Today we come with a unique idea i.e. Photodiode using LED. This project utilized an LED to detect and respond to the presence and absence of light. Before starting the description and working of circuit DIY Light Detector Circuit using LED, we would like to describe a few electronics components.
LED: Light-emitting diode is a photoelectric device that converts electrical energy into light energy. It is a heavily doped P-N junction diode that under forwarding bias emits spontaneous radiation.
Photodiode: Photodiode is an optoelectronic device in which current carriers are generated by photons through photoexcitation i.e., photo conducting by light. A photodiode is a special type P-N junction diode made of photosensitive semiconducting material.
From the above description, we came to conclude that LED is for emitting light where the photodiode is for detecting light. Both of these semiconductor devices work on the opposite principle. But here in this project, we had to use LED as a photodiode i.e. detect the presence and absence of light. The LED used here is a general-purpose 5mm LED and can be any color. But in the project DIY Light Detector Circuit using LED we had used RED color LED because RED LED is more sensitive to ambient light.
Circuit Description of DIY Light Detector Circuit using LED:
The circuit of DIY Light Detector Circuit using LED is designed using a high-performance BIMOS operational amplifier IC CA3140, two NPN general-purpose transistors BC547 (T1) and BC337(T2), and a few passive electronic components i.e. resistors and capacitor.
Here in this project, we are going to show how one can use general-purpose LED can be used as light-emitting as well as light-detecting circuit. In the presence of light, the heavily doped P-N junction becomes forward-biased and a voltage of 1V with the current of 0.030mA is generated.
According to the datasheet of op-amp CA3140, it consists of high impedance MOSFET input and low impedance bi-polar output. The input impedance is about 1.5 tera-ohm which is very high and as a result, it changes its output on a very low current (in a range of few pico-ampere i.e. 10-12 Ampere). Thus, the output of LED is enough to drive this MOSFET used in the input of op-amp CA1340.
LED1 (used as a photodiode) is connected to op-amp input in reverse bias mode as shown in figure 1 where resistor R1 is a biasing resistor. Resistor R2 is a feedback resistor that decides the feedback current of op-amp configured in trans-impedance mode (i.e. current-to-voltage converter).
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Operations of DIY Light Detector using LED
When light is exposed to LED1, it generates a current which is reverse. The reversing current increases voltage at inverting input pin as a result output becomes low. This low output of op-amp keeps T1 and T2 in off state and LED2 does not glow.
In dark, no current is generated from LED1 thus both the input pin has the same voltage. As a result, the output of IC1 becomes high and drive transistor T1 and T2 in conduction mode, and LED2 starts to glow.
NOTE: The project DIY Light Detector Circuit using LED posted here can be used as a night lamp. To do so, LED2 was replaced with the relay to control AC load.
PARTS LIST OF DIY LIGHT DETECTOR CIRCUIT USING LED
| Resistors (all ¼-watt, ± 5% Carbon) |
| R1 = 1 MΩ
R2 = 10 MΩ R3, R4 = 1 KΩ R5 = 100 Ω |
| Capacitor |
| C1 = 100 µF, 16V (Electrolytic capacitor) |
| Semiconductors |
| IC1 = CA3140 (BiMOS Operational Amplifier)
T1 = BC547 (NPN Transistor) T2 = BC337 (NPN Transistor) LED1 = 5mm RED LED LED2 = 5mm Any color LED |
| Miscellaneous |
| B1 = 9V Battery |