simulate this circuit – Schematic created using CircuitLab

In this schematic I made I'm trying to light up a a series of LED's but not in sequence "not yet" what I want is to raise the start of with the lowest voltage a LED can operate on which will be sorta dim but this is ok and expected "im counting on that" and as you raise the voltage the second LED will light up BUT (here is where my question coming in!) not light up the first one so on and so on, maybe I want to turn LED #6 and NOT LED's 1 through 5 ?!

And yes I know I'm using a voltage divider and thats because I dont want all LED's to light up with a the same wavelength or same intensity

enter image description here

Using this schematic I'm not sure how to incorporate the photo-resistor so I cant include it in there but the idea is that when a particular LED lights up above a threshold the photo-diode will help meet the condition in a algorithm

Before anyone starts to yell at me talking about that there are other "conventional ways" I KNOW and while practical they might be doing it the conventional way is only practical for that application not mines specially where the objective is to use less I/O pins on the given MCU. So if there is and I know theres gotta be a way to make this happen given the fact that we introduce the right components I know is possible, I can do this software wise but not hardware wise and this is why I'm here looking for help.

  • 1
    \$\begingroup\$ It's worth pointing out that wavelength is not dependent on voltage or current, only LED type/construction and temperature (for the most part). \$\endgroup\$ – uint128_t Feb 19 '16 at 3:41

You need a voltage comparator. Or multiple voltage comparators. Ideally, you'll just use a linear (or logarithmic) led driver like the LM3914 in dot mode. Which is just a bunch of comparators chained together.

enter image description here

  • \$\begingroup\$ thanks for the prompt reply but i updated a "better?" version of what i need using circuitLab, like i said i needed to add a photodiode but im not sure i even have it facing the led correctly im really new to this programs and by voltage comparator you mean a op amp or a 555 timer? \$\endgroup\$ – Frank Feb 19 '16 at 2:24
  • \$\begingroup\$ @frank a timer is not a comparator. Op amps can be wired as comparators, but there are dedicated comparator ICs that can be used. I have no idea what you want to do with those photodiode \$\endgroup\$ – Passerby Feb 19 '16 at 3:13
  • \$\begingroup\$ can you please confirm that this is what you mean i need please ? ebay.com/itm/… \$\endgroup\$ – Frank Feb 19 '16 at 5:13
  • \$\begingroup\$ @Frank yes, that's supposedly a lm3914, not sure if its real or counterfeit, but that's a linear led bar/dot graph IC. You can google or youtube lm3914 for plenty of examples on how to wire it and how it looks in dot mode. youtube.com/watch?v=gxio_dP_ARg \$\endgroup\$ – Passerby Feb 19 '16 at 5:17
  • \$\begingroup\$ thanks, is because i keep seeing the LM3917N and its throwing me off cuz you said the LM3917 not 3917N \$\endgroup\$ – Frank Feb 19 '16 at 5:42

Not the answer you're looking for? Browse other questions tagged or ask your own question.