In this tutorial, we are going to make a Dark Activated Timer switch. It will switch on the relay for a preset time duration during darkness. You can connect any AC/DC device with this relay to control it with dark activated timer. This circuit will not re-trigger until one timing cycle is complete. It is an ideal circuit for a lot of automatic light-sensing projects.
Hardware Components
The following components are required to make Dark 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. | Resistors | 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 |
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
Dark Activated Timer Circuit
Working Explanation
The operating voltage of this circuit is 4.5-12V. This circuit is divided into two parts. In the first part, a dark sensor circuit is built around two transistors Q1 and Q2. The second part is a long-duration non-re-trigger type timer circuit built with a 555 timer IC.
In the first part, we used an LDR to sense the dark. The sensitivity of this sensor is adjusted through a 50K variable resistor. When there will be no light falling onto the LDR its resistance will be maximized due to which the first transistor starts conducting. This transistor’s output will activate the second transistor Q2. When Q2 activates its output goes from high to low and it triggers the 555 timer IC.
The time duration of this circuit depends on the variable resistor and the value of the capacitor. In this circuit, we have used 10uF and it is providing a duration of 10-20 seconds, if we change its value to 100uF it will provide a duration for a longer period. Make sure to use the relay with the same value as the operating voltage. A diode (1N4007) is used with a relay to protect the IC from the back EMF of the relay.
Applications and Uses
This circuit has a variety of applications. It can be used in your houses, as a bedside lamp or a porch light, etc.
