I have to blame my pal Danno for this. He wanted a 70's vintage "color organ" one of those things that used to be a box that had Christmas tree lights on the inside and that frazzled plastic they put over fluorescent light fixtures on the front.
I figured replace the Christmas tree lights with LED's and call it a day. I have to make my life more complicated though.
I started thinking of the color changing LED's so I got a pack of the fast and slow changing ones. They are pretty neat. They have a Vf of about 3V and I have a 15V 1A power supply, 5 of the LED's in series and about 1.5K gives them a nice brightness. If you run them at even 10 ma they are way too bright and IMHO the colors are not as deep.
The problem with these is they are on all the time and while the color chaning is cool, the static placement of the lights is not. But, if I could sequence the lights, perhaps make different shapes or like a circle expanding out, that would be cool.
My first thoughts turned to a micro controller, but that would not be happy with my 15V supply and would probably not be happy driving the LED's directly, and that would lead to transistors or drivers or what not. More goop. My second thought tuned to either a CMOS based inverter acting with an RC as an oscillator or god forbid, a much hated 555. This could drive a CMOS decimal counter. Only this has a lot of parts and if you want the outputs to build up to all 10 being on at once it would also require flip flops. Urg. Can my life never be simple?
Than it hit me. Dead assed simple. An old National linear chip: The LM3914. This was designed to drive bargraph displays. I was able to get 10 of them for under $2 on eBay. This will run off of up to 15V, this has a constant current drive for the LED's (just one R to set the current for all 10 LED's), this can do bar or dot displays (just open one pin up), it has an internal 10 step divider and the hi and low sides are pins, it can take up to Vcc -3V. The input to the comparitors is a high impedance. Hmm.
My first test with the new chip was 10 red led's and a circuit right out of Nationals app notes for the part. They have a voltage divider to make it respond full scale to 5V. I did not have the exact resistors. When I first turned it on, all hell broke loose. Everything flashing and strobeing, but I did not have the input tied to anything. I tied it to ground and poof all the lights went out. So far so good. I hooked up a pot between ground and my +15V and put a DVM on the wiper and verified that it was at zero and I could sweep it up from there. Cool. I hooked the input to the pot and I was happy to see the LED;s coming on in sequence. I was also happy to see when I opened up the dot/bar pin the display type changed. Cool beans.
So, the next iteration was a 100Uf cap (rated at 16V, yea getting close there...) and a 1 meg resistor between V+ and ground with my dvm and the input to the bargraph across the cap. Powering on, no lights, but as the voltage made it's way up near .5V, the first light came on. Than the second, than the third, all the way to #10. Yahoo. Moving the V+ side of the resistor on the RC to ground reversed the process. Too cool.
Next, I switched out the LED's to the color changers. I was wondering if they would have too much digital noise and cause issues, and I had wanted to start simple for 2 reasons. One is it is just a good idea, and two, I was mildly questioning if the chips I bought actually had any guts in them at the price I paid. I was very happy they passed the simple test...
When I powered up the fast color changing LED's all hell broke loose. Shit. I tried bypassing the LED's like they suggested in the app note but with no luck. ...Read more »