/elec/propeller-clock

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 (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 resistence 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.