PIR Sensor Circuits

Find out more about using PIR sensors within renewable energy applications

In our article titled PIR Sensors (1) 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 (2) which is in turn charged by wind (3), solar (4), or other renewable energy sources.

In this article we will look at the ways in which the excellent Honeywell IS-215T (5) 12 Volt DC PIR sensor (available in the REUK Shop (6)) 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 (7).

12 Volt PIR Sensor

Look inside a PIR sensor at the connections

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:

PIR sensor transistor inverter NOT gate circuit diagram

...with the signal from the PIR sensor being generated as shown below:

Connecting the PIR sensor to the inverter circuit.

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 (8), a 12 Volt LED spotlight (9), 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 (10).

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 (11).

PIR Sensor Timer Circuit

In our introductory article to PIR Sensors (12), 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 (13) 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 (14) and transistor (15) 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 (16) as a complete unit in the REUK Shop (17) designed and tested for use with our PIR sensors (18).

Web Link References

(1) http://www.reuk.co.uk/PIR-Sensors.htm
(2) http://www.reuk.co.uk/storage.htm
(3) http://www.reuk.co.uk/wind.htm
(4) http://www.reuk.co.uk/solar.htm
(5) http://www.reuk.co.uk/buy-12V-PIR-SENSOR.htm
(6) http://www.reuk.co.uk/REUK-Products-For-Sale.htm
(7) Email address: neil@reuk.co.uk
(8) http://www.reuk.co.uk/Relays-and-Renewable-Energy.htm
(9) http://www.reuk.co.uk/buy-12V-LED-SPOTLIGHT-BULB.htm
(10) http://www.reuk.co.uk/Solar-Powered-Wireless-CCTV.htm
(11) http://www.reuk.co.uk/Multiple-PIR-Sensor-Circuits.htm
(12) http://www.reuk.co.uk/PIR-Sensors.htm
(13) http://www.reuk.co.uk/buy-VARIABLE-RESISTORS.htm
(14) http://www.reuk.co.uk/buy-12V-10A-RELAY.htm
(15) http://www.reuk.co.uk/What-is-a-Transistor.htm
(16) http://www.reuk.co.uk/buy-PIR-RELAY-TIMER.htm
(17) http://www.reuk.co.uk/REUK-Products-For-Sale.htm
(18) http://www.reuk.co.uk/buy-12V-PIR-SENSOR.htm