temperature sensor circuit which is built around two 2N4401 transistors which are working as switches and a 10K thermistor which is a heat or temperature sensor. The 200K variable resistor used in this stage is to adjust the level of heat on which you want to activate the circuit. Using low ohms thermistor than 10K can increase the sensitivity of the circuit.
The second stage is a UM66 IC. UM66 is a widely used CMOS melody integrated chip manufactured in TO-92 transistor package and available in many different melodies. Each melody has a different code that is written on the IC, for example UM66T02 generates the melody of "Jingle bells". You will find more details of codes in the datasheet of UM66 IC.
The third stage is an FM transmitter circuit that will broadcast the melody signal of the UM66 IC to an FM receiver at a distance of 50 meters. The 10K variable resistor in this stage is used to control the volume of the broadcasting melody. The oscillator stage of the circuit is built with the 2N3904 transistor. L1 is equal to 2 turns of # 24 enameled wire wound on 5mm form. TR is equal to 1 to 30 PF trimmer capacitor. For antenna use a 12 inch piece of wire or for maximum range use 30 inch wire and keep it vertical.
After building the circuit first make the circuit ready to perform by adjusting the 200K variable resistor on the place where it will activate the circuit on the room temperature. Once the circuit is activated and broadcasting the melody then switch ON your FM receiver and set it on a blank spot after 100 MHZ and place it 5 to 10 meters away from the transmitter circuit. Now slightly adjust the trimmer of the transmitter until you hear any melody signal on your FM radio. Once you have detected the signal on your FM radio the circuit will ready to perform. Now adjust the 200K variable resistor again to make the circuit activate on your desired level of heat.
A wireless temperature sensor circuit is a very useful device to detect the temperature of any remote place. The wireless temperature alarm circuit shown here is an ideal circuit to use to detect a level of temperature or heat from distant place.