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Series resistor calculation

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captainate

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I apologize in advance, I am a noob. I can't help it.

I need to pick these resistors up ASAP, and I don't know how to calculate them in PARALLEL (edit). The schematic I have uses different resistor values than I could get, and is not very "in depth". Original "schematic" is included as attachment if anyone's interested.

Also, DC input is 12V

**broken link removed**
 

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  • AB_switcher.pdf
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I apologize in advance, I am a noob. I can't help it.

No problemo; this is a place where noobs can ask questions. No shame in that.

I need to pick these resistors up ASAP, and I don't know how to calculate them in PARALLEL (edit). The schematic I have uses different resistor values than I could get, and is not very "in depth". Original "schematic" is included as attachment if anyone's interested.

Also, DC input is 12V

**broken link removed**

Not sure what you're asking. I looked at your PFD, and it only shows 2 resistors, not in parallel with each other. (They're apparently being used as current limiters for the LEDs.) Too bad they didn't provide a proper schematic (circuit drawing) so we could see what they're up to.

It looks like what this box does is switch between 2 musical instrument inputs through the 1/4" jacks, and at the same time illuminate the LED corresponding to that input so you know which one is active. Is that correct? Pretty simple.

OK, figured it out. for the red LED, since you need to "drop" 10 volts, you need a 500Ω resistor (10/0.02): for the other LED, a 435Ω one. This is a non-standard value, so use the next larger standard one (470). These values are not critical. They simply limit the amount of current that can flow through the diodes. So two 500Ω resistors will be fine.

If you can't find these values, get the next larger size or so.

So why does your document give such large values for these resistors?
 
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There are two possible reasons to use a resistor larger than needed to deliver the rated current of an LED.

1) Reduce the brightness of the LEDs if they are used in reduced lighting situations (may be particularly needed for high brightness type LEDs)

2) Reduce the power consumption for battery powered LEDs.
 
Hey thanks for the help, everybody. Carbon, you are right. It's an instrument switcher. With my level of experience (little) I figured it would be a good project to get started with. I followed all the directions and ended up with a faulty pedal.

The channel on the right (red LED) will pass signal when active, but there's some serious level of ground loop hum with it. The left channel (white LED) does not pass signal. Is the schematic wrong? Did I do something bad?

IMG 1

IMG 2
 
You posted a very, very good image. After looking and looking at it everything looks correct but I may have missed something. You are sure the signal in is a single channel that you are switching to either the A or B outputs? You may want to look at the wiring of your input plug to make sure it matches up with your input jack connections. Again, squinting at images isn't my forte but it looks correct.

Ron
 
Ron, thanks for the compliment. After taking the first one with just flash, the shadows were confusing so I added a second light!

Also, I built it with the intention of using it in reverse, with the two channels being inputs, so I can play my Rhodes and guitar through the same rig with the flip of a footswitch. However, anticipating this might not work that way, I tried it in its intended use as a splitter, which had the same problem. I think there is just something wrong with the circuit, and I was hoping someone would be able to tell me what!
 
Maybe tomorrow if I get some time I'll put it into an actual schematic form. Nearly as I can see yours looked just like theirs. I won't rule out a design flaw but one would think they would have tested it and used it?

Ron
 
Yeah the place I got it made it seem like it had been used by lots of people. I don't know how I could have screwed it up. The LED's work, and the audio path is about as simple as it gets!!
 
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