Simple Darkness Detector Circuit

LED turn on by LDR when dark.

LDR and resistor connect in voltage divider circuit.

In bright light transistor base get low voltage so transistor stay OFF.

When dark come LDR resistance goes down and base voltage goes up with transistor turn ON and LED light up.

This Simple Darkness Detector Circuit is easy to make and needs few parts only.

Below is list of parts for this circuit.

Circuit Working:

Parts List:

LDR is main part of this circuit.

It work like dimmer switch and changes resistance with light.

In bright light LDR has high resistance and in dark it has low resistance.

To make voltage divider LDR connect with resistor R2.

Together they split voltage from power source.

In bright light LDR block voltage to transistor base so transistor stays OFF.

Transistor work like switch and this small voltage at base can control big current.

When at dark the LDR resistance goes down and more voltage goes to transistor base.

Transistor turns ON and completes the path and LED lights up.

So LDR controls transistor like switch.

When dark come the LDR turns ON the LED.

Formula:

Voltage at transistor base is decided by voltage divider like LDR and fixed resistor.

Use this formula to find base voltage:

Vout = Vin × (R2 / (LDR + R2))

where:

• Vout is the voltage at transistor base
• Vin is the voltage from power supply
• R2 is the fixed resistor like 47k
• LDR is the LDR resistance which changes with light

How to Build:

To build a Simple Darkness Detector Circuit follow the below mentioned steps for connections:

• Take all parts shown in circuit diagram.
• Connect transistor Q1 collector to +9V through resistor R1 and LED.
• Connect transistor base between resistor R2 and LDR.
• Connect emitter of Q1 to ground.
• Connect resistor R2 and LDR from +9V to ground.

Conclusion:

This Simple Darkness Detector Circuit is easy to make and is quite useful.

This circuit is good for first electronics project which need few parts only.

We can use this small circuit in other big projects too.

Hope with this project it gives us an idea to make our own darkness detector.

References:

Dark Sensor Using LDR