Blink an LED?

[BrownPanick]

Not really a DIY question, but it might be useful to others.  I've got a chorus pedal that I am modding with true bypass and setting the LED to blink based on the "Rate" setting on the pedal.  What I'd like to do instead of blinking the light is to put in a bicolor LED.  The particular LED I have has two legs and glows red when current is flowing one way and green when it is flowing the other way.  AC voltage applied to it causes it to appear yellow.

Ideally I'd like to have it go between yellow and red, but red->green is fine.  I guess what I would do is use the power that is going from +1.8v to 0v based on the rate to control the frequency of a polarity shift of current going through the LED.  Does anyone know of an IC that will do that or a relatively easy method of doing this with discrete components?  There's not a ton of room to build a separate controller board, so I may just have to stick with blinking, but I think switching from red to green or yellow would clearly make my guitar sound better. 

Thanks in advance!

[Gurner]

The easy way to do this is with pwm ...send the bi-colour a 100% duty cycle & your LED will be red, send a 50% duty cycle it'll look yellow(ish) , send it 0% duty cycle it'll look green. Sweep the PWM duty cycle between 50% (yellow) and 100% (red) at a rate that's proportional to the rate on the pedal & you've satisfied your requirements. With a PIC, this is a piece of p1ss ...using analog, you'll likely end up with a high component count. Does an custom IC exist that would suit? Dunno ...but I doubt it.

[Electron Tornado]

Are you doing this with the status LED or a separate one?

On which make/model of chorus are you doing this? I've added a separate rate LED to some pedals, but not a bicolor LED.

[senko]

Find a better LED if the current one isn't working.  That'll save you a boatload of time that you would have been spending on a workaround. 

So, the AC YELLOW just seems to be a blend of RED and GREEN.  That makes sense.  If you want RED and YELLOW to be your two values, I'd suggest using a 555 timer *maybe*.  This is all theory and thought without trial, but an Astable 555's output would be a YELLOW, and a *maybe* a 0V output would be RED.  When your signal goes high (1.8v), there will be circuitry (buffer the signal) that will enable the 555 to a free-running 50% duty cycle output, resulting in YELLOW (AC signal), or a signal that drives the 555's RESET signal low, therefore a 0V output.  I just thought of this, so there are probably holes in this.

I made a pic, though. 
Maybe that will make what I am picturing clearer...  SWED
 

[Jdansti]

Beware that if you're successful, you run the risk of introducing clicking noises to your output depending on the method you choose. I've played with enough 555s to know that they are very difficult to shut up. A 555CMOS might be better than the standard 555, but I've had problems with them too.

If I may offer a suggestion that I used on a computer to send two colors (white and blue) through one hole (one color for the power and another for the hard drive):  Instead of having an LED bezel or the tip of an LED sticking up through the enclosure, design your enclosure and artwork so that the light will shine up through a small hole in the enclosure and shine through the artwork medium that is covering the hole. Imagine a piece of paper covering a hole with a light underneath the hole. This gives the added effect of having a nice smooth surface that lights up instead of something protruding.

To get the alternating yellow and red effect, wire a red LED so that it is always on when there is power (see option #1 below). Wire a green LED to the LFO.  Hot glue both LEDs to the inside if the enclosure so that they're both shining up through the hole. As the light from both LEDs shines up through the surface of the artwork, you'll see alternating red and yellow light. (Test this before modifying a good enclosure!!!!!!!!! )

Options/Tips:
1) The red LED can be wired so that it is always on when there is power, or only on when the effect is active.
2) Use a trimmer pot on each LED instead of a fixed resistor so you adjust the intensity for the best yellow effect.
3) You'll get better color mixing by using a light pipe to transfer the light from the LEDs to the surface.  I haven't found any through hole LED light pipes that combine the light from two LEDs, but a 6mm tube like this one might work: https://www.sparkfun.com/products/10695, or else scavenge one and adapt it, or make your own from clear acrylic. You also might be able to make a light pipe by overfilling the hole with hot glue and then sanding and polishing the extra material until it is flush with the top of the enclosure and very smooth.

[BrownPanick]

Thanks for the tips!  One thing I thought of right after I posted this question is that the supply voltage going to the LED is 9v, not 1.8 (it's got a voltage limiting resistor further up the chain, attached to the LED).  Not sure if that matters or not.

Gurner - the mod I was looking at took the status light (effect on or off) and made it so the indicator blinked at the frequency dictated by the rate control.

senko - Thanks for the schematic.  I'm going to run up to the store to buy a 555 or two to play around with.  I was expecting to actually have to use something like this, and I think there's enough room to fit something like this in the enclosure assuming it works.

Jdanti - This sounds like it might have potential.  All I'd have to do is run the constant 9v+ along with the up/down 9v+ supply.

I'm not sure why I was making this so complicated.  I started thinking about it and it now seems that all I have to do is grab on of the RGB LED's and use that.  It has a common cathode which will go to ground, run the 9v+ to the red anode, and then run the up/down signal to the blue anode (purple will look cooler than yellow). 

[Jdansti]

Keep in mind that my idea with the two LEDs is theoretical at this point. No money back guarantees!

Are you familiar with using current limiting resistors to avoid burning out LEDs?  You'll be ok with 9v on an LED as long as you limit the current. Straight 9v on an LED = crispy critter. .  In general, you're safe with at least 680Ω, but this usually results in a painfully bright output. A lot of people use 1.4k, but it really depends on the LED as to what the best resistance is. This is why I recommended using trimmer pots for the current limiters instead of fixed resistors. Place a cheap 10k trimmer in series with EACH LED and adjust them until you're satisfied. It's generally a good idea to have separate current limiters for multiple LEDs.

[duck_arse]

I'm glad you've come up with something simple, brown. my mind started whirring with 555's driving transistors and inverters and all sorts of complexications. my standard operating mode really.

in my experience, the bi-colour leds have a much lower light output than single colour superbrites. I go blind with anything less than 10k on mine, and I usually go higher still just to reduce battery consumption. it seems wasteful to me when the led uses 10 times the current of the audio section ....