Every project idea begins with inspiration, and I take mine from the PDP-8, the iconic minicomputer dating from 1965, used DTL with germanium PNP transistors.

Apparently it used 1409 discrete transistors and 10,148 diodes.

If they could make a minicomputer from transistors and diodes 60 years ago, then it should be possible to recreate one today.

In 1965 a single transistor cost the equivalent of $20, so they were used sparingly, with most of the logic being done with diode gates and the transistors only being used for signal inversion.

With cheap 4-layer pcbs from China and transistors costing a cent or two, making a machine similar to the PDP-8 becomes a realistic proposition.

The PDP-8.

Logic was laid out on small pcbs known as "FlipChips", which were about 63.5mm x 127mm (2.5" x 5").

Logic density was low, as a result of component size and pcb manufacturing practice, 60 years ago.

PCB layout was done by hand, using black crepe tape on mylar draughting film.  Tracks were in the order of 1.6mm wide.

The original PDP-8 used over 200 of these flipchips was contained in a cabinet about the size of a European refrigerator, where two wirewrapped backplanes, opened outwards, like the leaves of a book.

These backplanes held a maximum of  6 rows of 36 FlipChip cards. One backplane held the CPU and the other held the Core memory and the associated drive circuits.

With about 2.8m2 (4320 square inches) of FlipChip boards, the PDP-8 was very cleverly packaged into a relatively small cabinet, allowing access to all boards and the all important backplanes.

Component density has increased somewhat with the availability of smaller components, multilayer pcbs and much finer tracking.

A quick estimate sugggests that a pcb the size of a FlipChip could hold 30 to 40 discrete through-hole transistors.

I decided to design a test circuit, not only to prove the concept of White Light Logic using NPN silicon transistors, but to create  a fundamental building block of any computational circuit. The edge triggered D-typre flipflop or DFF.