Optical Delay Dynamic RAM (ODDRAM!)

[jpanhalt]

Won't 4 hours, or even 30 minutes, be a bit long to show the bytes changing?  The BB's were much shorter.  So far as I know, there is no way to reversibly quench phosphorescence.

John

[jamesglanville]

I think you could skip the hall sensor:

You just need to center on a glowy spot, so turn the light on for a second, then off then rotate back and forward to find the maximum brightness

[wobbly]

Won't 4 hours, or even 30 minutes, be a bit long to show the bytes changing?

Don't get the wrong end of the stick, mate.  The half-life is only (I'd estimate off the top of my head) 1 minute (the dynamic range of the human eye is HUGE).  Ignore the fact that the eye can detect a state retained for 4-hours in a little bit of plastic.
My sensor can do a MUCH better job on a MUCH SHORTER time scale (tens or hundreds of milliseconds).  I know this for a fact, I can see the frequency of the PWM output change smoothly and rather quickly in real time on my scope.

Consider the size of the wheel... 40 bits.  AND it takes time to charge up each bit as well!  We have time to burn!

People are apparently neglecting the other variables we have under our control:

*Charge time -- arbitrary down to the microsecond.
*Charge intensity -- a UV LED running at 30 mA.

By definition, the brightness change will be most easily detectable when the brightness gradient is (falling) steepest.  That is immediately after charging!

If my eye can see a state held for 4 hours, then a closely-coupled sensor should be able to do the same after 1 minute or however long the wheel takes to turn (less than that)!

Maybe I can read a bit, advance the wheel and charge a bit and advance the wheel again in less than 1 second.  That's 40-ish seconds per revolution, without data loss.  In a fairly dark room I suppose.

[wobbly]

I think you could skip the hall sensor:

You just need to center on a glowy spot, so turn the light on for a second, then off then rotate back and forward to find the maximum brightness

Where does the glowy spot come from?  The LED might be between bits at startup (e.g. no glowy spot at all, or perhaps two half-glowy spots).   Hence needing calibration.

[PCB.Wiz]

I think you could skip the hall sensor:

You just need to center on a glowy spot, so turn the light on for a second, then off then rotate back and forward to find the maximum brightness

Where does the glowy spot come from?  The LED might be between bits at startup (e.g. no glowy spot at all, or perhaps two half-glowy spots).   Hence needing calibration.

True, but a smarter calibrate could work.
If you drive the led and wobble over 2-3 bits, you always have one bit full bright plus 2 bits of possible fractional bright.
You then sweep across the 3 bits and calibrate to the brightest plateau.

An advantage is this would not need a full turn to seek the Hall trip.

 You could extend this angle calibrate to sensor calibrate too, by drive the wheel the 'wrong way', you can vary the LED energy and choose an operating point of brightness, by reading a few dots as they go past. This locks angle and gain.
 
This could self adjust for daylight/nighttime.

[jpanhalt]

I am familiar with half-life.  An optical filter with an OD of 1.0 (10% transmission, about 3.3 half-lives) appears quite dark.  5 half-lives is 1/(2^5) or about only 3% as bright.  Six half-lives is 1.5% as bright.  The BBs were only a few seconds.  In a dim, not dark room, I suspect you will need about 5 to 6 half-lives for an observer to preserve O's instead of 1's.

[wobbly]

@PCB.wiz,
Yeah that's true.  But I could also build a rocketship to explore Neptune.  In the interests of actually finishing the project I think I will just use a Hall sensor 😊

[wobbly]

I am familiar with half-life.  An optical filter with an OD of 1.0 (10% transmission, about 3.3 half-lives) appears quite dark.  5 half-lives is 1/(2^5) or about only 3% as bright.  Six half-lives is 1.5% as bright.  The BBs were only a few seconds.  In a dim, not dark room, I suspect you will need about 5 to 6 half-lives for an observer to preserve O's instead of 1's.

You are talking about the "observer?
 Do you mean a human?  There is no human in my electronics.  Observation might be done either in person or with a digital camera perhaps.

People, please give me a chance to build this thing before finding obstacles for it.  I'm learning too!

[Kleinstein]

Changing the time for charging or LED intensity should not have much effect on the speed. The problem is reliably seeing the difference between a dot charges 1 turn and 2+3+4 turns ago. This wants enough (e.g. 1 half life, as 1/2+1/4+1/8 is already quite close to 1) of a decay for one turn. So it gets tricky with less than 1 half life. If really needed one could gain a little from using less power for refreshing a bit than writing a new bright dot, but this compicates things.

One the other hand one wants enough light left, like no more than some 6 half lifes to get enough light left even with relatively short charging time.

The intesity may not follow a strict exponential law, but could be a bit stretched from a mix of lifetimes.

[Phil1977]

Yes. The time constant of the decay (or the half-life-time) definitely need to be in the same order of magnitude than the period time of the wheel.

If the decay time is too high, you can't reset bits.

If it´s too low, you have little SNR for readout.

I´d set the threshold for readout in a way that a visibly lighting dot is recognized by the photodiode. Afterwards I would try to find two critical speeds of the wheel:

- V1: The minimum speed when an activated lighting bit just triggers the photodiode with its next revolution
- V2: The maximum speed when an activated lighting bit just not triggers the photodiode after two revolutions

The average (V1+V2)/2 should be a good starting speed for operation...

[3roomlab]

acoustic delay line memory ?

[wobbly]

Not had time to do very much today, but I did get the 8-tooth gear made (see pic), which will make the maths a bit neater in the firmware.

Ordered some matte-black paint too, which should arrive tomorrow.

Maybe I'll have something worth photographing (videoing even?) by the weekend. 

By the way I keep forgetting to mention - the glow powder I'm using is this stuff: https://www.amazon.co.uk/dp/B0788NLQKZ?th=1
It mixes beautifully with optically clear two-part Polyurethane epoxy resin and works best when in thin layers up to about 3mm thickness.

[wobbly]

@Phil1977, did you develop your multi-channel "Drum" memory version any further?