To understand the circuit functioning of this homemade frequency meter, let’s go through the following explanation:
IC 555 forms the main part of the circuit and is wired as a monostable multivibrator.
Its frequency is determined by the external components R2, VR1 and C3. The setting of VR1 is important and may be used to adjust the measuring range of the frequency meter.
The frequency in question is applied to the base of transistor T1 via resistor R6. T1 conducts only during the positive peaks of the input oscillations.
During these conductions of T1, capacitor C2 is forced to discharge quickly through R7 and T1. Also, during the negative peaks of the input oscillations, T1 is cut OFF and now C2 charges via R1 but at a fairly slow rate.
Due to this, a sharp negative pulse appears at pin 2 of the IC through the capacitor C1. Resistor R3 makes it sure that the pulse is narrow and only just triggers the IC.
The IC immediately responds to the trigger generating a pulse of a constant period set by VR1 at its output pin 3.
This pulse is smoothed and integrated by R4, R5 and C5, C6 to produce an average value of the pulses. A moving coil type meter can be used to indicate this integrated value.
The magnitude of these pulses will linearly vary with the input frequency and thus can be directly measured over the meter.
Waveform Image Credit: http://www.bbc.co.uk/scotland/learning/bitesize/standard/physics/images/waveform2.gif