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Created July 20, 2013
Last modified July 20, 2013
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Crude linear or log signal amplitude to LED brightness conversion.


Assuming Audio outputs are low impedance (< 1k) then if audio signal is at approx 0V with zero signal then V(base) = Q2 Vbe above 0V i.e. approx 0.6V.

Hence Q2 wired as diode cancels Q1 Vbe drop.

Therefore even small signal swings above ground that are << Q1 Vbe, start to turn Q1 on.

Q1 acts as voltage to current converter with: iLED = V(base)/R2

Remembering that V(base) is V(in) + Vbe then V(emitter) is approximately equal to V(in).

Thus LED brightness is approximately linearly related to audio amplitude.

For linear amplitude to brightness conversion, R3 = 0R (i.e. it's a piece of wire!).

Note that perceived brightness is on a roughly log scale so linear actual brightness may not be the best way to represent the signal amplitude.

For a very crude log scale conversion, set R2 = 0 and R3 = 100R.

Simulate > DC Sweep > Run DC Sweep

Simulate > Time Domain > Run Time-Domain Simulation


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