PIR Sensor Circuits
PIR Sensor Circuits
Find out more about using PIR sensors within renewable energy applications
electronics | electric circuit | general | solarIn our article titled
PIR Sensors we introduced
passive infrared (PIR) sensors and summarised their uses. Of particular interest to us are
12 Volt DC powered PIR sensors and associated circuits since these can be powered directly from a
12 Volt battery which is in turn charged by
wind,
solar, or other
renewable energy sources.
In this article we will look at the ways in which the excellent
Honeywell IS-215T 12 Volt DC PIR sensor (available in the
REUK Shop) can be used by the renewable energy enthusiast with
lighting and security systems.
There is not much by way of instructions for use literature for this (or any other) 12 Volt PIR sensor on the market. Therefore everything in this article has been
discovered experimentally and solutions
invented as and when necessary. During this process three transistors and two variable resistors were destroyed in clouds of smoke, one diode was melted, and smoke poured out of the PIR sensor unit for ten seconds without doing it any long term harm! We would be
very interested to hear from anybody with more experience using these sensors and their recommendations. Please email
neil@reuk.co.uk.
12 Volt PIR Sensor
When the cover of the
PIR Sensor unit is popped off it appears as shown above. A row of terminal strip at the top right of the circuit board is used for making the necessary connections - two connections to provide power to the unit (labelled + and
-) and two for the
relay (labelled C and NO).
When the PIR sensor detects the movement of a warm body the integrated
LED Indicator lights up for a second or two. When the LED is not lit,
C and
NC are connected. When the LED
is lit
C and
NC are
not connected. Everything would be a lot easier for us if it worked the otherway around - i.e.
C and
NC only connected when motion is detected, however a simple
transistor inverter NOT gate can be used to
invert the output signal as shown below:
...with the signal from the
PIR sensor being generated as shown below:
So, when the PIR sensor unit detects motion, the LED lights up, the PIR Sensor signal is 0V which is then
inverted to an output signal of +12V (by our simple NOT gate). When no motion is detected, the LED is not lit, the PIR sensor signal is +12V which is then inverted to an output signal of 0V.
The output signal can then be connected across the coil of a
relay, a 12 Volt
LED spotlight, or a buzzer/alarm for example. However, the device (or circuit) connected to the output from the NOT gate circuit will only receive power for as long as the LED indicator is lit on the PIR sensor unit - a couple of seconds at most.
If a light needs to be switched on and left on for a fixed time interval, or if a video camera * is to record for say 30 seconds every time the PIR sensor unit is triggered, then we need to put together a simple
timer circuit.
*
See our article on Solar Powered Wireless CCTV.
NEW If you would like to connect more than one
PIR sensors into the same circuit - for example, one sensor at the front of a property and one at the rear, click here to read our article:
Multiple PIR Sensor Circuits.
PIR Sensor Timer Circuit
In our introductory article to
PIR Sensors, the schematic circuit pictured above was provided. A few modifications have been made to make it work with our PIR sensor.
Changes to the circuit diagram are marked in red.
When the 1M
variable resistor is turned to 0, the relay stays on for 4-5 seconds. When the variable resistor is turned to around 1M, the relay stays on for around 55 seconds *. Therefore the timer can easily be adjusted to 10 seconds or 30 seconds etc as required by the application.
*
The exact timings depend particularly on the choice of relay and transistor used.
Buy a Complete PIR Relay Timer Circuit
NEW - If you do not have the necessary tools, skills, time, or experience to put together a
relay timer circuit for yourself, do not worry - we now sell the
PIR Relay Timer as a complete unit in the
REUK Shop designed and tested for use with our
PIR sensors.
Article Last Modified: 11:32, 3rd Nov 2007
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