Adding LEDs to your Micropolis Module
BtbB-2014_Lighting.pdf
(Lighting Effects for LEGO MOCs presentation, Bricks by the Bay 2014)
Adding 5 volt power under our modules will make it easy to add LEDs for lighting, special-function LEDs (flickering flame, blinking, etc.), and even power small motors for movement.
| In the most simple method, a single rectifier diode can drop the 5 volt supply down to 4.4 volts. White and blue LEDs like to operate at 4.5 volts, so this is sufficient. You could get away without current limiting resistors, which would simplify wiring. |
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However, adding current limiting resistors is a Real Good Idea. If an LED goes bad, you don't want it to pull too much power (because then the wires might get hot, and LEGO could melt). Adding the resistor(s) can also preserve the battery capacity (so that aging parts do not draw more than you expect, and you get the longest run-time when you are running on battery).
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Ohms Law
This describes the relationship between Voltage (E), Current (I), and Resistance (R). If you know two values, you have the calculation to determine the third.
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E = Volts, I = current in Amps,
R = resistance in Ohms
Volts / Current = Resistance
Volts / Resistance = Current
Current * Resistance = Volts
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"E over I * R"
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Using this formula, with 5 volts as our power source, and using the values for common resistor values, we made a table that shows how much current will be passed by each resistor value.
In the case of a red LED, at 2.3 volts and wanting 15 ma for power, we would use a 330 ohm resistor. The resistor drops part of the voltage as it's limiting the current, so that the LED doesn't burn out.
Values with the orange highlight should be 1/2-watt. The rest can be 1/4-watt resistors.
|
|
Resistor
(ohms)
|
current
(ma)
|
Resistor
(ohms)
|
current
(ma)
|
| 1200 |
4 |
100 |
50 |
| 1000 |
5 |
82 |
61 |
| 820 |
6 |
68 |
74 |
| 680 |
7 |
56 |
89 |
| 560 |
9 |
47 |
106 |
| 470 |
11 |
39 |
128 |
| 390 |
13 |
33 |
152 |
| 330 |
15 |
27 |
185 |
| 270 |
19 |
22 |
227 |
| 220 |
23 |
18 |
278 |
| 180 |
28 |
15 |
333 |
| 150 |
33 |
12 |
417 |
| 120 |
42 |
10 |
500 |
Pick your resistor value, based on the current you want for your piece.
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