Elenco XP&trade - Manual (Page 54)

Project B22 Digital Voltmeter
Build the circuit as shown. Turn on the
slide switch (S1). Load program Digital
Voltmeter into the microcontroller (U21)
using the programming instructions in
project B1. Press the F8 key or select
Terminal under the PICAXE menu to open
a terminal window for communication with
the microcontroller, set the baud rate to
2400, and select Refresh.
The microcontroller will measure the
voltage at input 1, which is set using the
adjustable resistor (RV). Set the lever on
RV to any position, then push the press
switch (S2) to measure and display the
voltage. Setting the RV lever to the top
position will measure the voltage from your
batteries (B3).
Optional:
-53-
This circuit uses part of the NPN transistor
(Q2) as a reference, to calibrate the ADC in
the microcontroller for correct voltage
measurements. This self-calibration
eliminates the error that would occur as the
batteries discharge, but your measurements
will still have some error.
The adjustable resistor (RV) sets the voltage
to be measured, which will be between 0V
and the battery voltage.