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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 acrss 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. So the resistor limits this.
Unfortunately, the resistor will also have a potentiometer effect
(with the resistence of the main circuit). 10Ω was chosen as a value
due to these rough workings: Lets say the arduino circuit takes 100mA.
if we aim to lose 1V across the resistor, that's 1V / 0.1A = 10Ω (from
V=IR). The 100μF was a guess! The problem with the capacitor is that
if it's can only hold a small charge, it won't be able to maintain a
reasonable charge 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 charges). 100μF seemed like a good value,
according to dad!
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