Digital Temperature Meter Chip HT7500 and Its Application

The pin description of HT7500 is listed in Table 2.

3 Electrical characteristics and features

(1) Electrical characteristics

The maximum rated parameters of HT7500 are:

Power supply voltage 0V～2V

Input voltage VSS-0?5V~VDD-1?5V

Working temperature: -20℃～+75℃

Storage temperature: -55℃～+125℃

The main parameters of HT7500 are listed in Table 3.

Figure 2 HT7500 pin configuration

　　The main features of HT7500 are as follows:

1) Single-chip CMOS structure, using a 1.5V battery to work; 2) The measurement range is +32℃～+42℃（+90～+108）, the measurement accuracy is ±0.1℃（±0.2）, and the resolution is 0.1℃（0.1）;

　　3) Celsius and Fahrenheit temperature measurement can be selected using the adapter;

　　4) Alarm in case of fever;

　　5) Maintain the highest temperature and automatically shut down after 8 minutes and 40 seconds;

　　6) On/off control with a push button switch;

　　7) Display the last measured temperature.

4Operation procedures and functions

　　The operating procedures and functions of HT7500 are as follows:

　　(1) When the switch is pressed to turn on the power, a buzzer sound will be generated (0.125s);

　　(2) Display all program segments (2s);

　　(3) Display the last measured temperature (2s); (4) Display L℃ or (0.75s); (5) Display the measured temperature, and the ℃ or mark flashes at a rate of 1Hz; (6) If the temperature is <32℃ (or 90), display L℃ (or); (7) If the temperature is ≥42℃ (or 108), display H℃ (or); (8) During temperature measurement, always display the higher temperature; (9) If the measured temperature does not change for more than 16s, the ℃ (or) mark flashes and stops; (10) If the temperature is higher than 37.5℃ (or 99.5) during the measurement process, the buzzer alarm sounds a "beep-beep..." sound for 4s;

Figure 3 Operation procedure

Figure 4 LCD electrode model

　　0.125s0.125s0.125s0.125s0.125s0.125s0.375s If the temperature is ≤37.5℃ (or 99.5), the buzzer will sound an alarm, making a “beep-beep…” sound for 4 seconds;

　　B1……

　　0.5s0.5s

　　(11) When the measurement reaches 8 minutes and 40 seconds, it will automatically shut down; (12) When the measurement is completed, if the temperature rises within 8 minutes and 40 seconds, the ℃ (or) mark will flash again (repeat twice to stop) and count again for 8 minutes and 40 seconds;

　　(13) When the beeping sound reaches 4 seconds, stop measuring;

　　(14) When the battery voltage is low, the battery mark "V*" flashes at a rate of 1Hz. In this case, the test result is inaccurate. The low voltage detection condition is: 1.35V±0.05V.

　　The HT7500 operation procedure is shown in Figure 3.

Figure 5 HT7500 typical application

Table 4 Relationship between HT7500 related drive pins and LCD program segments

5 Typical Applications

HT7500 only needs a few external components to form a high-performance digital thermometer. Figure 5 shows the typical application circuit of HT7500. In Figure 5, the external sensor on the RS pin of HT7500 uses a 503ET thermistor; the external capacitors on the VEE pin, CAP pin, and C512 pin (both 0.022μF) are used to stabilize VEE (-1?5V); the external buzzers on the BZ1 and BZ2 pins are used to generate alarm sounds; the frequency is 5?3kHz; the external adjustable resistor (400kΩ) on the LOWC pin is used to adjust the level of the low voltage detector, and the pin is open when not in use; the external resistor ROSC = 820kΩ on the OSCI and OSCO pins is used to form an RC oscillator with the IC built-in capacitor to generate a 32kHz system clock; the external resistors and capacitors on the RS, RF, and SC pins form an alternating RC oscillator, and only one oscillator (RS or RF) is allowed to be activated within a certain period of time. The external resistor REF on the RF pin is the reference resistor, and its value is equal to the resistance value of the 503ET thermistor at 37.5℃. In the actual production process, in order to adjust the reference resistor REF, the pin TEST2 should be suspended, and the measured temperature will be the accurate value of the ambient temperature. The selection switch is connected to the IC pin 9. When the pin 9 is suspended, ℃ is displayed; when the pin 9 is connected to VDD, it is displayed.