UPDATE: my counter came in.
I took the plunge and bought the cheapest digital counter I could find, and it seems to have worked out, so hopefully this will be useful info:
I tested the unit, and it works fine. Initial testing shows that it counts 5-6 times faster than the hacked calculator counter that I had previously been using. The included sensor/switch and magnet perform well, and I was able to get accurate results up to RPMs over 1000. I am not sure of the upper limit on speed, but I have a 3000rpm drill that outran the counter, but that is fine since I plan to wind between 500-1000 rpm.
There are no included instructions — and the connection labels on the back are in Chinese — BUT you can see that the characters for ‘reset’ match the characters on the back-left terminals, and the characters on the right do not match anything else, so I figured that logically: the left is remote reset and the right is the counter-sensor connection. I tried it out and that is indeed the case. The reset terminals seem to take any kind of momentary switch connection.
I bought the last unit from droking.com, but you can get the same one (or a fancy LED version) elsewhere online for $10 or $20 respectively + shipping.
I dug up some specs online:
-Switch/sensor model: VJ12-D10K,
-maximum counting speed: 20 times / sec
-Power supply: 1.5V – AA batteries
-cleared: panel / External
-Counting range: 0 — 99999
-Remote clear the minimum pulse width: 0.1 seconds
-Counting the highest frequency: 20HZ
-Counting minimum pulse width: 10mS
-Contacts connected maximum resistance: ≤ 10KΩ hole size: 61 * 36 mm
-Contact Disconnect minimum resistance: ≥ 500KΩ Weight: about 50 grams