Proximity Switch Using One Transistor with UTSOURCE Service

We are going to build a simple proximity switch using just one transistor as the main component. In this circuit we are going to use the optical proximity method. In this circuit we have a IR led to send IR light towards the target object and a IR photo diode or a receiver to receive the IR signal sent from the IR LED reflected from the object. Although the IR detector detects the reflected light from the object, the current provided by the receiver it self will be not enough to drive a output. To overcome this problem, we are using a small NPN transistor. For the indication of the proximity of a object we will use a normal LED.

Components required:

Let’s see what components are required for this

1. UTSOURCE 220Ω Resistor

2. UTSOURCE 100Ω Resistor

3. IR LED

4. IR photodiode or receiver (2pin)

5. Normal LED

6. UTSOURCE BC547 NPN Transistor

7. 3.7V battery

This is the basic circuit diagram of the proximity circuit. As you can see a 100Ω resistor is used in series with the IR LED and a 220Ω resistor is used in series with the normal LED. Both these resistors are acting as current limiting resistors to the corresponding LEDs.

Step to follow:

Step 1: Arrange the components

Step 2: Solder positive pin of the photodetector (longer Pin) to the base pin of the BC547 transistor ( middle pin of the BC547)

Step 3: Solder two negative lead of the IR LED to the emitter of the BC547 transistor

Step 4: Connect 100Ω resistor to positive of the IR LED and the negative of the photodiode.

Step 5: Solder 220Ω resistor to the collector pin of the BC547 transistor

Step 6: Connect the negative pin of the normal LED to the remaining end of the 220Ω resistor and the positive lead to the negative pin of the photodiode.

Step 7:

Now connect a 3.7V battery. Positive pin of the battery to the negative pin of the photodiode and the negative of the battery to the emitter of the transistor

Step 8: Circuit is now ready. You can test it

How It Works

When the circuit is powered IR LED will emit IR (infrared) light as the IR LED and the photodetector is in parallel, there is no chance to IR detector to receive the IR light from the IR LED directly. When an object come near the IR LED and the photodiode, IR light reflected from the object will be detected by the photodiode and produce a small current as a result. This current is enough to trigger the base of the BC547 transistor. So, the normal LED will be turned on. You can observe when a object get more closer to the IR LED and the photodiode the brightness of the normal LED will be increased. This is due to closer the object higher the IR light detected by the photodiode and higher the current produced by the photodiode so BC547 will allow more current to flow across the normal LED.

Conclusion:

We can use these types of circuits to create a automatic light circuits which tuns on when a person comes near to it.