The post explains a simple automatic PIR controlled fan circuit for school college use, which responds and switches ON only in the presence of a human (students) in the classroom. The idea was requested by Mr. Souren Bhattacharya.

Technical Specifications

I am, souren bhattacharya, a high school teacher in west bengal.

To reduce electricity used in my school classroom can u please make a circuit which can switch offon fans (3/4 fans) in the classroom as per routine with a facility to manual override.

For example each class has one computer class and one physical education class in a week. we want to switch off fans when whole class is empty.

If you give ur contact no in my email inbox i can explain in better way in ur idle time.

my email id is please help us.

The Design 

The design will require some kind of human IR sensor to be included, for example a PIR sensor device which looks to be the most efficient and effective for the proposed application.

Incorporating a PIR sensor makes the design pretty simple since most of the complex circuitry is handled within the unit itself. The sensor just needs to be integrated with a triggering stage and a correctly rated power supply as shown in the following diagram.

Circuit Diagram

In the given diagram we are able to see a standard preprogrammed PIR module, a 7805 voltage regulator IC stage for supplying the PIR, and a simple 12 V transistor/relay driver stage.

The PIR Module

The PIR module has three terminals, the right one is the ground terminal, center one is the positive +3.3V or +5V, and the left terminal is the responsive output lead of the device.

When the particular assigned (+) and (-) terminals of the PIR device are connected to the specified supply voltages, the device instantly becomes responsive and begins "thinking".

No human presence or motion should be created in front of the unit's lens during this initial switch ON period for about a minute or so, until the device locks ON and puts itself into an alert or a ready stand by position.

The unit now becomes ready and responds to even the slightest human motion or presence in front of its lens by generating a positive supply at its output terminal, this high at its output terminal persists as long as a human presence is detected within a radial range of around 20 meters in front of the PIR device.

 Sensing Human Presence

The output turns into a zero voltage as soon as the human presence moves away or is removed.

The above well defined high/low voltage response at the output lead becomes ideally suited or accessible for a transistor relay driver stage as shown in the diagram.

When the PIR output is high due to the presence of a human (children in classroom), the transistor BC547 base receives the +3.3V out from the relevant lead of the device and quickly switches ON the relay.

The relay in turn switches ON the fan and the system stays ON as long as the students occupy the premise.

When the students leave and vacate the premise, the PIR instantly switches OFF its output to a zero voltage level, however the presence of the 470uF/25V capacitor at the output lead of the PIR prevents the BC547 from getting switched off instantly rather keeps it ON for a few seconds more after the PIR has reverted its output to zero.

After this delay the BC547 also gets deactivated, switching OFF the relay and the fan or any other desired load whatsoever that may be wired with the relay.

The above circuit may be effectively modified as given  below for operating lights, with a feature making sure that it's implemented only during the night time and not during the daytime when ample daylight is accessible. The idea was requested by Mr. Sham.

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