I'm working on a project that uses three 7-segment displays. My aim is to show the words 'CAT', 'DOG', and 'FOX' using the 3 displays, one word at at time with a 1 second delay in between.
To achieve this, I'm using a 74160 4-bit decade counter and a 555 Timer IC.
The 74160 will sequence the words, and the 555 Timer IC will handle the 1-second intervals. I'm looking for a simple way to program these words, as I find working with EEPROMs quite challenging.
Diode ROM can do this, use stripboard and put a movie on in the background or something, it takes longer than you think. Use one strip for each "word" input line and one strip for each segment output line and make up the appropriate links with diodes. @Bryan's comment on 14-segment displays is also a good idea.
If you decode the four words so you have one high of four, you can make an 'or' (actually 'nor') gate with resistors and BJTs. For example you could use a 74HC4017 for both counter and decoder. Assuming a common anode display, you would just use a resistor wherever an 'on' segment is desired. To display this:
You would need about 50 resistors for the 'ROM' (and 28 more for segment current limiting, plus 28 NPN transistors- actually segment b in the middle and right digits are not used so make that 26 of each rather than 28)
simulate this circuit – Schematic created using CircuitLab
Personally, I'd use a small microcontroller and an internally multiplexed display, an Arduino would be well suited if you don't have that option available. It would only take the Arduino, a ULN2003, 3x 2N4003 and 10 resistors.
If you wanted to use just gates rather than resistors or diodes, you could use 26x 3-input NOR gates (eg. 74HC27, 9x 14-pin packages) and 26 segment current-limiting resistors, plus the 74HC4017 and 555.
You have two inputs bit 0 and bit 1 from the counter and 21 outputs (a-g) for each 7 segment.
however there are only 8 possible patterns of illumination for each segment.
that seems like a lot but two of those are constant, two of those map directly to input signalsm, to of those map directly to the inverted form of input signals, so there's only two where you will need to combine the two input signals using logic gates.
so decode the binary input to give a signal matching each pattern, and then connect each segment to the signal with the appropriate pattern.
as they will be driving multiple segments you may need to boost the strength of the signals a bit, (eg using ULN7003 - it only has only 7 outputs but "never on" doesn't need a wire to any place to stay dark)
use a resistor for each segment.
The ULN7003 may be able to double as an inverter for some of the signals if you add pull-up resistors for the signal you need to invert, given that, a diode-or should get you all the combinatorial logic you need.
You don't have to use the ULN7003, you could use individual transistors instead, but the ULN7003 is more compact.
