Here is an Ultrasensitive Heat Sensor to monitor high temperature in Electronic devices. It can be placed inside the electronic gadget that usually generates heat during its operation. The circuit also functions as a sensitive Fire Alarm. This heat sensor uses the piezoelectric property to sense heat.
The sensor element is the ordinary piezo disc found in buzzer. The middle part of the piezo disc is coated with a layer of piezo electric material called Lead Zirconate. The crystals in this material are capable of dis-orientation and re-orientation when it is subjected to mechanical, electrical and heat stress. The Direct Piezo electric property of the piezoelectric crystals is the ability to generate electric signals when the crystals dis-orient and re-orient following a stress. This property is used here to sense temperature.
A high gain type OP-Amp is used to sense the electrical signals from the piezo disc. The inverting (pin2) and non – inverting (pin3) inputs of the Op-Amp IC CA3140 are shorted through the capacitor C1, so that both the inputs will be in a balanced state. When this happens, output of IC1 remains low. When the piezo disc is subjected to heat stress, it generates a very small current that is enough for upsetting the balance of the inputs of IC1. This changes the output of IC1 from low to high. This high output is used to activate the alarm circuit based on the ROM IC UM3561.
UM3561 is a ROM IC used to generate alarm tones like Fire engine, Ambulance, Machine gun, Police sirens etc. These sounds depend on its pin connection (pin6). Here it is used as a Fire Brigade Siren by leaving its pin6 unconnected. UM3561 is a low power IC requires 3 volts. So a Zener diode is used to reduce the 9 volt supply to 3 volts. Resistor R4 (220K) is important to maintain the oscillating frequency of the ROM IC. Since the output from IC2 is too weak, transistor T1 is used to amplify the sound for the speaker. Capacitors C2 and C3 are included as buffers so that, they will keep the voltage level for IC2 for some time even if the output of IC1 turns low.
Glue the fine side of the Piezo disc on the cabinet of the equipment close to the Heat sink or similar heat generating devices. No adjustment is necessary. Power (9-12 volt DC) can be obtained from the equipment itself.