This here is a simple but versatile pulse counting/smoothing integrator circuit with an NE555 as the shaper, and a little LCD display as the output. I have originally done it for counting pulses from my Geiger counters, so I have included a piezo speaker combined with an LED for indication, but it can also have a variety of other applications, like revolution counters, track/lap counters and so on.
The circuit consists of two main parts, the first part is the wave shaper which transforms input pulses to a more square-wave-like waveform. I have used a NE555 since I have a plenty of them, however nothing prevents you from doing this with discrete parts… As mentioned above, my purpose was to integrate the counter with a radiation meter, so the shaped output goes also to a piezoelectric speaker and to a LED diode. I have to say that the audible “ticks” have enough volume to be heard even from a distance.
For the pulse counting itself, this is where my (creative) laziness kicked in. To keep things small and simple, I did not want to bother with a driver circuit for a separate display. Then I found out on eBay that there’s a multitude of the so called “tally counters” with a cheap price per piece; approx. 1 EUR. These counters run from a single button cell and they switch themselves off after minutes of inactivity. You need to hack them from the inside and inspect the “COUNT” button; mine had been connected directly to the positive terminal of the cell and the other pin went to the internal circuitry. This is where the output of the shaper gets connected to.
The above schematic works for similar counters of the above type. In some cases you might need to tinker with the coupling capacitors C1 and C2, especially if a single pulse gets registered as two, three, etc – having an oscilloscope handy will be a great help.
Connected to the IT-65 geiger counter
I also recommend gutting the internal battery out and placing it within easy reach at repairs. With normal usage, both the 9 volt and the 1.5 volt batteries should last well over a year. I haven’t tried this, but I think that with an appropriate resistor the counter itself should run normally even from the 9 volt battery.
As for the pulse registration itself, this counter has had no problems registering a rate of 300 pulses per second, just the display gets dim at such a rate, and restores to normal contrast when idle.
Pulse counter in action. The circuit is still the same, just the housing had been re-done.