PIC capacitance meter measures charging time
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
The software for the PIC and more information on the circuit is
available by browsing to www.wasdomain.co.nr
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