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Simple Shadow Detector Security Alarm Circuit

This Simple Shadow Detector Security Circuit is like one guard with very sharp eyes.

It uses one light sensor like photodiode to look shadow in one place.

If something stop the light and make shadow then the circuit think someone is there and start an alarm.

Alarm can be loud sound, blinking light or something to indicate one.

This type of circuit is good for security because it sees movement without big camera or costly sensor.

So if anyone want to watch their place, this circuit will help them by turning shadow into warning!

Circuit Working:

Parts List:

Component TypeDescriptionQuantity
Resistors100k1
100Ω1
Preset 10k1
SemiconductorsIC CA31301
Buzzer1
LED Red 5mm 20mA1
Photodiode1

This shadow alarm circuit sees movement in small area using light sensor called PIN photodiode.

It can help stop thief by making loud sound when someone come near.

The circuit works because photodiode make small current when light fall on it and it is forward biased.

IC1 is voltage checker as a comparator its pin 3 at non-inverting input connects to resistor R1 and VR (a variable resistor) to make fixed voltage.

By turning VR we can set how much voltage goes to pin 3.

Photodiode is connected to pin 2 at inverting input of IC1.

This IC called CA 3130 is fast op-amp with special MOSFET inside so it works good even with very small current.

It has high input resistance and very small input current like 10 picoamps so it is very fast and looks good for low current jobs.

CA 3130A and CA 3130 are mix CMOS and bipolar tech.

Its input part have special MOSFETs that give very high resistance, low current and fast speed.

Also it can work with input voltage that goes almost to negative supply which is good for single battery use

Its output part goes very close to both power lines but there should be only 10mV gap so one can get almost full swing voltage out.

This IC works with 5V to 16V power which needs only one capacitor for phase setup and can adjust offset if needed.

Now under normal light as set by VR, photodiode send voltage to pin 2 of IC1.

This voltage is more than voltage at pin 3 which is set by VR so output of IC1 is low and buzzer and LED stay OFF.

But when someone comes close and make shadow on photodiode, light drop, current drop and voltage at pin 2 goes down.

Now voltage at pin 2 is less than pin 3 so output of IC1 goes high and buzzer turn ON to give alarm.

Formula:

In this shadow alarm circuit we have used one CA 3130 op-amp and one photodiode to find change in light.

These parts help circuit know if light goes down like when shadow comes, then it makes sound for alarm.

Exact math for this circuit depend on how we want alarm to work.

But some general formulas can help understand how circuit works.

Voltage divider formula (for resistors):

This formula help find reference voltage for op-amp.

It tells how much voltage come out from middle point between two resistors:

Vout = Vin × (R2 / (R1 + R2))

where,

This formula help decide voltage going to one pin of op-amp.

Comparator formula for op-amp:

In this circuit op-amp work like comparator.

It look at two voltages one from photodiode and one from VR (reference voltage).

If voltage from photodiode at pin 2 is less than voltage at pin 3 then output goes high with an alarm ON.

If voltage at pin 2 is more than pin 3 then output stay low with an alarm OFF.

Not exact big formula here but comparator only check which voltage is higher.

Output goes ON or OFF depending on that.

So this simple math help anyone know what circuit decide when to turn alarm ON or OFF using light and shadow.

How to Build:

To build a Simple Shadow Detector Security Alarm Circuit follow the below steps:

Power Connection:

Set Reference Voltage:

Photodiode Connection:

Output Connection:

Testing:

Adjustment:

Note:

Conclusion:

This simple shadow detector alarm circuit is very useful in security.

It uses light sensor to see shadow and give and alarm alert when something moves or someone comes.

This help warn people if there is intruder or any wrong movement.

References:

Datasheet IC CA3130

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