Automatic Pest Repellent

Automatic Pest Repellent is a non-chemical, clean, harmless and effective solution to unwanted pests. The circuit described here is nothing but a passive infrared (PIR) motion sensor activated water spray to deter animals from your lawn and/or garden. Needless to say, this system is ideal for repelling deer, dogs, raccoons, groundhogs, opossum, skunks, cats, rabbits, squirrels, and geese.

At the heart of the circuit is one readily-available passive infrared (PIR) motion sensor module. PIR (Passive Infra-Red) Sensor is a pyroelectric device that detects motion by measuring changes in the infrared levels emitted by surrounding objects. PIR sensor have elements made of a crystalline material that generates an electric charge when exposed to infrared radiation. The changes in the amount of infrared striking the element change the voltages generated, which are measured by an on-board electronics circuit. The device contains a special filter called a Fresnel lens, which focuses the infrared signals onto the element. As the ambient infrared signals change rapidly, the on-board amplifier activates the output to indicate a valid-motion.

The PIR Sensor has a range of approximately 20 feet. This can vary with environmental conditions. The sensor is designed to adjust to slowly changing conditions that would happen normally as the day progresses and the environmental conditions change, but responds by making its output active when sudden changes occur, such as when there is a “real” motion.

Pir sensor

The PIR sensor module have a 3-pin connection: Vcc, Output and GND. The pin-out may vary, so it is suggested to check manufacturer’s datasheet in order to confirm the pins. Sometimes, they have markings on the PCB next to the pins. As well, PIR sensor modules also have a 2-pin jumper selection for single or continuous trigger output mode. The two positions are labelled as ‘H’ and ‘L.’ When the jumper is at “Retrigger” position ‘H,’ the output remains high whenever the sensor is retriggered repeatedly. In position ‘L,’ the output goes high and low every time the sensor is triggered. So a continuous motion will give repeated high/low pulses in this “Normal” mode.

The circuit has three parts – the first for for driving a 6VDC /SPST electro-magnetic relay with the help of the PIR sensor module, and the second is a 6V DC selfpriming water pump for automated water spray function. Final part is the 6VDC power supply. The built in re-chargeable battery pack (6V/4.5 Ah SMF Battery) of the power supply can be charged using any suitable AC mains adaptor and/or a Solar Panel.

APR   FIG   2

In the circuit, switch S1 is the system on/off switch and the LED 1 is a “power status” indicator. In standby mode, if the PIR sensor module (SEN 1) detects a valid movement it will give a high (3.3V) output of variable width. This signal output goes to the relay driver transistor 2N2222A (T1) through R2. The 6V relay then activates the 6V water pump. One active piezo-buzzer (BZ1) is added for effective aural indication. For best results, try to attach a small sprinkler head at the outlet of the water pump.

Note: The PIR Sensor requires a ‘warm-up’ time in order to function properly. This is due to the settling time involved in ‘learning’ its environment. This could be anywhere from 10-60 seconds. During this time there should be as little motion as possible in the sensors field of view.

Schematic of the Pest Repellent Circuit

Automatic pest repellent


PIR Sensor Module – 1
1K ¼ W resistor – 2
100uF/16V Capacitor – 1
10nF/100V Capacitor – 1
Red LED 5mm – 1
1N4004 diode – 1
6VDC SPST Electro Magnetic Relay – 1
6V/4.5 Ah SMF Battery – 1
SPST On/Off Switch – 1
6VDC Water Pump – 1
Piezo Buzzer – 1

APR   FIG   4

Lab Note:

In my lab, the prototype was first tested with 6VDC power supply available from a power unit consists of AC230V to 6V step-down transformer and associated parts. Partial view of the prototype is included here!

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