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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

PIC capacitance meter measures charging time

Click for larger image

This circuit measures the charging time of a capacitor, using the PIC microcontroller as an accurate timer.

As can be seen, the circuit consists simply of a PIC16F628A microcontroller, an LCD panel and not much else. The capacitor is charged via one of three resistors which are manually selected by the second pole of switch S1. The other section of switch S1 is connected to the RA3 & RA4 inputs of IC1, telling it which capacitance range to test for.

When charging is started, the PIC starts a 16-bit timer and then stops it when input RB2 detects that the threshold voltage has been reached. This stops the counter. IC1 then computes and displays the capacitance value on the LCD panel. Transistor Q1 is used to discharge the capacitor before its next test.

Calibration involves selecting a capacitor of known value and then adjusting the value of the associated charging resistor until the display-ed reading is correct. The circuit works for values between 10pF and 470mF.

The software for the PIC and more information on the circuit is available by browsing to

William Andrew,

Lane Cove, NSW. ($40)

Mobile phone can power PICAXE circuits

Many small circuits such as those using PICAXE micros can run on low-voltage supplies such as 3.7V from mobile phone batteries. These make good cheap power supplies if you have an old phone with a reasonable battery lying around.

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