Using a regular LED as a basic LDR

(Many of you may already know this, but I didn’t. So I tested it out)

One of my friends told me that we can measure the amount of ambient light by reverse biasing a regular red LED. This didn’t seemed to be quite possible to me at start. So I wanted to see for myself as to how far we can use an LED as an LDR.

To test this hypothesis I set up the following apparatus using my National Instruments myDAQ.

The 2 wires are connected to the analogue input ports on the myDAQ unit such that the negative end of the LED is connected to the positive analog input and the positive end of the LED is connected to the negative analog input.

Using the oscilloscope I initially took the readings from different sources of light.

 

9 White LEDs held at around 3 inches from the red LED

This gave me a reading of about 443mV. The Voltage generated was not stationary. It moved up and down very slowly with a voltage of 1.93mV. Overall this was a very small swing. However I was surprised that there was a change in the reading. I continued with my next experiment.

 

9 UV Black Light LEDs held at around 3 inches from the red LED

The UV light gave the LED a much greater swing of around 1.245V.

 

1 Red LEDs held very close to the LED at test

The voltage change due to this was very insignificant. It was around 131mV. I replaced the forward biased Red LED with a yellow LED. This gave a swing of 109mV. Then I added another yellow LED to increase the change in voltage to 214mV. Adding another yellow LED increased the difference to 320mV.

 

Knowing that the reversed biased LED changes its voltage due to intensity I tested the voltage change while bringing my 9 white light LEDs closer and closer to the red LED.

I observed the following graph

 

A few Observations and Conclusions

• The Voltage swing obtained from a red LED connected in reverse bias is very small. (high inaccuracy)
• It takes a ling time for the voltage to rise up to its steady state voltage
• The voltage at steady state never really is steady. There is always some oscillation that is also very slow.
• Connecting up a red LED in reverse bias will behave quite similar to a LDR however there are many disadvantages to using this substitute.

 

To find out more about this just set it up yourself and have a go at it. You do not need a power source nor any resistors. All you need is a regular LED and a voltmeter, multimeter or any other voltage reading devices.

Thanks for reading.