Nuts & Volts - September 2006

●●●●

For several years

now, I have been

wanting to design

and build a period

counter with

totalizing function.

I finally made time

to do just that. My

initial design was

quite basic, but

then I started the

what-ifs, and the

original design grew

to twice its size.

■ Front view of the counter.

A WIDE RANGE

PERIOD

COUNTER/TOTALIZER

When I finished the first prototype, I sat back and studied it. Then I asked myself what would I want to strip to make the design simpler. The answer was, “nothing.” It was very functional the way it stood, and it’s a dandy.

The pleasure and convenience of use in the coming years far outweigh the few extra hours needed in construction. So there: I had what I wanted — another “mighty weapon” to add to my arsenal of bench test equipment. A simplifica-tion in design, with a slight loss in overall accuracy, is included near the end of this article for those who prefer to have less wiring to do.

You may be thinking, “Why build a period counter when I already own a frequency counter?” For starters,

there are many situations where it’s just more convenient to count electrical signals and read out the actual time (μs, ms, s) rather than frequency. Some frequency counters will also read out the period, but almost all of them fall short for slow repetition rates or long periods between uses. Their lower limit is around 10 Hz.

The counter described here also has additional important features. In addition to its wide range of counts (from one to one billion), it will also capture and display pulse width, either positive or negative. At its extreme range, it will capture and display a single pulse as narrow as 1 μs, even if it only occurred once a day — try doing that with your oscilloscope!

On the other end of its range, it will count rates as slow as 1,000 seconds or as fast as high-speed pulse trains from a function generator’s burst mode of operation. And finally, a totalizing function was incorporated into it to count electrical events and sum them (up to 100,000).

Although operation of counters are similar, they differ in several respects. A frequency counter counts an unknown clock rate of input cycles in a known gate time, usually one second, and displays the count in Hz/sec.

A period counter counts a known clock rate in an unknown gate time (input signal) and displays this as the actual time of occurrence. For this reason, period counters are a little more complex.