Multiple PIR Sensor Circuits
Multiple PIR Sensor Circuits
Connecting more than one PIR sensors in a circuit
electronics | electric circuit | generalIn our article
PIR Sensor Circuits we looked at how a
PIR Sensor can be used in an automatic security or lighting system. We recommend you read that article before reading on.
Connecting Two PIR Sensors
There are many situations in which it is necessary to use more than one
PIR sensor within a system - for example, where lights along the side of a house are controlled by one PIR sensor at the rear and another PIR sensor at the front. Unfortunately, because of the way that PIR sensors work, it is not possible to simply join together the outputs from more than one sensor.
n.b. This article is written for 12 Volt DC powered PIR sensors such as the Honeywell IS-215T (for sale in the REUK Shop). Different PIR sensors - particularly those powered by mains electricity may operate differently.
When a
PIR sensor detects motion, its output is 0V - the rest of the time it is 12V. Therefore, if two PIR sensors are connected in
series, if one PIR sensor detects motion and the other not, the output would be 12V - indicating no motion detected. This is not what we want. We need a circuit to automatically output 0V if EITHER of the PIR sensors has detected motion (and therefore outputted 0V).
AND Gate Logic
In
logic high voltage is represented by
1, and low/no voltage by
0. Therefore, the output from a PIR sensor upon detecting motion is
0, and is
1 the rest of the time. We want an output of
0 when
either of the PIR sensors is outputting a
0. This can be summarised by the following
truth table:
| PIR #1 | PIR #2 | PIR #1 & #2 |
| 1 | 1 | 1 |
| 1 | 0 | 0 |
| 0 | 1 | 0 |
| 0 | 0 | 0 |
This truth table is exactly the same as that for an
AND gate - a logic device which gives an output of
1 only if its two inputs are
both 1. The traditional symbol for an AND gate is shown below:
As mentioned previously in our
PIR Sensor Circuits article - in order for us to make direct use of the output signal from a
PIR sensor we need to
invert it - turning a 12V output into 0V, and a 0V output into 12V. When motion is detected by a PIR sensor, the 0V sensor signal is therefore inverted into a 12V signal which can be put across the coil of a
relay or a
transistor to control lights or other appliances.
The device used to invert the signal is known as a NOT gate - inverting an input of 1 to 0, and visa versa.
The NAND Gate
When connecting multiple PIR sensors in a circuit it is not necessary to put the PIR sensor signals first through an
AND gate, and then through a
NOT gate; instead we can use something called a
NAND gate. The truth table for a NAND gate is shown below:
| PIR #1 | PIR #2 | PIR #1 & #2 |
| 1 | 1 | 0 |
| 1 | 0 | 1 |
| 0 | 1 | 1 |
| 0 | 0 | 1 |
So, putting the sensor signals from two PIR sensors through a
NAND gate results in a 12V output
unless both PIR sensor signals are 12V (the situation when neither has detected motion) in which case the outout is 0V. Exactly what we want!
The traditional symbol for a
NAND gate is illustrated above.
Practical Applications with PIR Sensors
What is a NAND gate, and how is it physically connected to the PIR sensors in a circuit? NAND gates can be purchased very cheaply as ICs (integrated circuits) - with the
4000 series versions suitable for use in
12 Volt DC systems.
The
4011B (schematic pictured above) is a
Quad 2-Input NAND Gate - a chip with 4 NAND gates, each one accepting two inputs. Click here to view a
4011B Datasheet (PDF 30Kb).
The maximum output current from a 4011b is only
5 to 10mA - just enough to light one
LED. Therefore, if the output must be amplified through a transistor so it can power lighting (up to a couple of hundred mA) or energise the coil of a
relay if a larger current lighting or other devices are to be powered.
Above is a schematic showing how two
PIR sensor outputs can be put through a NAND gate and the resulting output signal amplified by a standard
NPN transistor (such as the
BC547B which has a max output 110mA - easily enough to energise the coil of a standard
12V relay).
Still Confused?
This is necessarily all a bit more
involved than our usual articles, and putting together such a multiple
PIR sensor system from scratch requires quite a bit of electronics knowledge. Therefore, if you need any assistance, or if you would like us to put together a suitable 'black-box' / 'plug-and-play'
circuit board for you, email your requirements to
neil@reuk.co.uk.
Pictured above is a circuit which takes as inputs the outputs from
two PIR sensors, triggering a relay switch when
either or both of the sensors detects motion. Click here to find out more and/or to purchase the REUK
Double PIR Relay Timer.
Article Last Modified: 16:43, 6th Nov 2007
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