Simple Light Activated Alarm Circuit

This Light Activated Alarm Circuit is like a super sensitive nightlight.

It uses a special light sensor LDR that acts like a tiny eye.

If it gets dark suddenly like someone might be sneaking around, the circuit triggers an alarm buzzer, light etc to let you know somethings up.

This circuit uses other parts transistors, resistors, capacitors to make everything work smoothly.

Circuit Working:

Parts List:

This simple musical light alarm circuit requires only 7 components including a light dependent resistor LDR and a 3.6V battery or 3 x 1.2V rechargeable batteries.

The well known UM66 is used as the sound generator, providing a pleasant wake up alarm.

As you may be aware, an LDRs resistance is normally very high up to 1M but it decreases significantly when exposed to light.

In this circuit, you can adjust the 220k preset to set the desired sensitivity which determines when the alarm begins to sound.

When light falls on the LDR, transistor T1 conducts powering the UM66 musical integrated circuit.

The musical note produced is amplified by transistor T2 and played through the 8Ω speaker.

The UM66 IC has different numbers, each corresponding to a different musical note in this example, UM66T is used.

While you can use 2 x 1.5V batteries, 3 x 1.2V NiCad or NiMH batteries are better because they are rechargeable.

Formulas:

The above light activated alarm circuit ,to detect variations in light levels, it makes use of an LDR (Light Dependent Resistor).

A transistor is triggered by a drop in light detected by the LDR, and this transistor then turns on a buzzer or speaker.

The formula for the circuit is:

V_out = V_in * (R2 / (R1 + R2))

where,

• V_out is the output voltage of the circuit
• V_in is the input voltage 3.6V.
• R1 is the resistance of the LDR
• R2 is the resistance of the 1k resistor

The voltage across the 1k resistor, which is used to activate the transistor, is calculated using this formula.

The likelihood that the transistor will be activated decreases with increasing LDR resistance since it lowers the voltage across the 1k resistor.

This formula represents a condensed version of the real circuit, which also has a speaker and a transistor in it.

The speaker produces an audible sound by amplifying the signal from the 1k resistor via the transistor.

How to Build:

To build a Simple Light Activated Alarm Circuit follow the below mentioned steps:

• Connect the positive terminal of the battery to the positive power supply of the PCB and the negative power supply to the ground.
• Connect pin 1 of the UM66 IC to the positive supply through the emitter of BC547 transistor.
• Connect pin 2 ground of the UM66 IC to the negative supply.
• Connect one terminal of the speaker to the collector of transistor T2 2N2222 and the other terminal to the positive supply.
• Connect the emitter of transistor T2 to the negative supply.
• Connect pin 3 of the UM66 IC to the base of transistor T2 through a 470Ω resistor.
• Connect the emitter of transistor T1 to the negative supply through a 220k resistor.
• Connect the junction of the LDR and the 220k resistor to the base of BC547 T1
• Adjust the 220k preset to set the desired sensitivity.

Testing:

• Illuminate the LDR with a light source, and the alarm should start producing sound through the speaker.
• Adjust the sensitivity using the preset resistor to control when the alarm triggers.

Note:

• Ensure proper polarity of the components and use caution when working with electrical circuits.

Conclusion:

The Light Activated Alarm Circuit is a simple yet effective way to trigger an alarm or activate a device based on changes in light levels.

By utilizing a Light Dependent Resistor LDR as the light sensor, the circuit can detect variations in light intensity and respond accordingly.

This circuit can be used in various applications such as security systems, automatic lighting systems and more making it a versatile and valuable electronic component.

References:

Datasheet IC UM66T