Light dependent resistor(LDR), also called as Cadmium Sulphide (CdS) cell or photoresistor is a resistor whose value depends upon the incident light intensity. In darkness, the resistanceof LDR becomes high and in light, the resistance gets low.
Dark Sensor Using 555
The following circuit works as a dark sensor. If the light falling on LDR is blocked, it switches on the LED D1. It has been configured in monostable mode of 555 timer which means that the circuit switches on the LED D1 for a specific time. The switching time is determined by the combination of resistance R1 and capacitor C1.
In this circuit, time (T)= 1.1xR1xC1 = 1.1x47x103x100x10-6= 5.17sec.
Touch Sensor using 555
We can make the previous dark sensor circuit work as a touch sensor by making small changes in the schematic. We just need to remove the LDR, put a small wire and connect it to the coupling capacitor. Note that the electrode wire connected to capacitor C2 should be around 2-5cm long. As we touch on the wire, the circuit triggers the LED for a specific time as determined by the resistor R1 and capacitor C1. You can connect the wire to a small metallic touch plate using an alligator clip.