A 3-in-1 Tester for Your Lab
This DIY piece of test equipment combines a variable DC voltage reference, a programmable function wave generator, and a fully functional frequency counter, and it costs less than $150.
Build a Frequency Counter
Frequency counters have been around for years as a standard piece of test equipment in both commercial and hobby labs. If you need one for your workshop, take a gander at the one presented here.