relay decoder for 7 segment display

296

04:16:32

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.

published 8 years ago

Thrashard96

8 years ago

Aren't transistors more practical than relays, though? I mean, you need to pass a certain voltage/current to activate a relay, much higher than transistors or even vacuum tubes at the tradeoff of higher output voltages, but at logic level you should go for about 1.2V, 3.3V or 5V.

user1644

8 years ago

This is for historic or amusement interest only, it's part of a design for a relay clock. Konrad Zuse created calculators and primitive computers using relays, but valves were preferred for practical computers used during WW2. Nowadays, you can buy single chip ICs to do decoding, or more commonly just do it in software.

hurz

8 years ago

5 for example is decoded sometimes wrong cuz there is additional voltage drop inserted and the hysteresis does cause a wrong on/off

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