I had a cheap pedometer from a cereal box and decided to use it as a digital counter to use with a Geiger counter. It could be used for any dgital counting application. The down side is that it can only count up to about 3 counts per second, if there are pulses faster than that it will miss counts. But hey, the price is right. For the Geiger counter application, I don't mind the slow count rate. I use it when checking for low level radiation, I leave the counter to count for a few minutes to see if the count rate is above background levels, so having the counter saves me from having to manually count each click.
Step 1: Open the Pedometer
My pedometer had special security screws but fortunately I have a set of security bits so I was able to open it. Inside there is a small weight on a spring arm which hits a spring contact to signal one count on the pedometer display. So it is only necessary to connect whatever digital signal you want to count to the arm and spring of the pedometer. A good quality pedometer has instead accelerometers, and unfortunately this type of pedometer cannot be hacked this way. Unfortunately I didn't take a photo of the inside of the pedometer, so I drew a diagram. I used a voltmeter to determine which of the arm or the spring had the higher voltage.
Step 2: Add Diode and Resistor
I added a diode between the arm and spring to prevent he input signal creating reverse votages on the input, which might harm the electronics. I also added a resistor to avoid overloading the pedometer electronics. I was planning to connect to the headphone jack of my Geiger counter which produces negative voltage pulses, so I connected the input to the spring contact which was the positive voltage in the pedometer. If your input signal produces positive pulses connect it to whatever contact has the negative voltage. The Geiger counter output is AC coupled, if your output is DC coupled you will want to also add a capacitor (maybe 10 uF) in series with the resistor.
Step 3: Connect It Up
I added a phone plug to plug into the Geiger counter headphone jack, and I put heat-shrink tubing around the resistor to prevent short circuits, and placed the resistor between the arm and spring contact to prevent any pedometer counts. Then I put the pedometer case back together, with the wires coming out holes that were originally for a belt clip. Done!