Light detector using LDR and astable mode of 555 timer

Many a times we need a device which should activate itself according to outside light (like when we open door of drawer it should lit up). We sometimes feel the need of some light activated device, such a device could be made from circuit presented here. This light detector circuit project uses a light sensitive element which activates the circuit in light and switches it off in absence of light.

The light detector project presented here can be used in following applications.

1. Such circuit can be used in refrigerators to automatically switch their light on or off corresponding to the status of door of refrigerator

2. It could be used as morning alarm, which switches on with morning sunlight.

3. In drawers and wardrobes, on opening their door some light should automatically switch on.

This circuit uses 555 in astable mode and produces a frequency of about 55.5 hertz. In this circuit, reset pin (pin4) which is active high is connected to VCC through a LDR (Light Dependence Resistor). LDR as stated light dependent resistor is a resistor whose resistance depends on light. Its resistance decreases to a few ohms in presence of light, and reaches to the order of mega ohms in dark. In this circuit as LDR is connected to reset pin (active high), so when light falls on it, its resistance decreases to large extent and reset pin takes high and circuit functions and produce pulse which could be used to drive a circuit (if it is a buzzer; frequency should be in audible range and if it is LED then frequency should be high to avoid flickering). In the absence of light, the resistance of LDR becomes very high and reset pin is at active low state and there is no output, thus forming light activated circuit. The resistance R4 connected between reset pin and ground is used as pull down resistor. Pull down resistor is the resistor which is used to pull down the voltage applied to a particular circuit not leaving it to floating state.