Note this is not the only possible way to wire it, because parts in series can be swapped around.

Beware the ground isn't at the bottom, because of how the relay is drawn.