The 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 reset. 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.
