A 4 bit binary to 7 segment display decoder created using relays and diodes.
ABCD are the input bits, where
A = bit 0 = 2^0 = blue
B = bit 1 = 2^1 = green
C = bit 2 = 2^2 = orange
D = bit 3 = 2^3 = red
This shows the decode for segment a of the 7 segment display. The circuit is equivalent to the boolean expression segment a = B OR D OR (A AND C) OR (~A AND ~C).
~ means NOT.
Relays are used for NOT and AND gates, diodes are used for wired OR. Note that the logic is negative, when A is TRUE then A is connected to ground. When A is FALSE, it is left floating.
This logic is more suitable for a common anode display, but here a relay is used to invert the final signal to drive the common cathode display.
For a real circuit, you would use current limiting resistors for the LEDs, and also diodes across relay coils to reduce inductive spikes.
