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


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.

  • \$\begingroup\$ With a microcontroller there are almost no boundaries in terms of blink pattern, etc. But maybe a MCU is overhead here.. How do you wish it to blink? Which at which frequency and duty cycle? \$\endgroup\$ – stowoda Oct 23 '18 at 10:16
  • \$\begingroup\$ Dear Stowoda, thanks! I want just a normal blinking, like the one in your car, with like a 0.5 second between blink. What do you recommend? thanks! \$\endgroup\$ – Stonemarten Oct 23 '18 at 12:42
  • \$\begingroup\$ @Stonemarten If you want an MCU board you could use this : ebay.co.uk/itm/… \$\endgroup\$ – F.Ahmed Oct 23 '18 at 17:59
  • \$\begingroup\$ Stonemarten, for exsample a en.wikipedia.org/wiki/Multivibrator or maybe a NE555N. There are many solutions to to let a LED blink. \$\endgroup\$ – stowoda Oct 24 '18 at 5:27

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.

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  • \$\begingroup\$ thanks Misunderstood!!! Sorry for the basic questions but I'll put 6 leds in parallel, and in the simulator I put that number... Do I need to consider them as single unit even if they share the current? \$\endgroup\$ – Stonemarten Oct 24 '18 at 5:26
  • \$\begingroup\$ the 5.5v come from an other output (for the ESP) because that chassis doesn't have the light OEM set up. This is what I've measured, the car is powered by 4 AAA batteries \$\endgroup\$ – Stonemarten Oct 24 '18 at 5:29
  • \$\begingroup\$ thanks for recommend the trinket! is one fine for all the leds? I need the blinking of 4 of the 6, not all. Since it is 3v, do I need to add a resistor to reduce the input? is it not a waste of energy? \$\endgroup\$ – Stonemarten Oct 24 '18 at 5:31
  • \$\begingroup\$ I'm using 2,550mAh Ni-MH Panasonic evolta black as batteries, not too expensive (here in Japan) and last around 30 minutes. Thanks a lot!!! \$\endgroup\$ – Stonemarten Oct 24 '18 at 5:37
  • \$\begingroup\$ There is a 5V trinket version. A resistor would not save energy. You would need a switching regulator to reduce the voltage efficiently, but that might add too much weight because it requires an inductor. I thought you may race them. Then the lithium would give you an advantage, \$\endgroup\$ – Misunderstood Oct 24 '18 at 5:42

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.

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  • \$\begingroup\$ I think you may have used the Vf off the schematic rather than the link. \$\endgroup\$ – Misunderstood Oct 23 '18 at 18:11
  • \$\begingroup\$ No I used the link \$\endgroup\$ – Damien Oct 24 '18 at 2:45
  • \$\begingroup\$ I was just wondering why you have different value than me. Any ideas? \$\endgroup\$ – Misunderstood Oct 24 '18 at 2:55
  • \$\begingroup\$ @Misunderstood I do not know what you considered but for a start you have 2 values while there are 3 LEDs, then you use 20mA while the spec provides different range for different LEDs. I used minima and maxima for the leds to stay withing the safe zone. \$\endgroup\$ – Damien Oct 25 '18 at 3:16
  • \$\begingroup\$ @Misunderstood I added the table with the numbers on the answer. \$\endgroup\$ – Damien Oct 25 '18 at 3:45

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.

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  • \$\begingroup\$ OP is not questioning that. The leds are in parallel with current limiting resistors per led, so the issue with parallel leds is not present here. This does not answer the question. \$\endgroup\$ – Passerby Mar 18 at 2:55

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