Light Logic

Using a Led and CdS LDR to form inverting Photonic logic gates.
The entire boolean logic set handled by resistors and diodes! Who'd'a thunk

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I always read that inverted logic as in a Not, Nor or Nand can not be made with just diodes and resistors. Well, this might just break out of that box. A inverting Not gate can be made with just one led and two resistors with one of the resistors being a CdS LDR. The real test will be in making a Nand gate in this fashion. Well, OK, the Nand now works as well .

The idea for this circuit came to my mind as a rough diagram back around October 5, 2018. I had been working on my #ColorChord - A Steampunk inspired creation  project and sat back thinking about the relationship between the led bar and the CdS cells. Then began thinking about the logic combinations possible and thought that perhaps a diode ( led ) / resistor ( ldr ) would provide the way to a inverting logic gate. In a flash the circuit was in my mind and a quick test set up proved it would work. The rest might be history......

Oct 13, 2018 -  I have continued to search for any company or anyone who has made the connection in using a basic opto-coupler / isolation  for use as  inverting logic and I find a lot of basic circuits for light sensing  but all come just a step short in noticing the logic functions at hand. This is something that in one form or another has been around for 80 plus years. I welcome any ones observations / comments about this.

  • 1 × Led Blue/White
  • 1 × CdS Photo Resistor LDR
  • 2 × Switching diode 1N914
  • 2 × Resistor 910 Ohm

  • Astable controlling a Nand gate

    Dr. Cockroacha day ago 0 comments

    Here is test number 3. The transistor-less astable multi-vibrator controlling a transistor-less Nand gate.

  • One less part

    Dr. Cockroacha day ago 1 comment

    After working with the Light Logic circuit for a few days, I have realized that there is one component that is not needed. Thank you @Yann Guidon / YGDES for the nudge. I decided to remove the resistor between positive and the CdS cell. This resistor was there in the event that a diode And or Or  gate was following the Light Logic gate. My thinking is that I will stick with the inv LL gates as the diode Or's loose some of their logic level. I can always add a Not to a Nand to make a And gate and maintain the logic level.

    I have also reduced the value of the resistor from positive to the led to 220 Ohms. In my case this gives stronger light and still protects the led at 3 volts.

  • Light Logic Astable

    Dr. Cockroach2 days ago 0 comments

    Second and more traditional circuit being tested.

    Again, the video does not show very well that the clock rate is stable.

    Led's D1 and D2 are not in this tested circuit. The lights are the gates themselves.

    Below is the first version that was a bit messy.

    First test using Light Logic Not gates wired up as a astable multivibrator. Poor video capture but the flash rate is indeed constant.

  • Where to go from here

    Dr. Cockroach5 days ago 0 comments

    I was thinking about what to do with Light Logic and the answer is fairly broad. I have had some comments from folks being sad that I have ended my work on IO the Cardboard Computer. I had already decided to design and build a better version and its name will be IO2. It's a long shot but IO2 will now use only Light Logic gates so IO2 will be a CPU / MCU without any transistors, relays or valves. That will be a very interesting light show indeed as each gate will light up when active.

    I am also interested in building a sound / music synth and would like to incorporate Light Logic into it as well.

    Bottom line, any circuit that requires a logic gate can use Light Logic.

  • Look, a dead bug

    Dr. Cockroach5 days ago 0 comments

    Three more little bugs on the way.

    Really indicates the size reduction.

    Tried to make the 2 input Nand as small as my fat fingers would allow and this is what I came up with...... Measures about 3/4 x 1/2 inch and about 1/2 inch tall.

    I have a few SMD Leds to mess around with. The CdS is glued directly onto the Led. The PCB is what the led came mounted on. The Led and CdS were scavenged from a night light.

  • Latch and Memory tests

    Dr. Cockroach7 days ago 0 comments

    October 10, 2018 -  The S-R latch is now working at 3 volts. Circuit is a basic Nand S-R latch with Not gates on the inputs.

