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Joined: Thu Jan 29, 2009 9:21 pm
Posts: 1
Post hello
wanted to start by saying thanks , for making cool stuff. i'm very much enjoying the tubehead i built. it works great, sounds great, and i feel great about it knowing that i built it myself.

last night i put in a blue led for the power indicator and it goes really nicely with the rest of the blue in the knobs and screen printing.
looks extra classy now.

but , its so hard to find some dim blue leds. the radio standard 5v led is still to bright and its the dimmest one i could find anywhere thats a good voltage match.
you have a source i can try ?


Sat Feb 21, 2009 1:31 pm
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Joined: Sun Nov 09, 2008 2:49 pm
Posts: 585
Post Re: hello, LED sub
In the TubeHead, the LED brightness can be varied according to the value of the resistor in its path. For the Power LED this is R2 a 330ohm. For the op-amp Clip LEDs they are Rs 37 and 58, 100ohms.

These resistances set the current flow through the LED for the voltage to them. The Power LED connects to the ac power source and current flows for the 8v or so the LED 'sees' from the 12v transformer. The LED forward voltage drop Vf is subtracted from the applied voltage with the result divided by the resistance for the current flow. For red LEDs the Vf is about 2v so the current through the LED is say, 6/330=0.018A which translates to 18mA. A bit more than is usual for a red LED but probably to cover possible inefficient parts that might be stocked. For a 5v blue LED, the equation is 3/330=0.009A or 9mA. This is about normal, but to get less intensity from the LED, a greater value resistor can be substituted for the 330ohm. Let's try a 470ohm: 3/470=0.006 or 6mA. That would probably do the job. Then for the op-amp Clip LEDs, the most voltage they'd 'see' is about 10v (minus the Vf 2v for a red LED) and with the 100ohms we have 8/100=0.080A or 80mA. High again, but in both the power and this clip indication, the on-time is partial. And driven by the op-amp the voltage source isn't as strong as from a power supply. Fro the 5v blue LEDs in the op-amp Clip circuits, we can try 220ohms: 5/220=0.023A or 23mA. In both instances I just made guesstimate at the resistance but for the best results, try jumpering in various resistances with alligator-clip test leads. You'll want to go incrementally higher for less intensity from the LED. Eventually, there won't be enough current flow through the LED for it to light. If it was the opposite situation, the resistance can be decreased for more intensity, but, where too little current results in no light, too much current results in a burned-out LED.

--sl

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Scott Lee
scott@paia.com


Mon Feb 23, 2009 2:10 pm
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