If you have seen the diyAudio schematic (currently #1 Google Images for
"discrete component flip flop"), you already noticed the notation at
every diode position: LED or 1N914. That notation has been there since
the original circuit was posted. Most people read it as a cosmetic option
-- LED if you want an indicator, signal diode if you don't.

That reading is wrong. The LED is the correct component. Not for
cosmetic reasons. For structural ones.

THE DIODE IS ALREADY THERE

In the DTL N-ary topology, the base path already requires a series diode.
This is not optional -- the series diode is what creates the immunity gap
between the active state and the inactive states. The contact potential of
the diode junction (Vf ~0.6V for 1N914, ~0.25V for BAT54 Schottky) is the
logic threshold. Remove the diode and the circuit loses its starvation
mechanism.

So the diode is structural. It is always there. It carries current every
time that position is active.

Now: an LED is a diode that emits light when current flows through it.
It has a forward voltage (Vf ~1.8-2.2V typical for visible red/green).
It has all the same electrical properties as a signal diode in this
circuit. It blocks in the reverse direction. It conducts in the forward
direction. It provides the series junction that creates the immunity gap.

When you substitute the LED for the 1N914, you get:

  Function 1 -- Series blocking diode: identical behavior, slightly
  higher Vf which WIDENS the immunity gap rather than narrowing it.
  The circuit works better, not worse.

  Function 2 -- State indicator: the LED lights when its transistor is
  active. One LED on at all times. Visual confirmation of which position
  holds the state. This was already in the original 1990s tube amplifier
  design -- one green LED indicating which channel was selected.

  Function 3 -- Optical bus emitter: the same current that flows through
  the LED as a series diode also drives photon emission. The LED fires
  the moment the transistor activates. No separate emitter needed. No
  separate trigger. The state change IS the optical emission.

Zero added components. Three simultaneous functions.

THE FLIP-FLOPTICOUPLER

When the LED output is received by a photoreceiver (photodiode or
phototransistor) at another cell's base input, the result is a
galvanically isolated logic coupling. The sender and receiver share
only light -- no electrical connection, no ground loop, no EMI coupling
between cells.

This is the Flip-Flopticoupler: a DTL cell whose LED emitter directly
drives the base of another cell through a photoreceiver. Multiple cells
can communicate without any copper connection between them.

Properties this enables:

  EMI immunity: light does not couple capacitively between traces.
  Hundreds of optical channels can run side-by-side with zero cross-talk.
  In high-voltage environments -- near motors, transformers, welding
  equipment -- where copper traces pick up interference, the optical
  path is completely immune.

  Galvanic isolation: no electrical path between sender and receiver.
  No ground loop. No voltage level mismatch problem between cells.

  3D stacking: optical vias through transparent substrate connect layers
  without drilling copper Through-Silicon Vias. Layer 1 can communicate
  with Layer 4 by shining light straight through.

  Phantom State optical: when no transistor is active, no LED emits.
  The optical bus is dark. Darkness IS the default state. Zero energy
  to maintain it. Zero energy to detect it.

THE SERIES DIODE SCALING LAW

The second property is independent of the optical extension and applies
to any version of the circuit -- LED or signal diode.

In the standard N-way circuit:

  Active collector:        0.2V (Vce saturation)
  Inactive node voltage:   0.2 + 0.6 = 0.8V (through one diode)
  Required to drive base:  Vf + Vbe = 1.2V minimum
  Immunity gap:            1.2 - 0.8 = 0.4V

At small N (2, 3, 6 positions) this 0.4V gap is sufficient. Noise
and leakage from 1-5 inactive branches is negligible.

At large N (256, 512 positions), you have hundreds of reverse-biased
junctions all connected to the shared bus. Each contributes nano-amps
to micro-amps of reverse leakage. The leakage accumulates. With enough
channels, combined leakage can threaten the 0.4V margin.

The fix: add one series diode to each base path.

With one extra diode in series from node to base:

  Required to drive base:  Vf + Vf + Vbe = 1.8V minimum
  Node still at:           0.8V (active collector unchanged)
  New immunity gap:        1.8 - 0.8 = 1.0V

The gap triples from 0.4V to 1.0V. No amount of passive leakage from
256 or even 1024 inactive branches can bridge 1.0V. The circuit scales
cleanly to any N.

