41099 Chap.8 Displays, motors and power budgets
Displays, motors and power budgets
What this chapter covers
- 01
Common anode and common cathode displays, and why the datasheet question comes first
- 02
Multiplexing a four digit display so the wiring stays manageable
- 03
Libraries, and the obligation to explain one you have chosen to use
- 04
Three kinds of motor and the different thing each needs from the board
- 05
Pulse width as a position command rather than an average
- 06
Current budgets at the worst moment, and runtime from stored capacity
Do the current and runtime sums before the demonstration rather than during it
- 3Check the three indicators against the per pin guidance and decide how to connect them.
- 3Plan for the worst moment rather than the comfortable case.
- 4Compute the average draw and turn it into a runtime, then qualify the answer.
Key terms
- Common anode
- A display package whose segment anodes share one connection, so a segment lights on a low output.
- Multiplexing
- Lighting shared display digits one at a time, fast enough to appear continuous.
- Library
- A collection of code adding functionality, whose behaviour you are expected to be able to explain.
- Stepper motor
- A brushless motor that divides a rotation into discrete step angles and can hold a position.
- Motor driver
- A board that supplies motor current so the microcontroller pins do not have to.
- Stall current
- The much larger current a motor draws when something prevents it from turning.
- Battery capacity
- The charge a pack can deliver over time, rated in amp hours or milliamp hours.
- Internal resistance
- The resistance within a cell that lowers its delivered voltage as current rises.
Displays, motors and power budgets FAQ
Why can a motor never be connected straight to an output pin?
Because the pin can supply only about 40 milliamps according to the course materials, which is suitable for indicators and small sensors and nowhere near what a motor draws. Connecting one directly does not produce a weak motor; it damages the board. The pin switches a transistor or a driver board, and that device switches the motor supply.
What is the first thing to read on a display datasheet?
Whether the package is common anode or common cathode. Both packages supplied in this kit are common anode, which means a segment lights when its pin is pulled low rather than high. Every line of the driving code depends on that answer, so establishing it before wiring saves rewriting the whole pattern table.
Is pulse width modulation the same thing as a positional control signal?
No, and the distinction is regularly tested. Modulation varies the fraction of a period spent high and relies on the load averaging it. A positional command sends one pulse per frame and the receiving device decodes how long that pulse lasted, so the width is the message rather than a means of producing an average.
What does the course expect if a library is used in an assignment?
That you understand its internals and can describe them on request. That turns the choice into a trade: the library saves real work and you take on the obligation to describe what its calls do to the pins. Read enough documentation to state that in one sentence before committing, or the library becomes a liability in a demonstration however well it works.
Why does a device that runs from a computer connection reset on batteries?
Because the starting current of a motor pulls the pack voltage below what the board tolerates, and the bench connection is a stiffer supply that hides the problem. Measure the supply voltage at the board while the motor starts: a dip at the moment of the reset confirms it. Remedies are a better pack, a separate motor supply sharing only ground, or a reservoir capacitor.
Which safety rule in this subject is not recoverable?
The lithium polymer battery rules. The course materials are unambiguous: never short circuit these packs, never cut off the connector and never solder directly to the battery. Everything else in the kit fails by ceasing to work, and this one does not.
Assessment move
For each output part, answer three questions before wiring: which way does current flow, how much of it, and who supplies it. Do the current sum at the worst moment and the runtime sum from the pack rating before the demonstration week. Test on the supply you will actually use, for the length of the session.
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