Circuit Analysis

This circuit is a 12V, 5A regulated power supply, based on the 7812 voltage regulator and BD434/436/438 transistors to increase the available output current. Let’s analyze how it works:

Rectification and Filtering The transformer provides a 15V-0-15V AC voltage with a current capacity of up to 5A.
The bridge rectifier converts the alternating current (AC) into direct current (DC).
Capacitor C1 (4700 µF, 25V) acts as a filter, smoothing out the rectified voltage and reducing ripple.

Voltage Regulation (7812) The 7812 voltage regulator is an integrated circuit (IC) that maintains a constant 12V output, but it has a current limit, typically around 1 to 1.5A.
To support 5A output, the circuit uses power transistors in a Darlington configuration.

Current Boost with BD434/436/438 Transistors Transistors Q1 and Q2 (BD434/436/438) function as current amplifiers.
The output current from the 7812 triggers the base of Q1, which in turn triggers Q2.
The 0.1Ω emitter resistors help balance the current between the transistors.

Output Capacitor (C2 – 100µF, 16V) This capacitor filters the output voltage, improving stability and reducing noise.

Component List

Summary

This circuit converts 15V AC into regulated 12V DC, supporting currents up to 5A. The 7812 ensures voltage regulation, while the transistors boost the current capacity. The capacitors assist in filtering, providing a more stable output.

The invisible theft alarm system consists of a phototransistor and an infrared LED. When the infrared beam is unobstructed, the alarm remains silent. However, if the infrared beam is interrupted, the alarm is triggered, emitting a sound. The system has an effective range of approximately 1 meter, provided that the phototransistor and infrared LED are properly enclosed in black tubes and correctly aligned.

How It Works

When the infrared rays hit the L14F1 phototransistor, it keeps the PNP transistor (BC557) in a non-conductive state, resulting in silence from the alarm. However, when the beam is interrupted, the phototransistor is deactivated, allowing the PNP transistor to conduct and activate the alarm sound.

To ensure proper functionality, the phototransistor and infrared LED must be securely positioned to avoid false alarms. Additionally, adjusting the variable resistor is critical to correctly bias the PNP transistor.

Although other phototransistors can be used as substitutes for the L14F1, this particular model is preferred due to its higher sensitivity.