LIGHT DETECTOR Circuit
This circuit
detects light falling on the Photo-cell (Light Dependent Resistor)
to turn on the 555 and create a tone that is delivered to the
speaker. Pin 4 must be held below 0.7v to turn the 555 off. Any
voltage above 0.7v will activate the circuit. The adjustable
sensitivity control is needed to set the level at which the circuit is
activated. When the sensitivity pot is turned so that it has the
lowest resistance (as shown in red), a large amount of light must be
detected by the LDR for its resistance to be low. This produces a
voltage-divider made up of the LDR and 4k7 resistor. As the
resistance of the LDR decreases, the voltage across the 4k7
increases and the circuit is activated.
When the sensitivity control is taken to the 0v rail, its resistance
increases and this effectively adds resistance to the 4k7. The
lower-part of the voltage-divider now has a larger resistance and
this is in series with the LDR. Less light is needed on the LDR for
it to raise the voltage on pin 4 to turn the 555 on.
View All Circuits
