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I'm currently trying to read an AH276 hall effect sensor using an AVR microcontroller. The sensor is built-in in a brushless fan I'm using and it's operating as the fan's controller. The sensor has two output pins which drive the four coils of the fan (two pairs). My guess is that, using one of the two output pins (doesn't matter which one), I can count the RPMs of the fan. Is my guess correct?

If yes, how can I read the RPM without using an external sensor? Should I use a transistor on either AH276's output pin or I can connect it directly to an IO pin of the microcontroller? I have figured out the software part, but I'm stuck on hardware.

enter image description here

Both the microcontroller and the fan run at 5V.

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The two output pins (2,3) are open collector NPN transistors. These will be switched on and off at a rate that sets the speed of the fan. So your guess is correct.

Coils can produce some spiky voltages so I wouldn't recommend directly connecting these pins to the input of a microcontroller. A simple resistor/transistor should do.enter image description here

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  • \$\begingroup\$ Perfect! I'm not an EE, can you please recommend me a transistor? Would a 2N2222 do the job? I guess that the resistor values should change accordingly to the transistor. Are the 10Ks OK or should I consider other values? \$\endgroup\$ – giannoug May 24 '13 at 11:14

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