A musical temperature alarm is an electronic circuit that detects when ambient temperature exceeds a preset limit and triggers a melody instead of a buzzer. This project uses an LM335 temperature sensor, TL071 op-amp comparator, and UM66 melody IC to provide an audible alert when overheating occurs.
Monitoring temperature is essential across many applications, from protecting sensitive electronic equipment to maintaining comfortable room conditions. Conventional temperature alarms typically use buzzers or LEDs, providing only visual, monophonic alerts. Here we present a musical temperature alarm that generates a pleasant melody, rather than a simple beep, when a preset temperature threshold is exceeded.
The circuit employs an LM335 temperature sensor for analogue sensing, a TL071 operational amplifier for signal comparison, and a UM66 melody IC for tone generation. When the ambient temperature crosses the set limit, the circuit activates a small loudspeaker to produce a musical alert. The design remains simple, compact, and cost-effective, making it suitable for DIY enthusiasts and electronics learners.
The system is suitable for use in rooms, incubators, and electronic enclosures where temperature monitoring is critical. Powered by a 9V battery, it consumes low current, ensuring portable and reliable continuous operation. Fig. 1 shows the author’s prototype.
| Parts List |
| Semiconductors: IC1 – LM335Z temperature sensor IC2 – TL071 op-amp IC3 – UM66 melody generator IC T1 – BC558 PNP transistor T2 – BC548 NPN transistor ZD1 – 3.1V zener diode LED1 – 5mm LED Resistors (all 1/4-watt, ±5% carbon): R1, R5, R6 – 1-kilo-ohm R2 – 4.7-kilo-ohm R3 – 10-kilo-ohm R4 – 2.2-kilo-ohm R7 – 330-ohm VR1, VR2 – 10-kilo-ohm potmeter Capacitors: C1 – 220µF, 25V electrolytic Miscellaneous: Batt.1 – 9V battery LS1 – 8-ohm, 0.5-watt speaker |
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Musical Temperature Alarm Circuit and Working
The LM335 is a precision temperature sensor that behaves like a temperature-dependent Zener diode. It generates an output voltage directly proportional to the absolute temperature (in Kelvin), with a scale factor of 10mV per Kelvin. As the ambient temperature increases, the output voltage increases linearly; it decreases as the temperature falls. For example, the LM335 produces approximately 2.98V at 25°C (298K) and about 3.03V at 30°C (303K). This linear voltage-temperature relationship makes the LM335 suitable for use with simple analogue processing circuits.
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