Temperature Range Monitoring System

The temperature tolerance monitoring system introduced in this article can monitor whether the operating temperature of equipment and systems exceeds the allowable temperature tolerance at any time. Once the maximum allowable temperature limit is exceeded, an audible and visual alarm signal will be automatically issued.

The monitoring system uses 10 LEDs. Each LED represents a temperature step (1°C), and the 10 LEDs form a temperature display bar. The last LED (LED10) in the display bar is used to indicate that the temperature has reached the maximum allowable temperature. In addition, the circuit also uses two buzzers: one is used to sound an alarm when the temperature reaches 50% of the temperature tolerance; the other is used to sound an alarm signal when LED10 lights up.

The chip LM35 (IC1) in the circuit is used as a temperature detector. It is directly calibrated with Celsius. Its output is proportional to the temperature. The monitoring sensitivity is 10mV/℃. The applicable temperature range is -55℃～+150℃. The chip LM3914 (IC2) is a single-chip integrated circuit. It is used to detect the analog voltage level to drive the LED temperature display bar as a linear analog display indication. As shown in the figure, the output of the temperature detector Ic1 is connected to the ⑤ pin of the LED driver chip Ic2. There are 10 identical comparators designed inside IC2, and the output of each comparator is connected to LED1～LED10.

NE555 (IC3) is connected in monostable trigger mode. When the temperature of the device exceeds the maximum allowable tolerance, LED10 lights up, PZ2 sounds an alarm, and pin ② of IC3 goes low, and IC3 enters the trigger state. As a result, pin ③ of IC3 goes high, which makes the second NE555 (IC4) oscillator work, and then generates an audio signal at its output pin ③. This signal is amplified by transistor T1 and drives speaker LS1 to sound an alarm. The frequency of the alarm signal can be adjusted by R7, R8 or C5.

Assuming you plan to use it in an instrument with a temperature tolerance of 10°C and a normal operating temperature of 55°C, first place the system near the instrument under test. Then adjust VR1 and VR2 to make LED1 light up at 55°C. Then increase the ambient temperature of the temperature detector IC1 to make the temperature rise. At this time, the corresponding LEDs should light up one after another. When LED6 lights up and PZ1 sounds an alarm, it means that the temperature rise of the instrument has reached 50% of the temperature tolerance.

At this time, the temperature is further increased to make LED10 light up. PZ2 issues an alarm, IC3 and IC4 start working one after another, and LS1 issues an audio alarm, indicating that the temperature tolerance has been reached, indicating that the instrument should be shut down in time.

Key parts in this article: LM35, LM3914, NE555, BC548