The circuit is combination of two sub circuits
- Sensor circuit
- Monostable multivibrator circuit using IC555
A sensor circuit is built using IR LED, photo diode and two NPN type transistors. IR LED and photo diode are placed nearby on the PCB pointing to same direction in such a way that IR light cannot fall directly on photo diode but only reflected light from any object or person falls on it. Photo diode is connected in reverse bias manner. The output of photo diode is given to 1st transistor Q1 that is connected in switch configuration. Again the output of Q1 is given as input to 2nd transistor Q2 which is again connected in switch configuration. Final output of sensor circuit is taken from output of transistor Q2 that is given to trigger input of IC555. IC555 is configured in monostable multivibrator mode. Resistor R9 and capacitor C3 are timing components of monostable multivibrator. The output of IC555 drives DC motor through Darlington amplifier TIP122. The complete circuit works on 5 V DC supply. DC motor is of also 5 V with 500 RPM.
If there is no body is in front of sensor circuit, IR light will not fall on photo diode and its output is high. So transistor Q1 is ON and its output is low. Because this output is applied as input to Q2 its output will be high. So the trigger input of IC555 is high. The IC555 will not get trigger and its output is low. So DC motor fan is off.
When a person (or any object) comes in front of sensor circuit, it reflects IR light that falls on photo diode. So its output goes low. So transistor Q1 input goes low and its output goes high. This makes input of Q2 high and its output low. As Q2 output goes low that applies trigger input to IC555. And its output goes high for the time period set by RC time components. Because this output drives motor, it starts running till the output remains high.
When again a person moves away from sensor photo diode output is high and finally the trigger input to IC555 is also high. So motor continuous to run for the time period given by R9 and C3 (adjustable from 1 to 10 sec) and then it automatically stops.
Thus the motor (fan) is ON only if any person is in front of the sensor. If it senses no one in front of it, it turns fan off.
There are few more applications of this circuit also.
The circuit can be used in automatic hand dryer that is used in wash rooms of super markets, multiplexes, hotels or malls. When any one places his/her hand in front of it, the internal motor starts that blows air to dry up hands.
With slight modification this circuit can be used in automatic tap that automatically starts water when a person places his hands below it.
Now let us do a simple modification in the circuit so that it can turn on/off 230 V light and fan simultaneously.
As shown in circuit tab 2, the few changes are
· In place of darlington amplifier there is simple NPN transistor
· The transistor drives 5 V 2 C/O (change over) type relay
· A fan and a bulb both are connected with two common terminals of relay and normally open terminals of relay are given direct 230 V AC supply
· The second terminal of fan and bulb is given to neutral of AC line
So now when sensor senses a person – it triggers IC555 - transistor becomes ON – it switches ON the relay – bulb and fan both are ON simultaneously.
So using this circuit in a lift, light and fan inside a lift will be ON only while lift is in use. When lift is idle light and fan both will be turned off automatically. SAVE ELECTRICITY – SAVE ENERGY