DISPLAY The display is split up in to seconds, each with 5 subdivisions (segments). That's a total of 300 segments per revolution. If the propeller spins at 2000RPM, that's 33.3 revolutions per second, or 30ms (30,000μs) per revolution. That means we'll be drawing 10,000 segments per second, which is 100μs per segment. With a clock speed of 16MHz, this is 1600 cycles per segment, which is plenty. SCEMATIC NOTES The diode (D14) across the fan's power connections is there because if the power across the fan breaks (due to the unreliable nature of the brushes), the motor in the fan has coils, which act like an inductor and will produce a back EMF (a huge negative voltage across the power connections) as the magnetic field collapses. This won't be good for the Arduino and could cause sparks on the brushes. The diode simply shorts the negative voltage. The capacitor (C1) and resistor (R14) are there to smooth the power supply from the unreliable brushes. The capacitor would discharge fairly slowly (due to the resistance of the circuit), but will charge very quickly. Potentially, it will charge so quickly that it'll pull too much current from the power supply (i.e., short the power supply and trip it). So the resistor limits this. Unfortunately, the resistor will also have a potentiometer effect (with the resistance of the main circuit). 10Ω was chosen as a value due to these rough workings: Lets say the Arduino circuit takes 500mA. If we aim to lose 1V across the resistor, that's 1V / 0.5A = 2Ω (from V=IR). The 100μF was a guess (from Dad), but "PCB" Mat suggested something larger, like 2200μF. So we went with 1000μF, which appears to power the board after power-off for a couple of revolutions. The factors here are that a capacitor that is only able to hold a small charge won't be able to maintain a current for a reasonable amount of time when the power breaks. If it's too large, it will take ages to charge and effectively short the power (save for the resistor) while it does.