Pyroelectric Fire Alarm System

This is definitely an ultra-sensitive fire sensor that exploits the direct piezoelectric property of an ordinary piezo component to recognize the fire. The lead zirconate titanate crystals within the piezo component have the property to deform and produce an electrical potential when heated, thus converting the piezo component into a heat sensor. The circuit mentioned right here is really sensitive. It provides a warning alarm in the event the area temperature raises over 10°C. The whole circuit has two modules: the sensor module and also the power supply module.

The Sensor circuit
The front end of the circuit contains a sensitive signal amplifier constructed close to IC1 (CA3130). It delivers a high output when temperature near the piezo component raises. IC CA3130 is really a CMOS operational amplifier with gate protected p-channel MOSFETs inside the inputs. It has high speed of operation and low input current specifications. There are certainly two inputs-the non-inverting input (pin 3) connected to the piezo component via diode D7 (OA71) that carries the voltage signal from the piezo component and the inverting input (pin 2) that gets a preset voltage through VR1.

By adjusting VR1, it is very easy to set the reference voltage level at pin 2. In normal condition, IC1 provides a low output and the remaining circuitry is within a standby state. Capacitor C2 keeps the non-inverting input of IC1 stable, so that even a slight change in voltage level in the inputs can change the output to high.

Typically, IC1 provides a low output, trying to keep transistor T1 non-conducting. Reseting pin 12 of IC2 (CD4060) connected to the collector of transistor T1 gets a high voltage via R5 and IC2 stays disabled. Once the piezo component gets heat from fire, asymmetry in its crystals causes a potential change, enabling capacitor C2 to discharge. It momentarily modifies the voltage level at pin 3 of IC1 and its output swings high. Transistor T1 conducts taking the reset pin 12 of IC2 to ground. IC2 is now enabled and gets going oscillating. With the shown values of the oscillating parts C3 (0.22µ) and R6 (1M), the first output (Q3) turns high right after several seconds along with a red LED2 starts flashing. If heat near the piezo persists, Q7 (pin 14) output of IC2 becomes high after one minute, and also the alarm will start beeping. If heat continues, Q9 (pin 15) turns high after 4 minutes and turns on the relay driver transistor T2. Simultaneously, diode D8 conducts and IC2 stops oscillating and toggles.

The solenoid pump connected to the N/O (normally opened) contact of the relay starts spraying the fire-ceasing foam or water to the probable sites of fire.

The power supply circuit

Power supply comprises a 0-12V, 1A step-down transformer having a standard full-wave rectifier formed by D1 through D4 and filter capacitor C1. A battery backup is supplied when the mains supply is cut-off because of short-circuit and fire. A 12V, 4.5Ah rechargeable battery is utilized for backup to provide enough current for the solenoid pump. When mains electrical power is available, diode D5 forward biases. It provides power towards the circuit and also charges the battery via resistor R2, and it limits the charging current to 120 mA. When power fails, diode D5 reverse biases and diode D6 forward biases, providing immediate backup for the circuit. LED1 signifies the availability of mains electrical power.

Assemble the circuit on a general purpose PCB and enclose it inside a appropriate case. Connect the piezo component to the circuit by using a thin insulated wire. Glue the flat side of the piezo component on a 30×30cm aluminium sheet to improve its sensitivity. Work on the sheet with the piezo sensor to the site in which protection is necessary. The remaining circuit could be fixed at a proper spot. If only the alarm generator is required, omit the relay driver section.

Download the circuit and explanation in PDF file: Pyroelectric Fire Alarm System