SCR/relay, returns to open state when power killed

[selsin]

I'm wiring a 12V home alarm system: magnetic door contact triggers lights/sirens...but when burglar closes door, I don't want sirens to stop - just continue until I hit a toggle switch that breaks the circuit. What would be best for that situation? SCR? Latching relay? Input to the SCR gate or latching relay would only be about 2.2V (powering an LED indicator light along the "gate" circuit so resistor will have to power it down to around 2.2V), but I'd need the SCR/relay to run a full 12V through it to power the sirens/lights. My concern with SCR is the gate being too sensitive that it may accidentally trigger from a power surge. My concern with a latching relay is I can only find two types: single or dual coil. Don't like that because I don't want to have to reverse polarity (single coil) or apply separate voltage to a reset coil (dual) to return it to default state - I just want it to "spring" back to open state when I kill the whole system. Any ideas?

 

[ronsimpson]

Door switch is normally open. When door switch closes, power goes to the relay coil. The relay closes and shorts across the door switch, keeping the power on to the relay coil. You will need to kill the power to reset.

 

[Colin]

Is that Ron Simpson siren? How does it work????

 

[alec_t]

How does it work????

Current through an internal resistor Ron triggers an oscillator.

 

[selsin]

Door switch is normally open. When door switch closes, power goes to the relay coil. The relay closes and shorts across the door switch, keeping the power on to the relay coil. You will need to kill the power to reset.
View attachment 112471

I think I understand your diagram and explanation. But what type of latching relay resets upon death? I thought both single and dual coil latching relays remained in their last state, even when current stops.

 

[ronsimpson]

It is not a latching relay! It is a normal relay.
The door turns on the relay. With the relay is on, it holds its self on. When the power is removed the relay falls back open.

 

[selsin]

OHHH! Please excuse my ignorance. So all I'd have to do is wire a relay such that it's coil would have two inputs: 1) door switch AND 2) current coming out of the relay (load current)? That way coil could stay energized by the door switch for as long as the door is left open, or when door is closed, the coil could be energize by the load circuit instead. So basically I'd be jerry-rigging the relay to behave as a latching relay...only with the bonus of resetting upon death.

 

[ronsimpson]

Yes.
There are several ways to think about it.
> The relay contacts are across the door contacts. So the door powers the relay coil. The relay contacts makes it look like one of the doors is open.
> Another way is to think of the relay as being a latching relay with "power down reset".
I can add a reset button if you want.

 

[selsin]

Thanks! I think I've got. Can't wait until the rest of the parts get in so I can wire this all up.

 

[selsin]

Got it all wired, works just as intended! Only thing is, with power switch off, LED continuously glows (very dim). It is wired to the Normally Closed pin on the relay (this way it lets me know when the system is armed). But with switch off, how is it glowing? Is it a bad switch that is actually letting a little current still flow through it. Also, fyi, instead of a battery, it's powered by an AC/DC adapter plugged into the wall.

 

[ChrisP58]

Got it all wired, works just as intended! Only thing is, with power switch off, LED continuously glows (very dim). It is wired to the Normally Closed pin on the relay (this way it lets me know when the system is armed). But with switch off, how is it glowing? Is it a bad switch that is actually letting a little current still flow through it. Also, fyi, instead of a battery, it's powered by an AC/DC adapter plugged into the wall.

Please post the schematic of your complete, as-built circuit.

 

[selsin]

See below. Could the LED somehow be getting a trickle of current from the negative pole of the power supply? (didn't know if LED's are so delicate that they can glow off of any hint of current, period)

 

[Pommie]

The only way the LED can glow is if S3 is faulty. What happens if you disconnect S3?

Mike.