95 lines
4.3 KiB
Markdown
95 lines
4.3 KiB
Markdown
Tutorial 5 - indicator LEDs
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===========================
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Keyboards often have LEDs to indicate CapsLock, NumLock, and other states.
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It's one of the first things we look at when a keyboard produces unexpected results.
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Adding LEDs to the basic breadboard keyboard
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The breadboard keyboard modifies the basic breadboard keyboard described in [tutorial_1_breadboard_keyboard.md](tutorial_1_breadboard_keyboard.md)
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Add components to the breadboard as shown in the picture.
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The three clear plastic cylinders are LEDs.
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LED anodes (the longer lead) are powered by 4.7k Ohm current limiting resistors connected to pins 16, 17, and 21.
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LED cathodes (the shorter lead) are grounded by a common terminal strip.
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keybrd sketch for driving LEDs
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------------------------------
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[keybrd_5_LEDs.ino](keybrd_5_LEDs/keybrd_5_LEDs.ino) is a simple sketch with three LEDs.
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The sketch will run on the above breadboard keyboard.
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As usual, the sketch annotations explain the code.
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LED brightness
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An LED's current limiting resistor value effects the brightness of the LED.
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Lets see how much visual difference resistance makes.
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Replace an LED's 4.7k Ohm resistor with a 270 Ohm resistor.
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It doesn't matter which end of the LED the resistor is on, the important thing is that the resistor and LED are in series.
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Less resistance makes the LED brighter.
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Too little resistance will burn out the LED.
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Connecting an LED directly to power will destroy the LED in a bright flash (do not look directly at the LED if you try this).
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2-mA LEDs are bright enough for keyboard indicator lights.
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Or you can use more resistance on a 20-mA LED to make it dimmer.
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LED current limiting resistor values
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------------------------------------
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Never connect an LED directly from ground to power. Doing so would destroy the LED.
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This formula calculates the minimum resistance for maximum LED brightness:
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```
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output-pin Supply Voltage Vs
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LED Forward Voltage Vf
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Forward Current If
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minimum current limiting restiance R = (Vs - Vf) / If
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```
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For Forward Current, use the smaller of:
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* Current capacity of output pin
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* Continuous Forward Current of LED
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Teensy LC output-pin capacities are:
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* four 20 mA pins (5, 16, 17, 21)
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* nineteen 5 mA pins
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* Teensy LC on-board LED is on pin 13.
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It has a current-limiting resistor on the board, and does not provide enough power for another LED.
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For Teensy LC 20 mA pin and the TT Electronics OVLLx8C7 LED:
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```
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output-pin Supply Voltage Vs = 3.3 volts
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LED Forward Voltage Vf = 2.2 volts
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max pin Current If = 20 mA
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max LED Current If = 20 mA
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minimum current limiting restiance R = (Vs - Vf) / If = 55 Ohms
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```
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It is safe to use more resistance.
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Calculating the resistance for the Teensy LC 5 mA pin is left as an exercise.
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Through-the-hole resistors have color coded bands that indicate resistance value.
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https://en.wikipedia.org/wiki/Electronic_color_code#Resistor_color-coding
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Exercises
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---------
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1) In this exercise you will calculate the minimum current limiting resistance needed for your output pin and LED.
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For your micro controller, find:
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* Supply Voltage coming out of the output pins
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* Current (mA) capacity of the output pins
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From your LED's datasheet, find:
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* Forward Voltage
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* Continuous Forward Current (mA)
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Calculate the minimum resistance needed for your LED and Supply Voltage.
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There are several "LED current limiting resistor calculators" on line.
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<br>
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<a rel="license" href="http://creativecommons.org/licenses/by/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by/4.0/88x31.png" /></a><br /><span xmlns:dct="http://purl.org/dc/terms/" property="dct:title">keybrd tutorial</span> by <a xmlns:cc="http://creativecommons.org/ns#" href="https://github.com/wolfv6/keybrd" property="cc:attributionName" rel="cc:attributionURL">Wolfram Volpi</a> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International License</a>.<br />Permissions beyond the scope of this license may be available at <a xmlns:cc="http://creativecommons.org/ns#" href="https://github.com/wolfv6/keybrd/issues/new" rel="cc:morePermissions">https://github.com/wolfv6/keybrd/issues/new</a>.
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