Let's Make Robots!

LED String

Hey All,

Let me present my next round of noob question to keep you all on your toes....

I'm trying to add AmandaLDR to my next project. Two eyes in front, two on the bottom for edge detection. The problem is that digital outs and space for wires are limited, so I was hoping to combine my SuperBright LEDs into a single output if at all possible.

When I connected the anode of one LED to the cathode of the next LED, only the second LED would work. Ditto for the third of 3, etc.

1. Is that because of the forward voltage of the LEDs?

Based on my positive experience with Oddbot's IR tracker, I though I could try to pull more power using a transistor:


...but with no luck. In fact, they don't light up at all with this approach. So ...


2. Why does this work for IR LEDs, but not regular LEDs?

3. Is 330 too much resitance?


...So I tried using the UNLxxx darlington chip and putting the LEDs in parallel rather than series. They both lit up, but it was a weak output. When I added a third LED, the output was too weak to even use. Ideas? Even 2 LEDs x 2 would work for me, as long as they were bright enough for the AmandaLDR to work.

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I've just posted a tip / walkthrough that will answer most of your questions about using LEDs as your not the only one having troubles. I'll do some more later on transistors, FETs and possibly "H" bridges.
so good to know that OddBot!!  can´t wait to read´em all!!!
Seriously Odd, you rule.

all y'all are experts as far as I'm concerned. I love that I can throw out a question and get three or four good answers in a few hours.

 as far as PNP vs NPN, I don't know the difference (looking it up as we speak). Just grabbed a picture and stuck it on there. What about the 2n222?

It's the fact that you just casually mention stuff like "In reality there is usually about 0.4V drop across the transistor." that never ceases to amaze me. You don't just give an answer to the question. You also give an education.


Thanx Mike, I often think the same of you and other programmers on this site when you casually mention how easy it would be to write an application for something like linking a robot to the net.

The IR LEDs only had a forward voltage drop of 1.2V each so four in series for example would have a combined voltage drop of 4.8V which is less than 6V

Super bright LEDs usually have a forward voltage drop in excess of 3V. Once your combined voltage drop exceeds that of the supply then the LEDs won't light. 

Ted's circuit above is a better approach. Your resistor value will be (6V-forward voltage of the LED)/current through the LED.

For example. If your superbright LED has a forward voltage of 3.4V and is designed to run at 30mA continuous then your resistor value would be (6 - 3.4) / 0.03A = 86 ohms

In reality there is usually about 0.4V drop across the transistor. This varies depending on the transistor and the current being conducted through it. You may want to include this as well although it usually isn't a problem.

(6 - 3.4 - 0.4) / 0.03A = 73 ohms


By the way, you've shown the symbol for a PNP transistor instead of a NPN. The BC549 is a NPN.

post them in parralell