Strange values while I'm measuring frequency with arduino

[OM222O]

yes because as I mentioned, you should choose the two resistors carefully. as I said before, use the second version for the website and the first version with your sensor. Also the TL081 which you chose will simply not work at all in this case as It has about 3v swing from each rail (look at the Vom : Maximum Peak Output Voltage Swing). That means at minimum you will need 6 Volts (3 volts above ground and 3 volts below VCC) for it to even operate properly! replace the op amp with the rail to rail ones which you mentioned. you should be able to buy MCP6002 from local suppliers very easily!

As you can see the only difference between the two versions of the circuit I have shown is the value of the resistors which I explained how you can choose.


 I have built and tested this circuit and it works properly. Good luck!