Study the Working Principle of the Circuit

Construct the simple light sensor circuit

On week 10th, (26/3/2018 - 1/4/2018)


This week I try to construct the simple light sensor circuit to show the working principles of the sensor. However, this circuit does not show any results or outcome. In my view, there maybe some errors on my circuit connection.

Working Principles

The IR Sensor Set consists of an IR transmitter and an IR receiver. With minimum interface and 5VDC power, it can be used as a reflective type IR sensor for mobile robot or low cost object detection sensor.

IR Transmitter will always transmit IR light (Infrared), it is not visible to human eyes. Since the transmitter and receiver is being arrange side by side, theoretically, the receiver should not receive any or in most cases, it will receive small amount of infrared emitted by the IR transmitter. The working concept of IR receiver is similar to transistor or LDR (Light Dependent Resistor). Just imagine this. Referring to above diagram, the IR Receiver is like a transistor with the ‘base’ controlled by the IR light received. When there is no IR light receive, the ‘collector’ of transistor does not allow current to sink to ’emitter’ further to ground of circuit. It is like very high resistance from ‘collector’ to ’emitter’, blocking current going to ground. In this case, the voltage at SENSOR1 node will be high, near to 5V.

When the IR receiver receives more IR light, it changes the resistance at ‘collector’ and allow more current to sink to ground, and this is similar to low resistance at the lower part of the circuit. So if you know the voltage divider formula, I guess you understand what will happen, the voltage at SENSOR1 will drop.

We utilize this voltage changes to IR light to “know” whether there is obstacle or not. Because when there is obstacle, IR light get  reflected to IR receiver further changes the voltage, monitoring the voltage changes will get you an obstacle detection sensor.