Written and Tested By: SSkopparthy
Here is a simple circuit that activates a relay when a living being (a human) is detected by the PIR sensor. Here PIR stands for Passive Infrared sensor. It doesn’t produce any infrared radiations to detect the presence of a living being but on the other hand it detects the infrared radiations released by them.
This circuit uses a HC - SR501 PIR motion sensor which is the heart of the circuit. Initially when the motion is detected by the sensor, it produces a small signal voltage(usually 3.3 volts) which is fed to the base of the transistor BC547 through a current control resistor and hence, its output goes high and it switches the relay on. This relay can be configured to be used with a electrical bulb or a tubelight, night lamp or anything else that works on 220VAC.
This circuit is mostly used in gardens, so that at night, when we go for a walk in the garden, the circuit switches on a light automatically and it remains lit until we are in the sensor’s vicinity and it gets turned off when we move away from that place and hence reducing the electricity costs. Here’s a back view of the sensor HC-SR501…
The sensor consists of two preset resistors which can be used to control the delay time and sensing range. The delay potentiometer can be adjusted to decide the time for which light remains on. The sensor when purchased, it comes with the default mode ‘H’ which means that the circuit switches on the light when motion is detected and it remains on for preset time and after the preset time lapses, if the sensor could still detect motion, it does not switch the light off and if it could not detect the motion, it switches off the light. Here are the technical details of the sensor HC-SR501…
Working voltage range: 4.5VDC to 12VDC.
Current Drain: <60uA
Voltage output: 3.3V TTL
Detection distance: 3 to 7 metres(can be adjusted)
Delay time: 5 to 200 seconds(can be adjusted)
One of the disadvantages of PIR motion sensors is that its output goes high even when a rat or a dog or some other animal moves in front of it and it switches on light unnecessarily. In cold countries, the sensor’s sensing range increases. Due to low temperature, infrared radiations released by humans travel more distances and hence causing unnecessary switching of lights. If installed in backyards, there are chances of activating of light when a car passes by because the radiations emitted by hot engine of car fools the sensor.
D1, D2 - 1N4007,
C1- 1000uf, 25V,
Q1 - BC547,
R1 - 10K,
R2 - 1K,
L1 - LED(green)
RY1 - Relay 12V
T1 – Transformer 0-12V.
After completing the construction of the circuit, enclose it in a suitable casing and use a separate casing for the sensor and connect the sensor to circuit using long wires so that you can place sensor at the place you wish like in a garden and circuit will be inside so that the circuit is protected from weather.
And remember to use a separate PCB for relay.
Also, don’t forget to use a suitable relay with correct current and voltage rating. You can use a terminal block which connects to the relay’s switching contacts, and arrange it as shown in image so that you can change the electrical device connected to relay contacts easily.
Usage of this sensors save electricity to great extents. It could reduce your electricity bills too!
“PLEASE SAVE THE POWER FOR THE NEXT HOUR!”
If the above design is intended to be used with an alarm and a lamp such that both the loads operate during night but the alarm only during day, then the diagram may be modified in the following manner. The idea was suggested by Mr. Manjunath