[sdiy] LED driver circuit

[ASSI]

On Saturday 24 November 2012, 12:22:15, David G Dixon wrote:
> I have updated my DIY 2164 4P LPF/LFO circuit with transistor-based LED
> drivers for each of the eight quadrature outputs.  The problem I'm having
> is that four of the LEDs are on at the same time, and I only want two on
> at the same time.  I have changed the current limiting resistor (to
> 49.9k) and now the brightness is where I want it.  However, I don't think
> I have the transistor base resistor sized correctly.  I started with 100k
> and recently increased it to 200k with little or no change in behaviour.

To have only a single LED on on four half-waves (IIUC) you would need to 
shift the switching threshold up considerably, to A*cos(pi/8) to be exact.  
Dividing down the input to the basis of the switching transistor so that at 
that point Vbe becomes about 0.7V works, but degenerating the emitter would 
likely be better (split the current setting resistor).  That makes the LED 
come on more gradually, which you may or may not want — but it helps reduce 
the switching noise from the LED.

An even better solution would be to steer the current to the max and min of 
the eight output phases (making it independent of the output amplitude and 
introducing no switching noise), but that requires a complete overhaul of 
the circuit.


Regards,
Achim.
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