Optical sensor to that triggers on the passing of the spoke of a tiny wheel (10mm wheel, with 1mm width spoke)


I am designing a detector to count the passing of spokes on small wheels. 

My plan is to use a light source like an LED and a receiver like a photo-diode to create a sort of break beam setup.

The signal will be sent through an amplifier with filtering, to the mic input of a PC to be used for measuring the passes per hour of the spoke of the wheel.

I am having a hard time finding the right product combination and circuit to make this happen.

The wheels are suspended so the spoke casts a shadow on the collector to trigger the signal as the wheel rotates / oscillates.   The wheels range in sizes from 8mm to 20mm and have spokes from 1mm to 2.5mm wide.

This should be a very simple device to build and I have made some but it is hard to get both a signal that has enough gain and low noise with the sensitivity to accurately trigger.  The make pulse would be around 30k per hour so the wheel is not moving very fast.

  • 0
    •  Analog Employees 
    on Apr 1, 2021 3:39 PM

    Hi there -

    The challenge for your proposed design is to achieve a narrow LED light beam so that the light is reflected from a single spoke in a reflective measurement or the light passes only one gap between two neighboring spokes in a transmissive setup. The choice of LED color depends on the material of the wheels.

    ADI offers photometric ASICs such as ADPD1080 and ADPD4100, which have integrated LED drivers and voltage references & analog channels to stimulate LEDs and measure signal from the photodiodes. ADPD1080 and ADPD4100 have integrated ADC for digital output. Check out these products at the links below:



    Glen B.