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4-20ma Current
Loop Tester: Using just a few components, this circuit can simulate
signals from various instruments, which source a constant current.
Low Value Capacitance Meter
This circuit was originally designed to measure the volume of fluid inside a
medical syringe. As designed, it produces a zero to 5 volt output,
corresponding to a capacitance change of about 10 picofarads. With a digital
voltmeter, at its output, it can resolve a capacitance change of 0.002
picofarads or 2 femtofarads.
Pump Motor Monitor This circuit can turn on a light or sound a
beeper, whenever an AC line powered pump motor turns on. It is designed to detect
the motor current using a small inductor placed on the outside of the motor’s power
cable. No direct wire contact is needed.
TEMPERATURE
SENSOR WITH 4 TO 20mA CURRENT LOOP
I designed a circuit similar to this one years ago to accurately measure the air
temperature inside a building 1000s of feet from a control room. The circuit uses
a very robust current loop method. It uses a highly accurate semiconductor
temperature sensor and an equally accurate voltage reference. The circuit includes a
diode bridge, so it is polarity independent. By using the component values
indicated, the circuit should not require calibration. It has a range from –40F to
+120F and an accuracy of +/- one degree F.
