In this tutorial, we are going to make a project on the Light-Activated Timer switch circuit. It can activate the timer only one time for a pre-set time period no matter how long the light keeps falling on LDR. It can not be re-triggered again until one timing cycle is completed. That’s why this circuit is ideal to be used for a variety of automatic light-sensing projects.
Hardware Components
The following components are required to make Light Activated Timer Circuit
| S.no | Components | Value | Qty |
|---|---|---|---|
| 1. | DC Supply | 4.5-12V | 1 |
| 2. | IC | NE555 timer | 1 |
| 3. | LDR | – | 1 |
| 4. | Variable Resistor | 50K, 2M | 1,1 |
| 5. | Resistor | 1K, 270K, 220K | 2, 1, 1 |
| 6. | Electrolytic Capacitor | 10µF | 1 |
| 7. | Transistor | 2N3904 | 2 |
| 8. | Diode | 1N4148 | 1 |
| 9. | Diode | 1N4007 | 1 |
| 10. | Relay | 12V | 1 |
| 11. | Ceramic Capacitor | 0.1µF | 2 |
| 12. | Breadboard | – | 1 |
Light Activated Timer Circuit
555 IC Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of 555 Timer
2N3904 Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of 2N3904
Working Explanation
The operating voltage of this circuit is 4.5-12V. It is divided into two parts. The first part is a light sensor circuit built around two 2N3904 transistors and an LDR. To adjust the light sensitivity we have used a 50K ohms variable resistor. When LDR receives light the first transistor starts to conduct and turns the second transistor ON. When the second transistor activates its output becomes low and it triggers the 555 timer IC present in the second part of this circuit.
The second part is a monostable vibrator circuit built with a 555-timer IC. After receiving a signal from the transistor this IC will give output at its Pin 3 and provides power to the relay. The relay will be switched on for a preset time period. The IC can not re-trigger until one timing cycle is completed and the output of Transistor 2 goes high to low again. If you want to activate the timer delay for three hours when the sun rises, then the timer will not re-trigger again after three hours until the LDR gets in the dark/night and light/day after that.
This is a circuit of long duration timer but it can be increased/decreased by changing the value of capacitor Cx used at the input of IC. We have used a 10uF capacitor, it will give 10-20 seconds of time duration. If you change its value to 100uF or 470uF it will give a time duration of several hours. This time duration is also dependent on the variable resistor 0f 20M ohms.
There is a relay at the output of this circuit so that you can connect any AC/DC device with it and control it.
