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A PIC-BASED DIGITAL VOM

by Duane Perkins

Start ý The Circuit ý Construction ý Testing ý Program Code ý Sources and PDF

There are lots of digital volt-ohm meters on the market and even the lowest-priced instrument (less than $20) works well for measuring DC volts. Measuring AC volts is another matter.

Usually, the AC is rectified and filtered to average DC, which is what is actually measured. The processor generates a reading based on the assumption of a sine wave but gives a false reading for any other wave form (true for analog multimeters as well).

So why not use a microcontroller to calculate the root-mean-square of any waveform that is being measured and display the true RMS voltage? This requires taking a large number of samples for the calculation. The samples can also be used to calculate the average DC voltage, allowing both the DC and AC components to be displayed simultaneously.

Figure 1ýHereýs a look at the basic ingredients for my digital VOM.

SYSTEM DESIGN

The design I came up with uses a PIC14000, which has a 16-bit analog-to-digital converter (ADC) as well as the capacity to perform the rather complex calculations. An analog front end is also necessary to shift the voltage to a level the PIC can accept. An ordinary dual op-amp with a few resistors does the trick. Add an LCD to view the measurements and a couple of rotary switches for function/range selection and you have it, as shown in Figure 1. Photo 1 shows the front panel and Photo 2 shows the PC board.

Photo 1ýThe interface panel is simple, but sufficient.

The digital VOM described here measures AC and DC volts in four decade ranges from ý0.53 to 530 V, plus a range of ý0.265 VDC and peak AC full scale. It also measures DC resistance in five decade ranges from 100 ohms to one megohm midscale. Voltage measurements are calculated as average DC volts and RMS AC volts, which are displayed simultaneously. A microcontroller samples the input 256 times and calculates the readings.

Photo 2ýLike the interface panel, the circuit board is simple and sufficient.

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