Thermometer Circuits

A thermostat doesn't try to maintain a constant temperature. In order to do so - it would have to keep switching on and off every few seconds. Instead - it keeps the temperature within a specific range. When the preset temperature has been reached - it switches off. And it only switches on again - when there has been a significant change in temperature.

The circuit presented here main- tains the temperature of the load (e.g., electric irons, heaters, etc) between two preset levels, i.e., a higher and a lower level which may be manually adjusted with the help of potentiometers VR1 and VR2 respectively. The load supply has been connected via a relay such that the load is ‘on’ only when the relay is energised. The circuit makes use of two negative temperature coefficient (NTC) thermistors having a resistance of approximately 3 kilo-ohms at room temperature. The thermistors should be placed in the vicinity of the load for sensing its temperature.

The circuit measures the car water temperature. CA3161 is a counter and 7segment driver to display amount of temperature on 7segments. The temperature sensor is a diode, 1N4148. This is set near of the Car Radiator. To obtain the 5 V power supply from Car Battery, you can use a LM7805, +5V low cost voltage regulator.

You can build a variety of simple machines with the same hardware and a different script : a charting thermometer, a vending machine that dials your number when empty, a leavening cell... thermometer with 1°F (0.5C) resolution four switch inputs, four relay outputs only an handful of cheap parts graphic LCD display

An economical and commercially available quartz temperature sensor, Y1 and IC1, an LTC-485 RS485 transceiver in transmitter mode, form a Pierce crystal oscillator. The sensor, an Epson HTS-206, presents a nominal frequency of 40 kHz at 25°C and a temperature coefficient of –29.6/ppm/°C (Reference 3). The transceiver's differential-line-driver outputs deliver a frequency-coded temperature signal over a twisted-pair cable at distances as far as 1000 ft.

The AD590 provides an output current proportional to absolute temperature (1 µA/K). In this application, the circuit offsets and scales the output to provide a full-scale range of 0 to 5V with a scale factor of 50 mV/°C over the chosen temperature range of 0°C—the freezing point of water—to 100°C, the boiling point of water. The AD8541 is a low-cost, low-power, rail-to-rail operational amplifier. It has a high common-mode voltage range and extremely low bias currents. You can calibrate out its 1-mV typical offset, the resistor, and AD590 errors. The output swing of the amplifier is 25 mV to 4.965V with a single 5V power supply, limiting the output by about 0.5°C on either end.

This project is easy enough for beginners, the only difficulties possibly arising from serial port hardware incompatibility from PC to PC. In the single-sensor version, you need only the sensor IC, a voltage regulator and and handful of diodes and resistors. Build it, and learn the secrets of IIC bus, how to implement IIC bus using only two resistors and a couple of zeners, how to drive it on a serial port using Visual Basic

The ICL7136 is the IC of the CMOS that the function to measure the voltage of the input correctly, the function to control the LCD(Liquid Crystal Display) and so on are housed in 1 chip. It is possible to measure the voltage to ±200 mV or ±2 V. The ICL7136 is the type which uses the LCD as the display. There is the ICL7137 in the type which uses the LED for the display. The control signal of the LCD is the square wave of the frequency is about 60 Hz(48KHz/800) and the voltage is 5.5Vp-p.

This circuit was built to stabilize a radio frequency VFO (Variable Frequency Oscillator) for ham radio applications. The circuit has also been used to lower the drift of a Ramsey FM10a micropower FM transmitter.

The circuit shown will provide a temperature sensitive output with both zero and scale factor independently selectable. Since the temperature transducer requires about 1.0 mA for normal operation, the thermometer is pulsed at a low duty cycle to reduce power consumption. A continuous output is obtained between pulses by a sample and hold. Since temperature does not usually change rapidly, the pulsed operation of the thermometer does not detract from its usefulness.