# Help with normally closed relay on motion sensor

Discussion in 'Electronic Basics' started by Elephant, Sep 29, 2007.

1. ### ElephantGuest

I have a motion sensor that has a normally closed relay. When motion is
detected, the relay opens. There is no option for a normally open
connection.

If I hook the contacts up to a buzzer, the buzzer of course will sound
all the time unil motion is detected.

What would be the best way to get around this. Another relay, an
opto-isolator.........??????

Any help would be appriciated. Don't want to have to construct too
complicated circuit.

2. ### Phil AllisonGuest

"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

3. ### Rich WebbGuest

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.

.----------------------.
battery or | | |
other source | | |
--- | |
- | |
| | |
| | | .-----.
=== | | | |
GND / \ | |
.----. o o .-----_ o o | .-.
| |----- )| | )| | ( X )
| | )| | )| | '-'
| |----- )| | .-_)| | |
'----' '-------' | '-----' |
| |
=== ===
GND GND

sensor NO relay NC relay alarm/buzzer
"on" = shut "on" = open
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

4. ### Sjouke BurryGuest

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.

5. ### JamieGuest

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.

6. ### terrySGuest

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!

7. ### ElephantGuest

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