Voltage Leaking back to controller

Precede the regulator with a 0.1µF capacitor and you should be good. If you want to be really conservative:

Precede the capacitor with a 100 ohm resistor,
In parallel with the capacitor place a 18V zener diode.

Looks good. It would be interesting to see if that will eliminate the need for the 0.1 cap on the Arduino. Could I trouble you for a sketch of how that gets wired in with the ST7805?

I cant seem to find ST7805 but LM7805 or MC7805 and other variants are available anywhere. Is there a significant difference.

Now I believe they all share common specifications.

in the below, C1 and C2 should be physically close to the L7805.

Thank you very much. I ordered an assortment of L7805 to L7824. The roboRed requires 7 to 20 V input. so should I use a L7808 or L7810 or L7812 instead. Also I assume that your diagram is +V on top. I believe other boards accept 5V input.

I just finally looked at the roboRed schematic. It will accept up to 23 Volts input (based on the MP2703 IC.

I would change from using the 7805 to putting the roboRed input where My schematic showed the 7805 then leave out the right hand cap.

Thanks I will save the 7805 for future projects. I should have the diode and 100ohm resister later this week.

Interesting results today. I received my new Yourdrino RoboRed board. Testing showed that with the MOSFET installed and running at 5V, the new board would reset itself without the need for using the reset pin and capacitor as required with the old board. Testing also showed that running the fan direct 5V PWM signal both new and old board required the reset pin and capacitor to reset. Testing at 3.3V with the MOSFET,does not required the reset pin and capacitor for both the new or old board. Note that running directly at 3.3V without a MOSFET cannot be performed because the voltage needs to be above 3.8 for the fan to run.

In conclusion I cannot say if I damaged the old board or are they operating on the edge of where they reset by themselves or not, and just a small differences in the boards could be putting one over the edge. But I believe the cap and reset pin is probably prudent to use.