Light Alarm (Sun Up Alarm)
This circuit designed to sound a loud alarm at the break of the dawn especially for those wh cannot wake up even with an alarm clock. Circuit can be modified as an intruder alarm or as a security alarm. Needless to say that 1.8 KHz astable multivibrator with 555 is used here again with a light dependent resistor used as a light sensor.
Light dependent resistor is made of Cadmium Sulphide and it offers very high resistance (about 1 mega ohm) when there is no light falling on it. If the light falls on it, resistance falls sharply (about 100 ohms). LDRs are popularly used in light sensor circuits. They are cheap and reliable and we can build a number of light sensing circuits around LDR
The circuit is shown in Schematic When there is no light falling on the LDR (R5), the transistor (Ql) is cut off as it is reverse biased by the variable resistor. Hence the reset pin
of the 555 is pulled low and the 555 stays reset. Resistance of LDR decreases as light falls on it and the base of the transistor becomes forward biased. Ql turns ON and pulls the reset pin 4 of the 555 high. This starts 555 as an oscillator and the sound produced at Pin 3 (OUTPUT) is further amplified by Q2, driving the speaker. Q2 is added here to increase the volume. You can still use this without amplification as in the earlier circuits. Variable resistor 100K (R3) is the sensitivity control. 10K resistor (R7) in series with protects the transistor Ql when the preset is set to zero resistance. Do not remove it.
You will find now that BC 157 transistor works as a switch and BEL 187 as an amplifier. BC 157 is a PNP transistor and BEL 187 is an NPN transistor.
If you have already built the earlier oscillator, construction of this should pose no problem. We have only added two transistors, a LDR and a few resistances. LDR should be conveniently placed near the outside light and away from room light or any other light to avoid false triggering. Use a Vero board. It is very important to follow the pin out of the transistors. As far as possible do not cut the leads short. Use sleeves on individual leads so that they do not short between each other.
The circuit can be modified to give alarm when the light goes out. Keep the transistor Ql and its connections intact along with LDR and R7.
LDR- Remove the end connected to ground and connect it now to power supply line.
R7- Remove end connected to power supply line and connect it now to ground.
Now by directing a light source straight into LDR, the alarm will be off as long as there is light falling on the LDR and anybody crosses and breaks the light beam, alarm will be on. This way the circuit can be modified as an intruder alarm or as a security alarm.
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