I came across an interesting yet challenging problem of finding the frequency of a series of pulses. The pulses can have a frequency anywhere from 1 to more than 100 Hz. The goal is to detect if the pulses have a frequency between 3 and 60 Hz. That is, raise a flag or light up an led if the frequency falls between 3 and 60 Hz. The constraints are -
- Using microcontrollers are not allowed
- Using a frequency-to-voltage converter IC is not allowed.
- False positives are allowed but false negatives are not allowed.
- (weak) The pulses can have a duty cycle less than or more than 50%, not necessarily equal to 50%.
Can you please suggest a method of doing so?
One method that I have designed and analysed is to use a counter IC and count the number of pulses, and reset the counter every 1 second (using a separate timing circuit like 555 timer). If the output number of pulses is between 3 and 60 (one can use a logic circuit to determine that) then raise the flag. The logic circuit required for this would be a bit tedious, and depends on the type of output of the counter IC.