Created by
Created February 28, 2016
Last modified March 10, 2016
Tags motor   photodiode  


Infrared tachometer used to measure fan speed by providing an interrupt to an Arduino. The goal is to synchronize two fans.


The sensor is an SFH213 photodiode aimed at a 10mm strip of reflective tape on the back of the fan hub. The hub is illuminated by two SFH4550 IR emitters, each with 15mA forward current. The sensor and emitters are spaced 5mm from the hub surface. The fan is a 10" Orion OD254AP-12MB.

Circuit notes:

'Noise' simulates a 2mm reflective contaminate on the fan hub.

'Ambient' represents maximum incident light illuminating the sensor and reflected from the fan hub.

'Pulse' is a half sine wave used to simulate the rise, peak and fall of the reflected light as the tape passes under the sensor.

SFH213 generates a 10uA pulse on top of 10uA from ambient light.

C1 AC-couples the pulse to the non-inverting input of comparator OA1.

C2 filters higher frequencies which may be caused by dust or other contaminants on the fan hub. This helps prevent random triggering of OA1.

InvBias is an adjustable (0-50mV) voltage divider which further prevents unwanted triggering by providing a relatively high threshold on the OA1 negative input.

OA1 is a rail-to-rail, high slew-rate amplifier.


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