# How to change frequency range of this circuit

I need to design a frequency to voltage converter and I found this circuit on internet: I removed the resistor R6 and the capacitor C5 because what I need is the PWM output it gives on the 555 timer. I have run a simulation and it seems to work pretty nice, where the output gives a PWM with 50% of the duty cycle on the lower frequency and almost 100% of duty cycle on the higher frequency of the input range. However, it is configured to work in a range if frequencies different from the one I need, and I don't know exactly what should I change because I have never seen such a 555 timer configuration. I guess it has to be related with R4, R5, C2 or C4 but I'm not really sure because the configuration I used before was with the pin 2 connected to pin 7, not 6.

I hope anyone can help me with changing the range of input frequencies that this circuit can measure. More precisely, what I need is a range that goes from 1Hz to 2Hz on the input, to get 50% to 100% of duty cycle on the output. If anyone knows any other circuit or alternative to do such conversion from frequency to voltage I would appreciate it too!

• In this schematic C1 and R1 together make a high pass filter, with cutoff frequency f = 1 / (2 * pi * R1 * C1) = 3.38 Hz This filter blocks frequencies less than approx 3 Hz. So if you are looking to change your input signal you need to modify that first. – shreyas Apr 24 '19 at 18:05
• Ok, that seems fine to change. Then what would be the next step? – Jaime_mc2 Apr 24 '19 at 18:35
• @Jaime_mc2 Pin 7 is the discharge pin and it's not really doing anything in that circuit except periodically discharging $C_4$. Since that node doesn't go anywhere, I think you could remove $C_4$ and $R_5$ and see no change in behavior on pin 3. Pin 2 and 6 are the 1/3rd and 2/3rd level comparator inputs and they are tied together. $Q_1$ periodically resets the voltage across $C_2$ and $R_4$ supplies current to it when not reset by $Q_1$. What I'm curious about is the amplitude and output impedance of your source? – jonk Apr 24 '19 at 21:15