This is a tutorial on a Light-activated LED using SCR. This circuit is very easy to build, it will turn on the LED when it senses light on the LDR, and SCR is used here for switching purposes. We usually use transistors in this type of circuit but in this project, we are using SCR instead of a transistor. Silicon Controlled Rectifiers or SCR is also known as thyristors look like transistors but their internal circuitry and working are completely different than transistors. SCR has four layers of semiconductors NPNP or PNPN, whereas transistors have only three layers of NPN or PNP.
SCR has three junctions, anode, cathode, and gate. It has three modes of operation, ON state, of state, and Reverse blocking. It works as a switch in the electronic circuits just like transistors but SCR is capable of handling high AC voltages easily unlike transistors which can be used for low DC voltages only.
Hardware Components
The following components are required to make Light Activated LED Circuit
| S.no | Component | Value | Qty |
|---|---|---|---|
| 1. | Battery | 6v | 1 |
| 2. | LDR | – | 1 |
| 3. | SCR Thyristor | 2P4M | 1 |
| 4. | Resistors | 330 ohm, 1K | 1,1 |
| 5. | Variable Resistors | – | 1 |
| 6. | LED | – | 1 |
2P4M Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of 2P4M
Light Activated LED Circuit
Working Explanation
The circuit is operating at 6 volts. When it’s dark the resistance of the LDR will be maximized and the LED will remain turned OFF until the LDR senses the presence of light and its resistance will start to minimize and at some point, it will let the current flow through it which will be fed into the SCR. It will activate and the LED will light up. The variable resistor is used to adjust the amount of light on which you want to switch ON the SCR and light up the LED.
Application and Uses
There are a lot of applications for this circuit, it can be used for lighting and decorative purposes. You can also use it as a lamp by doing a few modifications in the circuit or use it at places for safety purposes.
