E1: Driving LED’s without resistors

This is part 1 of a 4 part series on the design and creation of a digital dice that can be used to teach children about electronics and/or programming. The design and training material will be available for free once it’s done.

While I was working on this “project”, I questioned everything and I found myself some unexpected answers. Let’s start with these questions and their answers. It allows you to determine whether or not you are interested in this series at all.

  1. Do we always have to use resistors when we use LED’s in battery powered devices?
    Answer: No.
  2. Is it difficult to program your Atmel chips without anything Arduino?
    Answer: No.
  3. Can we reduce the standby power consumption of an ATTiny by so much, that it’s cheaper to not add a power button and keep it on standby forever?
    Answer: Yes.
  4. Can I use basic components to create a device, that allows me to measure current draw in nano amps very easily?
    Answer: Yes.

I will use these questions as the guideline for this series. In every post I will try to cover one question and provide you with a more detailed answer.

This week it’s about driving LED’s without resistors.

Context

The device has to be cheap and not contain components that we do not need. If we can do without resistors, I can cut on both the costs and the complexity. Now can we?

Answer

Yes we definitely can, as long as we select the right LED. The device is powered by a CR2032 3V battery. Thus, if we select a type of LED which has a Forward Voltages (Vf) above 3V, we should be safe.

When we look at the following graph from www.electronics-tutorials.ws we can easily see that a green coloured LED would fit our project perfectly.
I/V Characteristics Curves showing the different colours available

The data sheet for the ultra bright green LED that I had lying around and current measurements I did, supported the graph. I never measured more than around 13 mA of current flowing at 3V.

The ATTiny that I had in mind to control the dice can drive 40mA per output pin and a total of 200mA, so we can run 6 LED’s perfectly fine within the specs and without resistors!

That’s it for this week. Next week I’ll cover programming an Atmel chip without Arduino which is a lot easier than I thought.

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