4 in 1 DIY kit schematic

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1. Dark sensor:

Description: As you block light falling on the LDR, the LED D1 switches ON. As soon as you put lights on the LDR, the LED goes off.

Extra experiment: The sensitivity of the circuit depends upon the value of resistor R3. If you replace the resistor R3 (4.7K) with some other resistor, for example 330 Ohm, it becomes highly sensitive to darkness and if you increase the resistance beyond 4.7K, it gets less sensitive to darkness. You can try it.

For more information:

http://www.buildcircuit.com/dark-sensor-using-two-transistors-buildcircuit/

2. Latch Circuit

Description: When you press the tactile switch, the LED D2 switches on and when you press it again, the LED switches OFF. This is bistable mode of NE555 timer.

See how to make this circuit on breadboard.

Extra experiment: You can do a simple experiment with relay also. You can make a light operated switch with this circuit.

The schematic for the circuit is given below:

In this experiment, when you press light falling on the LDR, the relay gets activated and switches on the LED2. Here, relay operates exactly like the tactile (momentary) switch.

3. Astable Mode:

Description: In this experiment, the LED3 blinks according to the values of R3, R1 and C2. It has been configured in the astable mode of 555 timer.