This design note explains how to design an invisible alarm system easily. Infrared Transmitter ( Tx) and Receiver ( Rx) LED pair is easily available which can be used for making this alarm system. Infrared transmitter LED emits infrared rays in the wavelength around 950 nm. This LED is a high current device that can handle more than 100 mA current. The IR receiver LED is a kind of Photodiode that generates a Photo current when it receives IR rays. So the IR LED pair is ideal to make an alarm system that operates both day and night.
The alarm system has two sections- Transmitter and Receiver sections.
The IR LED is directly connected to the 9 volt battery through the current limiting resistor R1 (68 Ohms 1W). So the current through the IR LED is 132 Milli Ampere which gives sufficient range for the transmitter. Capacitor C1 acts as a buffer to give steady voltage to IR LED. The IR LED gives continuous IR rays for the receiver.Fig.1 shows the transmitter circuit.
Fig.2 shows the design of Receiver section. The receiver uses the IR receiver (Rx) LED to receive the IR rays from the transmitter. When it receives the IR rays, Photo current develops in it. Since the photo current from IR receiver LED is too low an amplifier using IC1 is used. CA3140 (IC1) is a high gain Op-Amp with JFET inputs that requires very low input current. Resistor R1 (10M) acts as the Feedback resistor. When the IR rays from the transmitter falls on the IR Receiver, it generates photo current and the output of IC1 turns high. This high output from IC1, saturates T1 and it conducts. At the same time, T2 and T3 will be out of conduction, since their base will be at ground potential. This keeps both Red LED and the buzzer off.Capacitor C1 gives a short delay to switch off T3.
When the path of IR rays breaks by a passing person, the output of IC1 turns low and T1 turns off. This allows T2 and T3 to conduct and buzzer beeps for a short period.
Fix Transmitter and Receiver LEDs face to face at the opposite sides of the door. When both are aligned exactly, buzzer and Red LED turn off. When the IR path breaks, alarm sounds.
Fig. 3 shows the design of the alarm system that gives continuous alarm once triggered. Here an SCR is used to drive the buzzer. SCR latches once triggered and remain conducting even if the gate current is removed. It can be switched off by removing the Anode current. For this, Push-to-Off switch S1 is used. By pressing this switch, alarm can be stopped.
Fig.4 shows the alarm system with SCR switch and Relay. The relay can be used to drive Bulbs, Electric alarm etc. Once the alarm is triggered, SCR latches and relay remains on. Relay can be switched off through S1.