PWM + I2C interference in fan controller

Hans H. Guest #6005269
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Hi,

I'm building a fan controller for a vintage computer and I'm facing some 
difficulties isolating the I2C bus and the motor PWM.  When I have a fan 
connected to the output, spikes are being introduced into the I2C bus 
which sometimes causes misreadings of the LM75A sensor data.  Can anyone 
suggest a simple way how I can decouple the two parts of the circuit 
better?

Please see the attached schematics and an image from my oscilloscope 
that shows the spikes.

Thanks!
Hans
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K. S. #6005660
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Hans H. wrote:
> Can anyone
> suggest a simple way how I can decouple the two parts of the circuit
> better?

show us your layout. I guess there is a significant problem with it like 
bad ground routing, no decoupling, signal paths next to power paths ...

2N7000 @ 3V3 GS can only drive <<100mA, it is only specified for 4.5V 
and its maximum threshold voltage is 3.0V. You need a real logic level 
Mosfet unless you use a toy ventilator with very minor current 
consumption (like a few mA max).
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