How to use an LED as a Light Detector Circuit

LEDs are usually used to make light, but did you know they can also sense it?

It is like a secret superpower, when light hits an LED in a special way, it actually creates a tiny bit of electricity.

This is called the photovoltaic effect, like a mini solar panel.

In a light detector circuit, we flip the LED around reverse bias and use this tiny electric signal to tell us how much light is there.

Circuit Working:

Parts List:

Component	Value/Type	Quantity
Resistors	300Ω, 220k, 47k	1 each
Semiconductors
Transistors	BC547, BC557	1 each
LED	Red 5mm 20mA	1
LED	White (bright)	1
Switch	ON/OFF	1

While most LEDs emit a specific color of light, some can also act as light detectors.

However, they are not quite as powerful as dedicated light sensors like solar cells, photocells or photoresistors.

Green LEDs can pick up on light, but high brightness red ones are the real stars.

They are about 100 times more sensitive than green ones.

There is a catch though the electrical signal they produce is very weak.

This circuit helps boost that signal making the LED a usable light detector.

Hold on, not all LEDs are created equal.

Most just are not sensitive enough for this purpose.

This circuit is designed specifically for the high brightness red variety.

Here is the cool part, this LED only detects light from the direction of its tip.

This makes it perfect for applications like solar trackers where light direction matters.

It is less suited for detecting regular room lighting unless the lamp is super close.

Formulas:

The following essential formulas and rules can aid in the construction and comprehension of the LED light detector circuit.

For Red LED (as light detector):

When the red LED is exposed to light, ohms law may be used to estimate the current flowing through it:

I_LED ​= V_supply​−V_LED​​ / R_LED​

where,

• The supply voltage, such as 6V, is represented by Vsupply.
• The forward voltage drop of the red LED is represented by VLED.
• The resistor value linked in series with the red LED is represented by RLED.

For White LED (as indicator):

Likewise, use ohms law to determine the current flowing through the white LED:

I_LED ​= V_supply​−V_LED​​ / R_LED​

where,

• The white LEDs forward voltage drop is represented by VLED.
• The resistor value linked in series with the white LED is represented by RLED.

Make that the resistor value RLED restricts the current passing through the LED to a safe working range, which is typically between 10 and 20 mA for ordinary LEDs.

Base Resistor Calculation:

In order to limit the base current IB, the base resistor RB for NPN transistors is commonly determined using the following formula:

R_B​ =​ V_in​−V_BE / I_B

where,

• The input voltage to the transistors base is represented by Vin.
• The base emitter voltage drop of a transistor is represented as VBE, which is usually around 0.7V for silicon transistors.
• IB is the intended base current.

One may get a typical value for IB using the following formula:

I_B ​= I_C / ​​β

where,

• The transistors collector current is represented by IC.
• The transistors current gain is represented by β (hFE).

Note:

A foundation for building and comprehending the LED light detector circuit using resistors, transistors, LEDs and a switch is provided by these formulae and recommendations.

Based on particular needs, such as LED kinds, transistor characteristics and desired circuit behavior, modify resistor values and component specifications.

Always double check computations using real component datasheets and hands on testing.​​

How to Build:

How to use an LED as a Light Detector Circuit follow the below mentioned steps:

• Connect the emitter of transistor BC547 to ground.
• Connect the collector of transistor BC547 to positive supply through series of resistor 330 ohm and a RED LED.
• Connect the base of transistor BC547 to junction of resistors 220k and 47k.
• Connect the collector of transistor BC557 to the 47k resistor.
• Connect base of transistor BC557 to one leg of white LED, and other leg to positive supply.
• Connect the emitter of transistor BC557 to positive supply.
• Connect a ON/OFF Switch to the positive supply of +6V DC.

Note:

• Take proper care while handing electrical works, if possible be in guidance of experts who has proper knowledge of electronics.

Conclusion.

To conclude, everyday LEDs can double as light detectors in a pinch.

While not as powerful as dedicated sensors, with some amplification magic, they can be surprisingly useful.

This directional detection makes them ideal for situations like solar trackers where light source matters more than overall brightness.

References:

Detecting light with an LED