This is the circuit of frequency meter. It measures the frequency of the input signal in terms of current passing through an ordinary ammeter. The main component of this circuit is the 555, a versatile, general-purpose timer IC. In this circuit, the 555 is configured as a monostable multivibrator that outputs a single pulse at pin 3 every time the input signal at pin 2 goes 'low'.
The width of the output pulse generated by the 555 is defined by R3 and C3. The larger the output pulse width, the longer is the time that pin 3 is high. Since pin 3 doesn't sink current when it is high, the current passing through the ammeter is higher when pin 3 is high.
This means that the larger the values of R3 and C3 are, the larger is the output pulse width, and the higher is the current passing through the ammeter for a given input frequency.For any given value of R3 and C3, on the other hand, the current passing through the ammeter increases as the input frequency increases.
This is because pin 2 is 'retriggered' more often as the input frequency increases, decreasing the total duration wherein the output is 'low'. Thus, the ammeter dial in this circuit would be a good indicator of the relative frequency of the input signal at pin 2. Good choices for the values of R3 and C3 to match the intended application as well as careful calibration through R5 are required to make this simple circuit work properly.