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1 0 0 • J A N U A R Y 2 0 0 6 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M
CMYK
D. MOHAN KUMAR
SHADOW ALARM SANI THEO
This opto-sensitive circuit soundsan alarm whenever a shadowfalls on it. So it can be used at
night by shopkeepers to protect thevaluables in their showrooms. A dimlighting in the room is necessary todetect the moving shadow. Unlikeopto-interruption alarms based onlight-dependent resistors (LDRs), itdoes not require an aligned light beamto illuminate the photo-sensor.
The circuit is powered by a 9V PP3battery and uses the most sensitivephoto-sensor L14F1 to detect shadows.It is portable and can be used at anyplace that is to be monitored.
Op-amp µA741 (IC1) is used as avoltage comparator. Its inverting in-put is biased by the voltage obtainedfrom the junction of 100k resistor R1and the collector of phototransistor T1.
The non-inverting input of IC1 gets acontrolled voltage from potential di-vider R2 and VR1.
In the presence of ambient light,the phototransistor conducts and theinverting input (pin 2) of IC1 gets alower voltage than its non-invertinginput (pin 3). This makes the outputof IC1 high, which is indicated by theglowing of LED1.
When a shadow falls on the photo-sensor, the output of IC1 goes low.This low pulse triggers the monostable
(IC2) designed for a delay of 51 sec-onds using R6 and C3. The output ofIC2 is used to light up LED2 and acti-vate the alarm.
Slide switch S2 is used to select ei-ther the buzzer or siren. When it istowards left the buzzer beeps, andwhen it is towards right IC UM3561(IC3) activates to give a loud alarmsimulating a police siren. Resistor R8
and zener diode ZD1 provide 3.1V DCto IC UM3561.
The circuit is easy to assemble asit requires only a few low-cost com-ponents. Enclose it in a cabinet withthe photo-sensor inside. Drill a 5mmhole on the front panel of the cabinetto let ambient light fall on the photo-sensor.
Adjust potmeter VR1 (47k) untilLED2 stops glowing and the buzzerstops beeping while LED1 glows. Thisis the position of VR1 to be main-
tained for that particularintensity of light. LED1 will continueto glow even when a shadow isdetected.
The circuit is now ready to use.To test it, move a paper in front ofthe unit. If LED2 glows along withthe beep of the buzzer, it means thatthe photo-sensor has detected ashadow.
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