|
Projeto de :
www.4qdtec.com
|
|
The third opamp stage gives a logarithmic scale: it does this by means of a 5
stage feedback circuit. The various stages of feedback are switched in by the
LED operating voltages and the diodes and resistors feeding back to the +ve
input os the op-amp.
Clearly the calibration is going to be affected by the LED operating
voltages: green LEDs are higher voltage than reds and high brightness LEDs drop
more voltage than low. However my experience is that any particular make/type of
LED are very consistent in voltage.
This circuit was evolved quite some while ago for an audio mixing desk and it
was put into production. We were at that time trying to find an IC which would
do the job. Once again the circuit using mostly discrete transistors worked out
easier. This was however about 15 years ago and there are newer ICs which will
do the job. However when I designed this circuit these were new and quite
expensive.
© J.Sabino 2003 - 2005