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Pull down resistor not working how I think it should

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tom g:
Hi I am working on a project to measure the RPM of my lathe using one of those light gate sensors from a printer. I am using a Node Mcu board to measure the pulses and display the result to an spi oled screen.

However, when I compare the result i get from my device to the result from a commercial rpm meter the device is measuring too low. At first I suspected the code but now I think that the signal from the sensor is not falling low enough and the interrupt is missing pulses. The signal oscillates between 3.3v and 2.6v which i don't think is low enough but the mcu seems to be triggering on some of the pulses. 
 
I tried using a pull down resistor to bring the pin low when it is not reading high from the sensor but it pulled both the high and the low values down (I tried resistor values between 1k and 40k). Is there a better way to achieve this ?  I included an oscilloscope trace and a sketch of the circuit

Thanks

Tom45:
Do you know the output circuit of the sensor?

tom g:
its one of those led light gates i think its just an led and a photo sensor (transistor ?)

Brumby:
That does not tell us a lot.

Do you have a part number for this sensor?

ahbushnell:


https://www.digikey.com/products/en/sensors-transducers/optical-sensors-reflective-logic-output/556?k=reflective+sensor&k=&pkeyword=reflective+sensor&sv=0&pv16=376802&sf=1&FV=1205%7C421129%2C-8%7C556%2C69%7C411897&quantity=&ColumnSort=0&page=1&stock=1&pageSize=100

For the LED put a resistor in series.

Use a pull up resistor on the output.

Or look around on Digikey (or Mouser). 

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