The ability of a Light Dependent Resistor (LDR) to alter its resistance in response to light intensity is used in a simple wheatstone bridge light detector circuit.
One arm of the wheatstone bridge has a series connection between the LDR and one of the fixed resistors.
In the opposing arm, a series connection is made between the potentiometer and the other fixed resistor.
After applying the voltage source across the bridges two opposing corners, the output voltage is measured between the two remaining corners.
You may use an operational amplifier op amp to increase the bridges modest output voltage.
An LED can then be driven by this voltage that has been amplified.
The LED turns on when light strikes the LDR and causes the output voltage to drop.
Circuit Working:
Parts List:
| Category | Item | Quantity | Power Rating |
|---|---|---|---|
| Resistors | 10k | 3 | 1/4 watt |
| 330Ω | 1 | 1/4 watt | |
| Potentiometer | 10k | 1 | – |
| LDR | LDR | 1 | – |
| Semiconductors | IC LM741 | 1 | – |
| Transistor BC547 | 1 | – | |
| LED (red) 5mm 20mA | 1 | – | |
| Components | 9V Battery | 1 | – |
Using light as the weight, our light detector circuit functions similarly to a balancing scale.
Think of the scale as having two sides.
We have an adjustable resistor (potentiometer) and a light sensitive resistor (LDR) on one side.
Two fixed resistors with the same value 10k ohms are located on the opposite side.
The LDR tilts the balance in the dark by acting as a large weight on one side of the scale.
The LDR grows lighter (lower resistance) when light shines on it, balancing the scale.
A unique chip called an op amp monitors the scale in the capacity of a referee.
It continuously monitors the voltage at points C and D on either side.
The weight of the LDR causes the scale to tilt in the dark, increasing the voltage on its side point C.
The imbalance is detected by the op amp.
The voltages on both sides C and D are comparable because light balances the scale when it strikes the LDR.
Once more, the op amp notes, this data is used by the op-amp to operate a small switch (transistor).
The LED illuminates when it detects darkness (high voltage on the LDR side) activating the switch.
When it detects light (balanced voltages), it turns the switch to turn off the LED, which serves as an indicator.
In simple words, this circuit turns an LED on (dark) or off (bright) based on the LDRs response to light.
What is Wheatstone bridge and Formula:
By balancing two legs of a bridge circuit, one of which contains the unknown component, an electrical circuit known as a wheatstone bridge may be used to measure an unknown electrical resistance.
The main advantage of the circuit is that, when compared to a basic voltage divider, it may offer incredibly precise readings.
It functions similarly to the original potentiometer.
The relationship between resistances R1 and R2 and resistances R3 and R4 is known to exist in wheatstone bridges, where R4 represents the unknown resistance R1 and R2 are known resistors and R3 is a potentiometer and the voltage drop between points C and D is zero.
Formula:
It is crucial to understand the idea of creating a balanced bridge, or equal voltage between points C and D.
The balance point of Wheatstone bridge is mentioned in below formula:
R1 / R2 = R3 / R4
The ratio of two resistors R1 and R2 in a balanced wheatstone bridge is equal to the ratio of the remaining two resistors R3 and R4.
This formula guarantees that there is no current flow via the output when the bridge is balanced, meaning that there is equal voltage across C and D.
How to Build:
To build a Simple Light Detector Circuit using Wheatstone Bridge below mentioned are the steps for connections:
- Assemble all the components as shown in above circuit diagram.
- Connect pin 2 of IC1 LM741 to the junction of R1 and R2 resistors.
- Connect pin 3 of IC1 LM741 to center leg of VR1 pot.
- Connect pin 4 of IC1 LM741 to ground.
- Connect pin 6 of IC1 LM741 to base of transistor Q1.
- Connect pin 7 of IC1 LM741 to positive supply of 9V battery.
- Connect VR1 pot upper leg to positive supply through LDR, connect center leg to pin 3 of IC1, connect lower leg to ground.
- Connect transistor Q1 collector to positive supply, connect base to pin 6 of IC1 through resistor R3, connect emitter to ground through resistor R4 and LED1.
Safety Measures:
- Op amps are susceptible to static energy, so handle them carefully.
- Prior to contacting the components ground yourself.
- Op amps can produce heat under some circumstances, thus use the appropriate heat sink.
- Use of a heat sink to disperse heat is something to think about if your circuit is running the op amp at its highest ratings.
- Erroneous wiring of components is a typical error.
- Before turning on, check your circuit diagram and make sure all connections are made.
- You may reduce the hazards associated with creating and experimenting with your light detector circuit by adhering to these safety precautions.
Conclusion:
To conclude, the light detector circuit using wheatstone bridge senses light using a deft balancing maneuver.
When it becomes dark, the light sensitive resistor tips the scales like a weight.
If an op amp is utilized it functions as a referee by activating an LED to show light or dark depending on the balance.
When building, never forget to put safety first.
Make sure you have the right equipment, a secure power supply and a non conductive surface.
Enjoy making your own light detector!
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