Finding frequency of a series of pulses (3 - 60 Hz) without using a microcontroller or frequency-to-voltage converter
Here's a hint: An edge-triggered retriggerable monostable multivibrator followed by a DFF makes a very effective frequency discriminator.1







simulate this circuit &ndash; Schematic created using CircuitLab

U1 is the one-shot. Its Q output goes high for 16.67 ms every time there's a rising edge on the input. If the output is still high when the next edge comes along, then the period must be less than 16.67 ms, which means that the frequency is greater than 60 Hz. The DFF captures this status &mdash; its output is continuously high for frequencies greater than 60 Hz and continuously low for frequencies less than that.

Build a second one of these with a period of 333.3 ms, and combine their outputs with a simple gate to produce the result you're looking for. You can build the entire setup on a breadboard with just three chips: a dual one-shot, a dual DFF, and a single 2-input NOR gate.



1 This has applications wherever FSK is used &mdash; anything from low-speed data modems for telephones, to data storage on cassette tapes, to early floppy disk drives, among many others.