    The logic for the above video.

    Wired up the first test memory board using Nand's. It has issues keeping it from changing state when I want it to and needs a lot of tinkering. I am going to try out Nor gates as well

  • Nor Gate ? No Problem

    Dr. Cockroach10/08/2018 at 14:08 0 comments

    The Not gate and Nand gate works great. Now onward to the Nor gate. The Nor gate will be simpler than the Nand as all I have to do is reverse the two 1N914 diodes and remove their 910 Ohm resistor. That will give me ALL the logic gates I will ever need using scrap parts without ever using a single transistor, relay or vacuum tube.

  • 2 Input Nand Gate

    Dr. Cockroach10/08/2018 at 11:26 0 comments

    2 Input Nand gate using only diodes and resistors.

  • Little fireflies

    Dr. Cockroach10/07/2018 at 23:39 0 comments

    Just made up four little fireflies. These are the heart of the gates.

  • Basic 2 input Nand works

    Dr. Cockroach10/07/2018 at 17:54 0 comments

    I have just finished testing a 2 input Nand gate by adding two 1N914 diodes as a And gate ahead of the inverter gate. Works just fine. This opens up a whole new world for me to play with.

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Yann Guidon / YGDES wrote 9 hours ago point

Did you see ?
It shows one example of ROM/decoder with CdS cells :-D

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Dr. Cockroach wrote 15 hours ago point

Thanks Yann for bringing me out of the darkness. Glad you caught this :-)

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Yann Guidon / YGDES wrote 12 hours ago point

it's a pleasure to see you hacking, and I'm glad your experiments are rewarded !

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esot.eric wrote 2 days ago point

Breaking all the rules! Perfect teaching-aide, too! Nicely done.

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Dr. Cockroach wrote 2 days ago point

Broke / Bend, I lost track :-) Thanks :-)

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Yann Guidon / YGDES wrote 2 days ago point

Is there an "optimisation" to merge the input diodes with the LED function ?

Just have the CdR as "high side" resistor, and the 1N4148 are replaced by LEDs that both shine on the photocell :-)

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Dr. Cockroach wrote 2 days ago point

I was planning on trying to mount real small SMD leds to the CdS and someone else mentioned using fiber optic cables to couple the gates to each other...

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Starhawk wrote 2 days ago point

You can get LED night lights with CdS cells inside at Dolla Tree, IIRC...

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Dr. Cockroach wrote 2 days ago point

My Dollar Tree keeps running out of them but I will keep checking my store :-)

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Kelly Heaton wrote 5 days ago point

What are the specs of your CdS Photo Resistor LDR (what resistance in light and dark)?

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Dr. Cockroach wrote 5 days ago point

I read about 40K blacked out and 200 Ohms very bright. These Ldrs are from night lights sold at Wal-Mart. @Morning.Star  sent me one of his that went to almost inf when blacked out and about 30 Ohms lighted but I misplaced it for the moment :-)
 For small purchases, the four pack of led night lights with the Ldr runs about $5 and besides the Ldr and Smd led there are a few more useful parts to hack out of them ;-)

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Kelly Heaton wrote 4 days ago point


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Peabody1929 wrote 6 days ago point

How about using fiber optic cable to connect the LED output to the photocell input?  A 2 input gate would have two cables over a single photocell.  The fiber optic cable could even be the "wiring" between gates.

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Dr. Cockroach wrote 6 days ago point

That has been mentioned as well as attaching very small SMD leds to the face of the CdS. Many ways to try this out and looking forward to testing them all :-) Thanks :-)

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Morning.Star wrote 10/09/2018 at 05:21 point

Thats what I've been told, not possible. Lol, sounds like a challenge :-D

This is impressive, low level theory and technology. Nice discovery dude. Nice work! :-)

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Dr. Cockroach wrote 7 days ago point

It looks as if I am going to have to write my own book on how this works. The rules have changed a little bit ;-)

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