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USB interface with leds

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alexan_e

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It is an opamp (operational amplifier), check post #16
R1=5K ohm
R2=1K ohm
R3=10 ohm
and the LED resistors are 1 ohm (other values can be used too), these resistors are used there to ensure that the current is equally shared between the leds, not to limit the current.
The current that will go to the leds is 80mA so if it is shared equally to 8 leds each one will get 10mA, if you want more or less then R1/R2 or R3 have to be changed.

Alex
 

fael097

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right,
so, the opamp has a specific value, like ohms or something, or I just buy a 8 pin one?
how about the pot, is it ok to use a 10k pot?
 

alexan_e

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A 10K pow will work fine, connect the two ends to +5v and gnd so that the center tap gives 0-5v depending on the position.
The opamp in an integrated circuit , it is described by a model number , the one I have used is LM358 (LM358 - Low Power Dual Operational Amplifier) which is cheap enough and is widely available but many others can be used.

The transistor I have used is BD139 but you can use a much smaller one like 2n2222 because the powers consumption on the transistor will be very low (less than 1/4 watt)

Alex
 

fael097

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right, the transistor. I thought it was the potentiometer in the scheme, misunderstood the symbol.

where'd u put the pot in the scheme?
 

alexan_e

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I have made you a new schematic to see the components better

led_constant-current2.gif
I have also added R11

Alex
 

fael097

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right, thanks. I got a BD139 instead of the 2n2222a, any problems?
also am I really going to need the R11?
 

alexan_e

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You can use BD139 or any other available NPN transistor without problem
You can remove R11, the circuit will work fine.

Alex
 

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