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Light circuit for RC car

I’m building a small light system for an RC car, this is the schematic:

Circuit

The power source is 5.5 V. These are the LEDs that I am using, KKHMF 100個 3mm LED 発光ダイオード LED電子部品バッグ 5種類 各20個 – two white, three red and one green.

I’ve used an online resistor simulator for calculate them, I hope that they are right

I would like to add a blinking capability to four of them, and I’ve understood that I need a microcontroller.

Here my questions:

  • Are the resistors correct?
  • What microcontroller can I use and how I can connect it?

The car is very small (1/30 scale) so all the parts have to be at very low consumption and very small, able to resist to mild vibration and some shocks.

Electrical Engineering Asked by Stonemarten on November 21, 2021

3 Answers

3 Answers

LEDs in parallel do not share the current unless you buy hundreds, measure them all and group them into piles having the same forward voltage.

Answered by Audioguru on November 21, 2021

Are the resistors correct?

No the resistors are not correct.
Could not read the description only the voltages.
enter image description here
Using the average voltages:
5.5V supply, 3.2Vf @ 20 mA = 115 Ω
5.5V supply, 2.0Vf @ 20 mA = 180 Ω

enter image description here
Source: LED Series Resistor Calculator

all the parts have to be at very low consumption and very small 66 vs 100

Why 5.5V? 3 AAA batteries is 4.5V.
3.3V, if available, will be more efficient.

You should use the brightest LEDs you can find.
The higher the mcd or lumen, the lower the current needed for the desired brightness.
Cannot read the specs on the LED you chose, but for example let's say the green is 100 mcd, 3.2V @ 20 mA.
There is a Cree C503B-GAS-CB0F0792 green LED, 5mm, 53,650 mcd @ 20 mA

To get 100 mcd out of the Cree LED. You would only need 100mcd/53,650mcd x 0.02A = 370 µA
That's 0.000851 watts 0.85 mW vs. 115 mW for 20 mA for a 13,529% less power.

Bottom line, use a bright LED at a lower current.


If you could use surface mount LED: The brightest and smallest: Luxeon Color C Line

What microcontroller can I use

Atmel is always a good choice.
Here is an ATtiny85 from Adafruit Adafruit Trinket - Mini Microcontroller.
Adafruit Trinket - Mini Microcontroller

Uses Arduino IDE
Smaller and lighter than the Arduino Nano.
Nano: 45 * 20 * 3mm 6 grams
Trinket: 27 x 15 x 4mm 1.85 grams

I could not find a smaller lighter LED blinker circuit.
They do exist, just could not find one.

This PCB was a blinking LED for a Logitech mouse package.
The blinker circuit is under the block blob.

enter image description here



Side note: If you are using AAA alkaline batteries, you will do much better with the lithium. The lithium starts with 1.6V, and at 400 mA for 1 hour, holds the voltage above 1.4V. Where the 1.5V alkaline would drop to 1.2V.

If using Li-ion use Lithium Nickel Manganese (NMC) for highest power.

Answered by Misunderstood on November 21, 2021

It seems your calculation are wrong.

To calculate the resistor for the LED you should do:

(VCC - Vled) / Iled.

With the max brightness and the lower diode limit you should use (rounded to E12 serie):

enter image description here

  • white 150 ohm
  • red 220 ohm
  • green 220 ohm

For blinking the LED, you have tons of possible way, MCU is probably what will give you most of the freedom.

A good and simple way to start is to use an Arduino which is made for people to learn easily.

Answered by Damien on November 21, 2021

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