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Fan Tacho
Measurements are done with a Yakoo brushless fan 12V 0,18A fan
We got similar results with a Ebmpapst 3412N /2GL fan. The frequency is 66Hz (period 15 ms)
We got similar results with a Cooler Master A8025-25BB-3BS-P1 fan. The frequency is 37Hz (period 27 ms). This fan is regulated. So the frequency may vary
Tacho output behaviour
Floating tacho output with the fan, SW1, off: 12V
Floating tacho output with 10k resistor to ground and the fan, SW1, off: 12V
When the fan is running, SW1 is closed, the floating tacho signal is a square wave between +400mV and -200mV
When the internal pull-up resistor of the ATmega328p is activated there is a square wave signal on the input pen of the ATmega328p with a high of 5,2V and a zero of -0,4V.
When the fan is switched off with SW1 the tacho signal goes to 10,6V. Connecting this to an ATmega328p input means that the internal protection diode gets activated. This is not desirable. Therefore an extra circuit is needed:
Circuit considerations
Make sure pen 4 of the ATmega328p has the internal pullup activated with pinMode(2, INPUT_PULLUP);
When the fan is switched on there is a symmetrical square wave on pen 4 of the ATmega328p. The top level is 5,2V and a low level is 0V. The frequency is 80Hz, a period of 12,5ms
The switch can be replaced with a transistor so the fan can be controlled electronically
Diode circuit
Transistor circuit
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