Optical Delay Dynamic RAM (ODDRAM!)

[Phil1977]

Would a laser be any better than a normal LED when shooting point-blank?  I don't know.  At least it's an excuse to buy lasers, so that's fine.

A laser would for sure be easy to focus onto the active bit - but regarding it´s power a usual LED should be more than enough.

In my setup I waste probable 90% of the LED power by a slit aperture. And another 90% by the PWM I have used. And I just used a clear 3mm-LED at 10mA. Anyhow, even at usual indoor daylight the activated phosphorescent dye has been clearly visible.

Do you use any lens with your LED? Even if you have no aperture you would strongly increase the irradiance by any focusing lens. Or just use a laser ;-)

[wobbly]

Do you use any lens with your LED?

Nothing other than the LED's own clear package. 

This is a roughly to-scale image of the LED in relation to the bits...


The orange part is a short bit of brass tube which is a snug fit for the LED.  I reckon that at least 75% of the LED's output reaches the phosphor dots, as a conservative estimate.  Might be as much as 90%.

Perhaps I can find a COB LED with a bit more grunt.

[wobbly]

I now have the TSL235R hooked up and Timer1 taking it's external clock from it.  Works great, and I can get usable readings from it.

As a test just now, I configured the sensor to be read for 1 second at each bit position.  And after each reading I'm sending the data out via the USART to my PC.  I then charged up a few of the bits with a flashlight and allowed them to move past the sensor...

Code: [Select]

Light:9
Light:6
Light:7
Light:10
Light:14
Light:20
Light:14
Light:15
Light:11
Light:10
Light:175
Light:209 <------ :) Brightest Bit passing the sensor!
Light:128
Light:94
Light:47
Light:9
Light:7
Light:7
Light:8
Light:7
Light:6
Light:6
Light:6
I'm quite pleased with this result.  The dynamic range isn't great (it's a 16-bit timer) but I think it is usable.  But this will depend on me finding a more powerful UV LED.  I was charging the bits from the "wrong" side using my flashlight, which does reduce the effectiveness a bit but as a first test I think this is quite encouraging.

[Kleinstein]

The resolution is not very high, but should still be sufficient. One is somewhat limited in the speed to still get at least a little reading for the dark dots.

An interesting test would be so look at the decay. So look at 1 originally bright dot for a longer time and record the decay over something like 1 or 5 minutes. The speed will tell how fast or slow one would have to run the thing. So maybe more time to write and one would also know the time available for the frequency measurement.

Brass is pretty absorbing in the blue / UV range, but this should not be that bad. Ideally the LED would have a relatively small opening angle to get more of the light actually to the target. Simply less distance and a 3 mm LED could also be an idea.

[Phil1977]

Another small step towards the optical memory revolution:

https://youtu.be/LTuTPwY1iZc

A non-collimated photodiode (BPW34) is already creating enough feedback for the 180°-assembly to periodically refresh the memory content.

The circuit between the Photodiode and the LED is currently just a 2-stage OpAmp-Amplifier with AC coupling and small hysteresis.

Next step will be to built a collimator for the photodiode, then - hopefully - the storage density will rise and more complex patterns may be feasible.

[wobbly]

@Phil1977, your setup is looking very promising.  It will be interesting to know what kind of bit-density you can squeeze onto that wheel.

I'm now wondering about how the "charging" light should best be applied.  I think that a high intensity light for a short amount of time, is greatly preferable to a lower intensity light for a longer time.  But there is a latency to the charging process!  When the LED turns on it takes a while for the phosphor to respond and this is actually visible - the LED goes to full brightness instantly, but the glow from the phosphor takes a little time to increase.  And then the same happens in reverse, when the LED goes off, the phosphor's natural decay is very obvious.

Following is a graph of my light sensor's output frequency (y-axis) over time (seconds, x-axis).  The phosphor was charged up for 1 second from a 30 mA 395 nm LED basically at point blank, and aimed at the same side of the phosphor that faces the sensor...


This tells me a few things:

• My LED sucks.  So I've ordered some hopefully more capable COB LEDs (3 Watts allegedly)
• The TSL235R is an awesome sensor.  Apart from it being unobtainable these days.
• I'm going to have to complete a full ring revolution in something like 15 seconds.  Including sensing and refreshing the bits.
• The half-life isn't constant!  It increases as it gets dimmer.  Hmm.

[Phil1977]

Thanks for naming your sensor - I needed quite some tries to find something appropriate.

But then the BPW34 together with a rather bad opamp proofed as more than sensitive enough.

The shown circuitry is only the very first try. AC-coupling is not optimized in any way yet, it was just the first breadboard setup and I was quite glad it did something.

 

D1 is the photodiode, D3 is the "writing" LED. LM324 was just used because it was laying around and fast opamps sometimes do stupid things on breadboards. The feedback loop of U1A is a little strange (filter cap outside of the loop) because it significantly reduced mains hum on this stages output.

For the next circuitry I want to superimpose a clock sine with the photodiode so that the memory content gets phase stable.

[Kleinstein]

Getting a non exponential decay and thus a half life that gets longer over time is not that surprising. One can naturally expect areas that are faster and slower, so a mix of different half lifes. After some time less of the fast part and more of the slow part is present.

I would not expect this to be an effect of the intensity, more a time effect. So first fast and later slow.

The time constant looks good. Form that side one should be able to use something like 4 seconds to 30 seconds for a cycle / rotation.

The intensity may not be greate, but looks like it is enough to measure, at least for the time used in the test. Faster writing may want more intensity.

For the charging / writing with the LED there should be little difference between short time with high intensity or longer with less intensity. It should be the energy delivered that matters.


For the TIA circuit the capacitor C1 is odd: one usually tries to keep that capacitance at a minimum. In simulation the capacitance is usually there to include parasitics and the capacity of the photodiode. It is more that one want a little capacitance in parallel to R2 to better tollerate capacitance of the detector.

[wobbly]

The time constant looks good. Form that side one should be able to use something like 4 seconds to 30 seconds for a cycle / rotation.

For my (not ideal) hardware I think that 20 to 30 seconds per revolution would be a good balance between robust operation and being something visually appealing to look at.

If I can really hammer the light into the phosphor quickly by using a high intensity pulse of strong UV, then I can also use a shorter sensor sampling time and thus increase the speed of the ring's rotation.

Another worry is that the phosphor degrades with repeated charge / discharge cycles and has a limited lifetime.  That would be a showstopper.

[Phil1977]

Another worry is that the phosphor degrades with repeated charge / discharge cycles and has a limited lifetime.  That would be a showstopper.

These phosphors are usually meant to be daylight-stable, and 12h of daylight will be many thousand hours of data storage...

For the TIA circuit the capacitor C1 is odd: one usually tries to keep that capacitance at a minimum. In simulation the capacitance is usually there to include parasitics and the capacity of the photodiode. It is more that one want a little capacitance in parallel to R2 to better tollerate capacitance of the detector.

Yep, C1 should get into the feedback loop. I´ll try it the next run. Thanks!

I also have a OPT-101 photodiode with integrated TIA in my box with parts to play with. I once scavenged it because it features an opamp in a very fancy crystal clear housing. Maybe it´s a good opportunity to use it...

[wobbly]

I'm sorry I've not been posting lately.  Shoulder pain problems continue  :'( , and I'm waiting for more powerful LEDs to arrive hopefully in the next day or two.

[jpanhalt]

That's OK.  I love the updates.  Just don't delay more that 7 days as that makes one weak.