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QualityKits Part #: QK120
QK120: Temperature Meter Kit
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Temperature Meter Kit without meter
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Price:  $26.95
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This kit uses a 3 ½ digit Digital Panel Meter (DPM) to display the temperature reading from two separate sensors. Part # PMLCD
DPM sold separately as PMLCD.
A switch is used to select between the two sensors.
The temperature sensors can be connected up to 30 feet (10 meters) from the display using a 3-wire cable. A 2-core screened cable, as used in audio applications, will do nicely. The user must supply this cable.
Temperature is measured and displayed in degrees Fahrenheit with the LM34 sensor; degrees centigrade with the LM35 sensor. You can get the data sheets from the National Semiconductor website at national.com. The kit is housed in a plastic box with a silk-screened front panel. Overall dimensions are 5.1 x 2.7 x 1.6 (13 x 6.7 x 4 cm). The kit can be powered from either a 9V battery or a 12V DC plug pack.
HOW IT WORKS
This discussion also applies to both the LM34 and LM35. The kit is based on a precision temperature sensor from National Semiconductor. Its output voltage is linearly proportional to the Fahrenheit or centigrade
temperature at a factor of +10.0mV/ºF or +10mV/ oC. This means that it can directly drive a digital voltmeter. And it means that one sensor on the kit can be LM34 and the other LM35.
The LM3x does not require any external calibration to provide typical accuracies of ± ½ °F at room temperature and ± 1½ °F over a full 20 to +199.9° temperature range.
It requires only three connections:
1. DC Voltage (5 to 20 volts)
2. Output
3. Ground
The display used is a 3 ½ digit Digital Panel Meter. It is ready to go with just a couple of changes to be made. These are detailed in the Assembly Instructions.
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QK120 is sold without Digital Panel Meter
DPM can be ordered separately as PMLCD
The circuit is quite straightforward. The temperature sensor output is connected to the DPM via switch SW2. This switch selects the temperature reading to display. The DPM is set to a full scale reading of 1.999V with resistors RA and RB. The sensors and the DPM can be powered from either a 9V battery or an external 12 volt DC supply. A power switch allows the unit to be turned off when not in use. External power is applied to IC1, an LM317L adjustable regulator. Diode D1 provides reverse polarity protection. The regulator output is set by two resistors, R1 and R2. The regulator maintains a constant 1.25 volts between the adjust terminal (pin 2) and the output terminal (pin 3). This causes a constant current to flow through R1. This current also flows through R2, producing a constant voltage drop across it. Some current from the adjust terminal also flows through R2 but this is negligible and can be ignored. Thus, the output voltage is given by
V OUT =1.25(1+R2/R2)=9.77V
Diode D2 prevents any current flow into the battery and allows the battery to remain connected even when external power is used. Similarly, diode D3 prevents any current flow back into the regulator circuit when only the battery is used. As mentioned before, you can also use this kit to measure temperature in degrees Celsius. In this case substitute LM35 sensors in place of the LM34s.
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Price: $26.95
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