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 as a Nor.

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 Green is best but white works well
  • 1 × CdS Photo Resistor LDR
  • 1 × Switching diode 1N914 or other switching diode
  • 1 × Resistor 220 Ohm minimum, can use higher values

  • Just messing around with a Pulser circuit

    Dr. Cockroach2 days ago 1 comment

    November 15, 2018 -  just messing around with Light Logic tonight and wired up the same clock pulser circuit that I had used in the first few weeks of the IO Cardboard Computer project. This time all Light Logic gates from the clock itself through to the final output led.

    Fed the clock output through three Not gates and then to one input of a And gate ( Nand + Not ) and then fed the clock directly to the other And gate input. Connected a led to the And gate output and the result was a very short pulse just like I had using DTL gates a year or so ago.

    Just proves more and more what the Light Logic gates can do in real applications.

  • Self latching Light Logic ? YES !

    Dr. Cockroach11/09/2018 at 01:48 7 comments

    It is possible. @matseng  forwarded an idea to me that does latch on and off every time. The led is a standard bright white and the CdS cell is a large 1 inch dia. cell hacked from some old 1960's equipment. Cell resistance in total dark is over 100k Ohms and when lighted comes down to less than 100 Ohms.  Also big thank you's to @Yann Guidon / YGDES  and @Morning.Star  for bringing ideas to make this all work as a package.

    View under the hood. The real business end. Yes, two led's and Ldr's in there.

    Here is the first video of the led latch in action.

  • Diode / Resistor D- Latch

    Dr. Cockroach11/05/2018 at 12:32 0 comments

    A working D-Latch 1 bit memory using only diodes and resistors.

  • Light Logic takes on the 74LS00

    Dr. Cockroach11/04/2018 at 22:47 8 comments

    Going to wire up a larger than life 74LS00 Quad 2 input NAND using Light Logic and keep it TTL compatible. If it works out well enough then it will be my go to Paper Nand Chip for all future Light Logic projects. Don't you just love my great soldering skills ;-)

  • Light Logic meets Audio

    Dr. Cockroach10/30/2018 at 22:37 2 comments

    Update Nov 2, 2018 - I decided I had better move some things around to hear if I was truly increasing signal gain with the Light Logic circuit. I connected a 8 Ohm speaker directly to the radio ( Walkman style ) output and had to place my ear directly on the speaker to hear anything. I next connected the transformer and speaker to the radio and could hear the audio but faintly from a few feet away. I at last connected the speaker and transformer to the Led/Ldr circuit and the audio signal can be heard across the room.  There is a clear case of signal gain going on here but I still need to accurately measure it and there is a slight bit of distortion on the final output from the speaker. The Led voltage has to be adjusted for best volume/quality, not the brightest setting and the Ldr voltage has a very broad range from about 4 volts to over 12 volts with very  little overall effect.


    This idea might not pan out but I had to try it and it shows promise. Using the Led and CdS cell to not only couple two audio stages but to perhaps amplify as well. The test feeds audio from a radio to the LED / LDR coupler stage and then sends it on to a normal amplifier but I need to perform further testing/measuring to know if any pre-amplification is taking place around the Led/Ldr circuit as voltage gain. There are several passive preamps / conditioners on the market but I want to mess with this circuit a little then get back to the logic games.

  • Not, Nand and Nor - All in a nut shell

    Dr. Cockroach10/28/2018 at 22:49 2 comments

    Light Logic

    By Mark Nesselhaus aka Dr. Cockroach - October 28, 2018

    A demonstration to prove out that diode resistor logic, DRL, can perform inverting Not, Nand and Nor logic functions.

    Using a LED and a CdS LDR to form inverting logic gates. The entire Boolean logic set can be handled entirely by resistors and diodes.

    I know that all this has been covered elsewhere in these logs but I wanted to get the basics on one page.

  • All one package

    Dr. Cockroach10/27/2018 at 13:26 0 comments

    Wired up another astable multi-vibrator and mounted it at the top of the master slave j-k flip flop. Almost ready to frame it and call it art.

  • Damn, those 0402 Leds are tiny

    Dr. Cockroach10/24/2018 at 10:46 0 comments

    Now that is a little bit more secure.

    With bare hands and my glasses off, I managed to attach a 0402 led onto a CdS LDR and solder tiny wires to the led.

    First I found out how much of a size difference there was between the two parts.

    Yes, the led is on top of the LDR

  • Astable wave forms

    Dr. Cockroach10/22/2018 at 14:10 0 comments

    I downloaded a good O-scope program for my netbook this morning and took a look at the wave forms from the astable multivibrator. Kind of expected what I saw and looking at ways to shape the wave form a little without using transistors.

