Posted on Feb 5, 2014

As shown, the circuit is powered by a 9-V battery, B1, but it can easily run off any 7- to 10-Vdc power supply. At the heart of the thermometer is IC1, an LM34 temperature sensor. That device produces a voltage between the Vout and GND terminals that is linearly proportional to temperature. Al-though the output is usually 10 mV/ °F, IC1 is connect

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ed in a resistor network (made up of R1 to R3) with a gain that provides an output of 40 mV/` °F Capacitor C1 is used as a noise bypass across R1 and R2. Because the voltage output by IC1 will be used by the rest of the circuit to "determine" what the temperature in the room is, potentiometer R1 will have to be calibrated exactly. The output of IC1 is fed to pin 5 of IC2, an LM3914 LED bar- or dot-graph driver, which is where the actual temperature-determining process occurs. IC2 has 10 internal comparators, the output pins of which are connected to LED1 to LED10. The voltage input to pin 5 is compared by IC2 to the voltages at pins 4 and 6; that process determines which LED or LEDs light. The LEDs can be set to light either one at a time (dot mode) or progressively (bar mode). When jumper JU1 is not installed, dot mode is enabled. When the jumper is installed, the chip is in bar mode. In dot mode, the LED that corresponds to the correct input voltage lights by itself. When the input voltage increases, an LED representing a higher temperature will light and the LED previously lit will extinguish. In bar mode, the LED representing the temperature will light and all of the lower LEDs will stay lit. Each mode has its advantages-the dot mode uses less current because only one LED is lit at a time, but the bar mode is easier to read at a glance. Resistor R8 and capacitor C2 provide decoupling for the LED-supply circuit. If...

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