Replace reed switch with relay closure driven by HC-SR04

Thread Starter

satx

Joined Aug 23, 2017
2
I have Simon XTi home alarm, with several PIR and door/window sensors. Suits my needs, almost.

While XTi does allow a NC alarm loop, there's no way to know which NC sensor, assuming more than one, is tripped.

For a sensor outdoors (PIR not recommended), I'd like to replace the reed switch of a door sensor with a relay closure, where the relay is activated by output of HC-SR04.

I don't want a microcontroller or c-code or computing, just a circuit that takes the output of the ultrasonic sensor and drives a NO relay to closed, in place of the door sensor's reed switch. A couple considerations are low-pass filtering to kill bounces, FPs, and some kind hold for up to 5 secs.

iow, an action just like the automatic door openers in, eg, grocery stores.

suggestions? circuits? :)
 

AlbertHall

Joined Jun 4, 2014
12,023
It will need a circuit to generate a 10uS trigger pulse repeated for each measurement.
The output pulse is always present whether the sensor detects anything or not. Therefore a circuit will be needed to test whether the echo is shorter than some limit.
Then a circuit to provide the 5 seconds hold will be needed.
And finally it will need to drive the relay.
Not too difficult but it is a fair bit of circuitry.
 

Thread Starter

satx

Joined Aug 23, 2017
2
Thanks

So the output pulses come with greater frequency with reflections.

If so, then maybe the pulse train could charge a capacitor who's level would vary with the output pulse rate
 

AlbertHall

Joined Jun 4, 2014
12,023
So the output pulses come with greater frequency with reflections.
No. There is an output pulse for each trigger pulse. The width of the echo pulse changes depending on the distance to the target. For a near target the pulse is short and for a distant target or no target at all the pulse is wider.
 
Top