Does anyone know why the schematic here has 4 diodes in series? Why
four? Wouldn't one be just as good?

http://www.paia.com/ProdArticles/drumsens.htm

Dave

In article
<77f5667d-80be-47f2-a0b1-e9690eb5515d@d21g2000prf.googlegroups.com>,
Kasterborus <kasterborus@yahoo.com> wrote:

> Does anyone know why the schematic here has 4 diodes in series? Why
> four? Wouldn't one be just as good?

Review the concept of a diode drop. Without wasting too many cycles on
analyzing the circuit, it looks like use is being made of getting 3
forward diode drops from the 5v supply to the junction of D3, D4 (which
is also light emitting) and D5.

--
Cats, coffee, chocolate...vices to live by

On Feb 10, 9:58 am, Kasterborus <kasterbo...@yahoo.com> wrote:
> Does anyone know why the schematic here has 4 diodes in series? Why
> four? Wouldn't one be just as good?
>
> http://www.paia.com/ProdArticles/drumsens.htm
>
> Dave

It's 3 diodes in series (D1, D2, and D3).
The simplistic explanation is that roughly 3V is required at the point
where D3, D4, and D5 are connected together. Each diode has a
roughly 0.7 V drop when conducting. With just one diode instead of 3,
there would be (5 - 0.7) or 4.3 V instead of the 3V they wanted.

Mark

"Kasterborus" <kasterborus@yahoo.com> wrote in message
news:77f5667d-80be-47f2-a0b1-e9690eb5515d@d21g2000prf.googlegroups.com...
> Does anyone know why the schematic here has 4 diodes in series? Why
> four? Wouldn't one be just as good?
>
> http://www.paia.com/ProdArticles/drumsens.htm

The circuit has an added feature that the web page describes as:
"The LED D4 provides a power indication and also serves as an indicator that
the sensitivity of the sensors has been set properly by blinking if the signal needs
to be turned down."

To create this feature, they are driving the LED by two different means. With no
sensor input, the LED is only driven by the path from +5v through diodes D1, D2,
and D3 (then through the LED D4 and finally the 100 ohm resistor to ground).
As someone else has indicated, D1 to D3 create a 2.1 volt drop so that the
voltage at the left side of D4 will be about 2.9 volts.

When the sensor input signal is large enough, the output of the LM324 will also
start to drive the LED via D5. This occurs when D5 is forward biased. This
requires that the output of the LM324 is at least 3.6 volts (2.9 volts plus 0.7 volts
across D5). So a large input to the LM324 (and thus a large output) will cause
the LED to get brighter. They need D1, D2, and D3 to drop the 'non bright'
voltage at the LED so that there is some room for the LM324 to drive the LED
for the 'bright' output.

A final note: The LM324 is set up a unity gain amplifier so its output is simply
equal to the signal from the sensitivity adjustment potentiometer R3.

Great, that helps a lot - thanks for the explanations