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- Tutorial 5a - indicator LEDs
- ============================
- Keyboards often have LEDs to indicate CapsLock, NumLock, and other states.
- It's one of the first things we check when a keyboard produces unexpected results.
-
- Adding LEDs to the basic breadboard keyboard
- --------------------------------------------
- The breadboard keyboard modifies the basic breadboard keyboard described in [tutorial_1_breadboard_keyboard.md](tutorial_1_breadboard_keyboard.md)
-
- Add components to the breadboard as shown in the picture.
-
- The three clear plastic cylinders are red and yellow LEDs.
- Each LED is in series with a 4.7k Ohm current limiting resistor.
- LED anodes (the longer lead) are powered by pins 16, 17, and 21.
- LED cathodes (the shorter lead) are connected to a grounded terminal strip.
-
- !["back"](keybrd_5a_LED_on_uC/back.JPG "back")
-
- keybrd sketch with LEDs
- -----------------------
- The [keybrd_5a_LED_on_uC.ino](keybrd_5a_LED_on_uC/keybrd_5a_LED_on_uC.ino) sketch will run on the above breadboard keyboard.
-
- LED forward voltage
- -------------------
- Forward voltage is the voltage used by the LED.
- Forward voltage is published in the LED's datasheet.
-
- Most blue and green LEDs have about 3.3 forward voltage, which does not reliably illuminate on Teensy LC's 3.3 volts.
- Most red and yellow LEDs have around 2.2 forward voltage.
- So use red and yellow LEDs on Teensy LC.
- Some low-current LEDs have 1.6 forward voltage.
-
- LED brightness
- --------------
- The amount of current going through an LED is directly proportional to how bright it appears.
- By picking the correct resistor, you have full control over how bright the LED appears.
-
- Less resistance makes the LED brighter.
- Too little resistance will burn out the LED.
- The current supplied to an LED should always be limited by a resistor or some other device.
-
- LED current-limiting resistor values
- ------------------------------------
- In the section we will compute the minimum resistor value for maximum LED brightness.
-
- This formula calculates the minimum resistance:
- ```
- output-pin Supply Voltage Vs
- LED Forward Voltage Vf
- Forward Current If
-
- From Ohm's Law, minimum current limiting restiance R = (Vs - Vf) / If
- ```
- Voltages and current capacities are published in datasheets and sometimes pinout diagrams.
-
- Teensy LC output-pin capacities are:
- * four 20 mA pins (5, 16, 17, 21)
- * nineteen 5 mA pins
- * Teensy LC on-board LED is on pin 13.
- It has a current-limiting resistor on the board, and does not provide enough power for another LED.
-
- Continuous Forward Current of TT Electronics OVLLx8C7 red or yellow LED is 30 mA.
-
- Plug the numbers into Ohm's Law:
- ```
- output-pin Supply Voltage Vs = 3.3 volts
- LED Forward Voltage Vf = 2.2 volts
-
- For Forward Current, use the smaller of:
- current capacity of output pin If = 20 mA
- continuous Forward Current of LED If = 30 mA
-
- minimum current limiting restiance is R = (Vs - Vf) / If = 55 Ohms
- ```
- Add a safety margin for resistor tolerances (1%, 2%, 5%, 10%), and round up to a standard value.
- http://www.rfcafe.com/references/electrical/resistor-values.htm
-
- ```
- 55 Ohms + (55 Ohms * 10%) = 60.5 Ohms < 68 Ohms
- ```
- It is safe to use more resistance.
-
- Calculating the resistance for the Teensy LC 5 mA pin is left as an exercise.
-
- Through-the-hole resistors have color coded bands that indicate resistance value.
- https://en.wikipedia.org/wiki/Electronic_color_code#Resistor_color-coding
-
- Exercises
- ---------
- 1) In this exercise you will calculate the minimum current limiting resistance needed for your output pin and LED.
-
- For your microcontroller, find:
- * Supply Voltage coming out of the output pins
- * Current (mA) capacity of the output pins
-
- From your LED's datasheet, find:
- * Forward Voltage
- * Continuous Forward Current (mA)
-
- Calculate the minimum resistance needed for your LED and Supply Voltage.
-
- 2) See how bright an LED can get.
- In the above breadboard keyboard, replace an LED's 4.7k Ohm resistor with a 68 Ohm resistor.
- Make sure use a controller pin with a 20mA capacity (Teensy LC pins 16, 17, 21 have a 20mA capacity).
-
- <br>
- <a rel="license" href="https://creativecommons.org/licenses/by/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://licensebuttons.net/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="https://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="https://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="https://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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