[sdiy] THAT 2155 on sale

[Tom Wiltshire]

> On 10 Apr 2020, at 16:17, Pete Hartman <pete.hartman at gmail.com> wrote:
> 
> 
> 
> On Fri, Apr 10, 2020 at 8:43 AM Oren Leavitt <obl64 at ix.netcom.com <mailto: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 it.  
> 
> I do have a bunch of Silonex Vactrols that would be nice to characterize….

Why not give the Arduino a DAC for output and then use that to drive X/Y channels on your oscilloscope. The X channel only needs to be a simple scanning ramp (so a basic incrementing count to the DAC) and then the Y channel displays your response value.

Tom

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