[sdiy] THAT 2155 on sale

Pete Hartman pete.hartman at gmail.com
Fri Apr 10 17:17:59 CEST 2020

On Fri, Apr 10, 2020 at 8:43 AM Oren Leavitt <obl64 at ix.netcom.com> wrote:

> I have an idea for "quick matching" photo cells.
> Basically one op-amp - a comparator with hysteresis (like that of a basic
> LFO)
> 1) Put the photo cell side in a resistor divider and connect to the
> comparator input.
> 2) Connect comparator output to to LED side thru appropriate current limit
> resistor.
> The circuit should self oscillate at an amplitude/frequency determined by
> the LDR's light/dark resistance and lag time characteristics. Plug
> different photocells into same circuit and sort them by matches in
> amplitude/frequency shown on oscilloscope.
> An idea..

I rather like it.  I've occasionally started putting on paper ideas for
using an Arduino where an analog output drives the LED and steps from 0 to
5V (with appropriate current limiting of course) and an analog input reads
from a voltage divider including the resistive side.  The idea being to get
a graph.  But then I got stuck on how to display the graph meaningfully
without going full blown OLED.  Considered simply sending the output to the
serial port and letting a computer read the details.  And that really omits
the time factor, although I could do a 0V -> 5V step and observe the time
it takes, I guess.  Overall, too complicated, so I never tried to implement

I do have a bunch of Silonex Vactrols that would be nice to characterize....

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