Joining several PIR sensors

Question

I wish to use some PIR (HC-SR501) sensors and give some response if at least one of them detects human movements.

For simplicity I wish to use only one input pin in an Arduino UNO. My idea is to connect all the output wires into a single Arduino digital input. Should it work?

I am in doubt because the following two reasons:

• The sensors must be far away from each other (about 5 sensors partially covering an area of about 200 m^2). That implies that wires are long and I do not know if this can have some adverse effect.
• I do not know if it is bad to supply the 3.5V of some sensor that is currently detecting movement to the output of the sensors that are not detecting anything.

Thanks in advance

• Delta_G: Don’t know about the first question but your second concern can be solved by a simple diode one each line to make sure current only goes one way.

Answer 1

The BISS0001 that your unit uses provides an "active high" output. To connect multiple devices together you just need to "OR" the signals.

There's many ways of doing that.

The simplest method is the "wired OR" circuit, which just involves one pulldown resistor plus one diode per device:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

You could also use a multiple-input OR gate chip. You can get these in 2- (74HC32), 3- (74HC4075), 4- (74HC4072) input versions.

• user1420303: Thank you! Could you comment why a resistor is needed and how to calculate its resistance as a function of the number of PIR employed?
• Majenko: The resistor is simply a pull-down. When all the inputs are LOW the diodes are reverse / not-at-all biased. When one goes high they become forward biased by the current through the diode and the resistor to ground. The value is pretty meaningless. The BISS0001 can source up to 10mA, so it wants to be larger than 500Ω. The number of PIR modules is irrelevant. If you have very long wires you might want to reduce the pull-down a little.
• user1420303: Thank you! I just read about pull-down. In practical terms, the resistor is needed to avoid false positives due to noise. Right?
• Majenko: It is needed to form part of the circuit. It's what provides the LOW when none of the PIRs are providing a HIGH.