Flow meter circuit and low pulse count

Question

^{simulate this circuit – Schematic created using CircuitLab}

I have a very simple circuit that uses a hall effect sensor to measure water flow which then in turn sends pulses to GPIO 13 on my PI. Problem is, for some reason the count of pulses is way below what I know the rate of speed of the impeller to be. Has anyone seen this issue? I have a simple setup of the ground direct to the board, the 5 V direct and pulse output through a voltage divider back to the PI. Its odd that I get a good reading, and a pretty consistent on the number of pulses, but its like 5 a minute.... yet the sensor has 4000GPH going through it.... Could my voltage divider be causing this? I am using a 10k and a 4.7k. Any other issues that anyone can think of before I assume a faulty magnet in the housing?

Answer 1

I'm going to guess what your circuit really looks like because the schematic does not look reasonable to me.

I'm guessing that you have used two resistors as a voltage divider to reduce the maximum voltage from the sensor (around 5V) to something that is compatible with the 3V inputs of the RPi. The resistor values look appropriate, but you have reversed their locations. The 10 kilohm resistor should be between the RPi input and ground, while the 4.7 kilohm resistor should be between the RPi input and the sensor output. I don't think you really have a voltage source V1.

You can calculate the RPi input voltage as

V_{PI} = V_{SENSOR} * R2/(R1+R2)