PC overheat alarm

When the temperature inside the computer exceeds the preset threshold, the speaker will sound an over-temperature alarm. The temperature threshold can be adjusted within the range of 0℃ to 100℃.

The computer overheat alarm unit can be made in the form of a small PC expansion card. Find a free one in the PC host. Plug it into the slot. The alarm unit consumes only 12mA and can be powered by the power supply in the PC.

The sensor LM35 (IC1) provides on-chip signal conditioning functions, including amplification, level shifting, and phase inversion. After signal conditioning, the output increases by 10mv for every 1°C rise in temperature. Therefore, for a temperature range of 0°C to 100°C, the corresponding output voltage range is 0V to 1V. The operational amplifier CA3130 (IC2) is the voltage detection stage, which compares the output voltage of the temperature sensor with the reference voltage preset by the comparator. If the output voltage of the temperature sensor exceeds the reference voltage, the output of the comparator becomes high, causing the low-frequency oscillator IC3 behind to work, which in turn activates the audio oscillator to drive the speaker LS1 to emit a "beep...beep" alarm sound. The size of the reference voltage determines the temperature that triggers the alarm.

The attached picture shows the alarm circuit. LM35 is a temperature sensor. IC2 is an operational amplifier used as a comparator. VR1 provides a reference voltage that can be set arbitrarily within 0-1V. The reference voltage is added to the inverting input of IC2, and the output of LM35 is coupled to the non-inverting input.

The low frequency oscillator IC3 is a standard 555 multivibrator with a frequency of 1Hz, and IC4 is another 555 multivibrator with a frequency of 2.5kHz. The output waveform is a burst pulse modulated by 1Hz, which is used to drive the speaker to sound.

Before setting VR1 to the appropriate temperature threshold, it is important to determine what temperature is most appropriate.

If it is assumed that the normal value of room temperature does not exceed 25℃, the temperature inside the computer may be as high as 35℃. In this case, the center tap voltage of VR1 should be set at 350mV.