circuit presented here wakes you up with a loud alarm
at the break of the daylight. Once again the 555 timer
is used here. It is working as an astable multivibrator
at a frequency of about 1kHz.
The circuit's operation
can be explained as follows:
When no light falls
on the LDR, the transistor is pulled high by the variable
resistor. Hence the transistor is OFF and the reset
pin of the 555 is pulled low. Due the this the 555 is
When light falls on the LDR, its resistance
decreases and pulls the base of the transistor low hence
turning it ON. This pulls the reset pin 4 of the 555
high and hence enables the 555 oscillator and a sound
is produced by the speaker.
The variable 100K
resistor has to be adjusted to set the light intensity
that triggers the alarm.