This simple circuit senses heat from electronic devices and gives warning alarm. It is an ideal add on circuit to monitor heat in CD players, Amplifiers etc.
The circuit uses 14 stage ripple free binary counter IC CD 4060. It has an internal oscillator and its frequency is set by C1, VR2 and R2. Reset pin 12 of IC determines its working. When pin12 is high, IC remains off and turns on when pin 12 becomes low. Variable resistor along with the NTC Thermister forms a potential divider to set the voltage level at pin12. When the temperature is normal, resistance of Thermister will be high so that IC1 remains off. When the temperature exceeds a level set by VR1, IC1 starts oscillating and Red LED blinks. After few seconds buzzer sounds indicating that temperature inside the device is increasing. So that the device can be switched off immediately to prevent damage. Enclose the circuit inside the device with thermister close to the heat generating area. Power to the circuit can be obtained from the device.