    As of 10/24/18 I have been able to get a almost clean square wave from the astable without transistors at 43 Hz and adjusting the voltage.

    This pic is at 19.8 Hz directly from the astable.

    The next pic is after the clock signal has been inverted by a Light Logic Not gate.

    This third pic is with the frequency bumped up to the upper limit at 42.2 Hz

    Clearly there is evidence of some capacitor action taking place in the circuit by way of the CdS internal structure. Lots of room to tinker with this circuit and looking into clamping and clipping without using transistors.

  • A Photonic Transistor ?

    Dr. Cockroach10/21/2018 at 22:34 0 comments

    I was making another Light Logic firefly gate and when I sat there looking at it, I understood why others said that it acted like a transistor. More or less, it even looks like a old style transistor from the 1950s and it does roughly have a Emitter, Base and Collector and functions like a NPN, well sort of...

    Is this an attempt to enter into DIY Quantum computing? Hardly, that's a whole different ball game on a whole different level.... Unless I can reduce the light required for switching down to a single photon and that's not going to happen at this level.......Unless.......

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matseng wrote 11/08/2018 at 14:35 point

-H-o-w- (EDIT: Have) you been able to make a self-latching circuit out of this?  I.E basically just a single LDR and one LED connected in series and pointed to each other?  

At full darkness the resistance of the LDR is high enough to not light up the LED, then you temporary short the LDR to light up the LED causing the resistance of the LDR to drop enough to keep the LED on permanently.

If you then temporary short the LED so it will go dark causing the LDR to be high-resistance keeping the LED in a permanent off-state.

(Some current limiting might be a good thing here...)

I played around with it a bit, but my LEDs are so sensitive that they light up enough to cascade-trigger the LDR to full-on.

If it would be possible then a memory matrix rivaling the first core memories in density could be within reach.

  Are you sure? yes | no

Dr. Cockroach wrote 11/08/2018 at 16:04 point

Hey there, The Not, Nor and Nand gates interconnect and work just like their transistor counterparts.... The only connection between the Led and Ldr is their common ground. The led is turned on and off by the previous gate by shunting to ground. So far my s-r latch and D latch are working fine as well as the master slave J-K using the clock signal from the astable that is also using the led/ldr combo :-) Excess room/sun light needs to be shielded to some extent. The self latching idea sounds like something for me to try out....Need to look into it :-)

  Are you sure? yes | no

Dr. Cockroach wrote 3 days ago point

It is very possible :-D

  Are you sure? yes | no

matseng wrote 10/29/2018 at 09:41 point

So this means that each gate have both voltage and current gain... Or else the signal would "fizzle out" after chain of a few gates.   So possibly it could be possible to make an (very bandwidth limited) audio amplifier out of LEDs and LDR's then.

[ The department of ancient knowledge wants to remind you all that the word fizzle meant "to break wind without making noise" back 500 years ago ;-) ]

  Are you sure? yes | no

Dr. Cockroach wrote 10/29/2018 at 10:45 point

Fizzle, I can still learn something new every day ;-)

Yep, each gate refreshes the output levels so I might give the gates a try at audio and hear what happens. I do not expect Hi-Fi quality though ;-)

One issue to contend with is the latency of the CdS material as that is limiting the speed of the gates.

  Are you sure? yes | no

Yann Guidon / YGDES wrote 10/15/2018 at 21:18 point

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

  Are you sure? yes | no

Dr. Cockroach wrote 10/15/2018 at 15:08 point

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

  Are you sure? yes | no

Yann Guidon / YGDES wrote 10/15/2018 at 18:41 point

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

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esot.eric wrote 10/14/2018 at 05:43 point

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

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Dr. Cockroach wrote 10/14/2018 at 07:31 point

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

  Are you sure? yes | no

Yann Guidon / YGDES wrote 10/14/2018 at 03:52 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 :-)

  Are you sure? yes | no

Dr. Cockroach wrote 10/14/2018 at 07:29 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...

  Are you sure? yes | no

Starhawk wrote 10/14/2018 at 01:50 point

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

  Are you sure? yes | no

Dr. Cockroach wrote 10/14/2018 at 07:30 point

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

  Are you sure? yes | no

Kelly Heaton wrote 10/11/2018 at 13:29 point

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

  Are you sure? yes | no

Dr. Cockroach wrote 10/11/2018 at 14:55 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 ;-)

  Are you sure? yes | no

Kelly Heaton wrote 10/12/2018 at 01:47 point


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Peabody1929 wrote 10/10/2018 at 15:51 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.

  Are you sure? yes | no

Dr. Cockroach wrote 10/10/2018 at 16:26 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 :-)

  Are you sure? yes | no

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! :-)

  Are you sure? yes | no

Dr. Cockroach wrote 10/09/2018 at 11:01 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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