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Electrical
The electrical
part of the Etch-A-Sketch design was perhaps the simplest. There
were five aspects:
Stepper motors
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Our stepper motors were unipolar stepper motors, but we figured
that we did not need the fine control that unipolar driving supplies.
So we drove our stepper motors bipolarly but ignoring the extra
center tap that makes unipolar stepper motors what they are.
The
stepper motors then had coil resistances of 140 ohms. We would be
driving them with 12v from the Handyboard, and from V=IR we see
that we would only need 85mA of current to drive them in steady-state.
The L239D H-Bridge chips on the Handyboard can supply up to 1A of
current. We were safe.
Analog joystick position input
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The analog joystick was prefabricated. The joystick shaft would
slide contacts along linear potentiometers when moved. We simply
cut off the leads coming from those linear potentiometers (pots)
and fed them into the sensor inputs on the Handyboard. The pots
would form a voltage divider with the 47k internal pull-up resistor
in the Handyboard. Our pots were not significantly large in resistance
compared to 47k, so we did not see a very large volatage drop across
the pot when at maximum resistance, but it was enough for our purposes.
We maxed out below 4 volts.
Button inputs
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The button inputs were simply digital inputs, connecting to ground
when pressed, otherwise allowing the internal pull-up resistor in
the Handyboard to pull the voltage high. We did not use the digital
inputs because we wanted to use the internal pull-up resistor.
LED outputs
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For each button input, we also had an LED that lit up to signal
that that particular function was running. We used the digital outputs
(Seethe programming section for digital output details) on the HandyBoard
to supply +5v. We wanted 20mA to run through the LEDs, so by V=IR
we used 250 ohm resistors in series with the LEDs.
Wiring
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The joystick came with a cable filled with wires. We used all of
them, plus four more wires run outside to cable together the joystick
and the Handyboard. We briefly considered putting the Handyboard
inside the joystick and only running four wires to the stepper motors,
so that we would not need the four extra wires external to the cable,
but realized that there was not enough room in the joystick for
the HandyBoard and its battery. : (
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