The active transistor is unaffected: its base is driven by all other
HIGH collectors through their diodes at roughly B+ - Vf = ~11.4V at
12V supply. One extra series diode costs 0.6V of that drive -- trivial.

The general law:

  k extra diodes per base path --> immunity gap = (k+1) x Vf - 0.2V
  k=0: 0.4V    k=1: 1.0V    k=2: 1.6V    k=3: 2.2V

Each extra diode adds 0.6V to the noise moat. The designer controls
the scaling margin by diode count, independent of N.

For the LED version: the LEDs already provide higher Vf than a signal
diode (1.8-2.2V vs 0.6V). A single LED in the base path already gives
a wider immunity gap than the standard 1N914 version, making the LED
substitution a net structural improvement before adding any extra diodes.

THE PRACTICAL SCALING GUIDE

  N=2 to 32:    Standard topology. One diode (or LED) per base.
                Leakage negligible. No extra diode needed.

  N=33 to 256:  One extra diode per base path. Gap: 1.0V.
                Handles accumulated leakage at large N.

  N=257 to 1024: Two extra diodes per base. Gap: 1.6V.

  N=1025+:      Three or more. Gap grows linearly.
                Supply headroom is the only limit.

THE BICOLOR INDICATOR -- SERIES DIODE PRIOR ART

Look at the bicolor indicator panel in the master schematic. Two LEDs
in opposite polarity, in series, sharing a resistor. Already in the
documented prior art schematic. Already showing two diode junctions in
series in the base path.

This is not a new circuit being claimed. It is an existing documented
circuit whose connection to the scaling law is being identified. The
bicolor indicator already demonstrates that multiple LED junctions in
series are a natural part of this topology -- prior art for the series
diode principle.

THE NO-TRANSISTOR FLIP-FLOPTICOUPLER

The panel labeled "flip flopticoupler" in the master schematic (no
driver transistor version, directly above the bicolor indicator) is
the optical coupling mechanism in its most primitive form. Two LED
paths from a shared node. No transistors. The path with fewer junctions
conducts. That LED emits. A photoreceiver at another cell receives the
light and drives that cell's base.

The entire optical bus mechanism -- galvanic isolation, EMI immunity,
no copper between sender and receiver -- is present in this
no-transistor circuit. Transistors add gain and holding force. The
optical principle requires neither.

THE PRIMITIVE DEMONSTRATION

The mechanism is visible in the simplest possible circuit -- no
transistors required:

  One pull-up resistor to a node.
  Path A: one LED to ground.
  Path B: two LEDs in series to ground.

Node settles where Path A conducts. Path B needs twice the forward
voltage -- it sees half, cannot conduct. Path A wins completely.
Path A LED lights. Path B LED is dark.

Add another LED to Path B: now Path B needs three times the forward
voltage. Even more immune. This is the entire scaling mechanism --
counting junctions. The fewer junctions to ground, the more it wins.

The bicolor indicator in the schematic IS this primitive demonstration
with both LEDs visible simultaneously -- one active, one dark,
determined entirely by junction count and path impedance.

PUBLIC DOMAIN DECLARATION

Alan Cyr (Wolf13), CYR Technologies, Chicago, releases to public domain:

  (A) The LED-as-series-diode substitution in the DTL N-ary topology
  (B) The Flip-Flopticoupler optical bus coupling mechanism
  (C) The series diode scaling law and Noise Moat
  (D) All N-way variations with LED base diodes

Free to use. Free to build. Free to sell. Free to patent improvements.
No license. No royalty. No attribution required.

Prior art: circuit 1990s. LED substitution and optical extension: 2026.
Series diode scaling law: 2026.

RELATED POSTS

Tri. Flop (Hackaday, August 2026):
  The 30-year tube amp circuit as a ternary neural network weight cell.
  {-1, 0, +1}. BitNet b1.58 compatible. One transistor per weight.

One LOW in 128 HIGH Lines (Hackaday, August 2026):
  Single transistor holds full ASCII character. N=128. Public domain.

FILES

cyr_onehot_storage.v    Parameterized Verilog RTL. Set N=3 for ternary.
CYR_OneHot_18way_v3.cir LTSpice 18-position netlist.

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Wolf13 / Alan Cyr * CYR Technologies * Chicago * September 2026
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