Concept: Four 555 timers into a gated D latch

No guarantees, but it worked in the simulator.

Today, figured out how to make a (possibly low-speed) transparent D latch out of an R-/S latch, plus a couple of logic gates, and an inverter. Each of these four things can be constructed from a 555 timer (or half of a 556). Alternatively, each of the gates can be replaced by three diodes[1] and a resistor, and the inverter by an NPN transistor and a resistor. Not pictured is the AC coupling[2] on the E input that would make this an edge-triggered flip-flop instead of a latch.

The significance of this is that a D flip-flop, a 1-bit static RAM and an important element of sequential logic, can be constructed very easily with stuff readily available from Radio Shack, which doesn’t stock logic ICs anymore but does carry 555s and 556s (as well as diodes, resistors, and transistors). Naturally, I have the best interests of the impatient experimenter in mind. :-)

  1. [1] With Schottky diodes you could probably get away with omitting the output diode.
  2. [2] In this case, an RC highpass is used, an inline capacitor of about 0.1µF followed by a 10kΩ pull-down resistor.

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