Shadow Alarm Circuit

This shadow alarm circuit can sense a moving shadow in a confined area. It can be used to protect things from theft. When somebody approaches the unit, it will give a loud alarm to abort the attempt of theft. The circuit uses the light sensing property of the Photo diode.

The circuit uses the light sensing property of the PIN Photodiode. The p-n junction of the photo diode gives light current when it is forward biased. IC1 is designed as a voltage comparator. Its non inverting input is connected to a potential divider R1 and VR. By adjusting VR, input current to pin3 can be set to a reference level. The inverting input of IC1 is connected to a photo diode. IC CA3130 is a 15 MHz BiMOS Operational amplifier with MOSFET inputs and bipolar output. The inputs contain MOSFET transistors to provide very high input impedance and very low input current as low as 10pA. It has high speed of performance and suitable for low input current applications.

CA3130A and CA3130 are op amps that combine the advantage of both CMOS and bipolar transistors. Gate-protected P-Channel MOSFET (PMOS) transistors are used in the input circuit to provide very-high-input impedance, very-low-input current and exceptional speed performance. The use of PMOS transistors in the input stage results in common-mode input-voltage capability down to 0.5V below the negative-supply terminal, an important attribute in single-supply applications.

A CMOS transistor-pair, capable of swinging the output voltage to within 10mV of either supply-voltage terminal (at very high values of load impedance), is employed as the output circuit.

The CA3130 Series circuits operate at supply voltages ranging from 5V to 16V, They can be phase compensated with a single external capacitor, and have terminals for adjustment of offset voltage for application requiring offset-null capability. Terminal provisions are also made to permit strobing of the output stage. The CA3130A offers superior input characteristics over those of the CA3130.

Normally in the light (as set by VR) Photodiode gives voltage to pin2 of IC1.Since this voltage is higher than the voltage set by VR at pin3 ,output of IC1 remains low keeping LED and buzzer off. When a person approaches the photodiode, the shadow causes a reduction in current through the photodiode so that voltage at pin2 decreases below that of pin3. Output of IC1 then goes high and Buzzer sounds.


The circuit triggers when the light intensity changes without a shadow. It is better to keep the unit in a place where constant light is available

Shadow Alarm Circuit diagram


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