Help with normally closed relay on motion sensor

"Elephant"

** Just connect a transistor ( say a TIP31 ) in series with the buzzer DC supply, with a resistor (of say 470 ohms ) from C to B so it is biased hard on.

Then connect the relay contacts from B to E .

Job done.

..... Phil

There is no best way, but better ways shade into more complicated solutions. A straightforward approach is to use a second relay with the opposite sense such that, when it is powered through the motion detector relay it will hold its contacts open. When the motion detector fires, its relay contacts open, allowing the second relay contacts to shut, powering the buzzer.

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sensor NO relay NC relay alarm/buzzer "on" = shut "on" = open (created by AACircuit v1.28.6 beta 04/19/05

The normally closed contact is ideal for adding to an alarm system. contacts at windows, doors etcetera are also closed, and you can just add the motion alarm in series to any alarm system. Then when a prowler is detected, your break in alarm will sound. Else, add another relay,to invert the motion alarm.

To keep it simple, have it drive another relay with NC contacts and drive that.

If you're trying to incorporate some electronics, I guess you could use those contacts to shunt the input of a simple biased switching transistor. The contacts would simply shunt the BASE bias resistor to common for example.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
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The original post only describes an 'undesired' condition; not the 'desired' one.

However making a presumption that the buzzer is to sound when motion is detected? But not while motion is not detected?

In a typical North American 115 volt situation.

Thinking about the same. I have two separate motion sensor fixtures that control two light bulbs each. One idea would be to connect a small front door bell transformer across the bulbs (usually a red wire out of the sensor!) the lo-volt output of the transformer would operate buzzer. And buzzing would persist for as long as the lights were timed to stay on.

While the sensors are rated for incandescent bulbs only don't think the small amount of inductive reactance of the small transformer in parallel with the the pure resistance of the two bulbs would be a problem.

One advantage in our situation is that the circuit can be installed completely by using standard electrical code approved parts in a standard manner. No special hook-ups or home made circuits are required.

We ensured that while installing the motion activated lights the third wire (usually red?), the one out of the sensor that turns on the lights, was brought back in a standard three conductor cable (White, black, red + ground) an octagon box. The low voltage transformer is to be mounted on that and low voltage wires will be run to the buzzer. Maybe will add a low voltage switch to turn off the buzzer if necessary.

Our sensors are rated for 300 watts (Incandescent only) but are normally fitted with two 75 watt floods ( 150 watts) per fixture. So one idea is to use that third wire to turn on a couple of other lights (up to a maximum of another 150 watts) in an adjacent area etc.

A further note: Our relative live 'next door' and we have low voltage communication wires running between the two houses, underground. These could be used to alert people in the other house that someone is around the first one setting off the motion sensor lights!

Thank you all for your suggestions. I used a 2nd relay and it works perfectly.