B
BlackMamba
Guest
Source: Precyzyjny termometr ( lekarski )
Project by robball
Presented thermometer is used for human body temperature measurements with the accuracy and resolution not lesser than 0.1 ‘C. Its operating range is much wider, from 0’C up to 100’C.
As a sensor LM35 chip was used. Output voltage is connected to AC converter ICL7135, which usually controls 4,5-digit LED display. In its application note is shown possibility of intercepting conversion results with only two data lines. Such interception, display control and power switchin is realized by 89C2051 microcontroller.
Control program was written in Bascom.
Power switching function of the devise is worth mentioning. Device is switched by one pushbutton. Pushbutton switches key controlling power, starts the microcontroller, which in turn switches the same key. So device is working when the pushbutton is released. When measurement is finished microcontroller switches off the key disconnecting power from the device. Thanks to such solution and emphasizing the power usage there is possible to power whole device from single 6F22 battery.. When working device consumes approximately 8mA, most of that current is consumed by microcontroller. There is a possibility to decrease power consumption by using controller sleep modes.
Calibration was made on the special laboratory stand equipped with heated double water tank and mercury thermometer with resolution 0.1 degree.
Please have a look at three transistors at the upper left schematics corner. They form switch circuit. T1 is a key. Pressing PSW causes switching the T1 key on and starting the microcontroller. Microcontroller sets “1” on the PSON output starting tat way two other transistor, and as a result T1 transistor is conducting independent on the PSW button state. When the measurement program ends up, the microcontroller sets PSON output low, T3 stops conducting and switch is off. Additionally T2 transistor stops conducting and prevents re-switching on due to accidental pulse from microcontroller.
Transducer is rather a good choice, reference voltage 1.2V allows internal resolution 0.012 deg, and ease of device calibration. ICL7135 is more friendly in non-standard applications than ICL7107. LM35 is very respectable sensor. In the range 32-45’C (calibration should be made in this range) transducer data conversion to result is based on linear function.