Seven-segment displays are easily salvaged from lots of electronics. They are somewhat limited in what they can display, but there are 128 possible glyphs, not counting the decimal point which usually accompanies them. Many of them are recognizable letters or punctuation.
The most popular 7-segment LED display on Digi-Key right now is http://www.digikey.com/product-detail/en/kingbright/ACSC56-41CGKWA-F01/754-1047-1-ND/1747764, a Kingbright 2.1V 20mA (pulsed to 150mA in 100μs pulses) common-cathode slightly slanted green LED display with a decimal point, with the totally ridiculously high price of US$2.70. Its datasheet “numbers” the segments as follows:
a
--
f|g |b
--
e| |c
--
d DP
with the totally absurd pinout 7, 6, 4, 2, 1, 9, 10, 5, with the last being DP.
If we make a the MSB of a 7-bit word, then the standard glyph for 0 is 0x7e, with 0x7f being 8. The numeric weights in hexadecimal are as follows:
40
--
2|1 |20
--
4| |10
--
8
A more complete font looks like this:
' ' 0x00 '⸣' 0x60
'-' 0x01 '⊃' 0x61
''' 0x02 '"' 0x22 '⸣' 0x42 'ⁿ' 0x62
',' 0x04 '⊂' 0x43 '°' 0x63
'r' 0x05
'?' 0x65
'|' 0x06 'Γ' 0x46
'⊢' 0x07 'μ' 0x27 'F' 0x47 'P' 0x67
'_' 0x08
'=' 0x09 '≡' 0x49 '⊇' 0x69
'⊆' 0x4b 'º' 0x6b
'⸤' 0x0c
'c' 0x0d '¿' 0x2d '2' 0x6d
'L' 0x0e '[' 0x4e
't' 0x0f 'E' 0x4f 'e' 0x6f
'i' 0x10 '1' 0x30 '7' 0x70
'¬' 0x11 '⊣' 0x31
'4' 0x33 'ς' 0x53 'q' 0x73
'n' 0x15 'ñ' 0x55
'∩' 0x76
'h' 0x17 'H' 0x37 'A' 0x77
'⸥' 0x18 ']' 0x78
'3' 0x79
'y' 0x3b '5' 0x5b '9' 0x7b
'u' 0x1c 'J' 0x3c 'ū' 0x5c
'o' 0x1d 'd' 0x3d 'ō' 0x5d 'a' 0x7d
'U' 0x3e 'G' 0x5e '0' 0x7e
'b' 0x1f '∀' 0x3f '6' 0x5f '8' 0x7f
I’ve tried to omit duplicates here. These 68 glyphs don’t include all the letters in even the English alphabet (missing are ‘k’, ‘m’, ‘v’, ‘w’, ‘x’, ‘z’, and very sadly ‘s’), but they include a fairly complete repertoire of logical operators (at least if we interpret set arithmetic as logic) except for ∃. They include a perhaps unreasonable number of bracketing characters.
The absence of ‘s’ and ‘w’ eliminates many of the most common English words: ‘is’, ‘was’, ‘with’, ‘have’, ‘this’, ‘his’, as well as most plurals. ‘m’ also eliminates ‘from’. So you can’t use this font to write in English, unless maybe you use ‘ς’ for ‘s’. You can't even write “sin” and “cos”.
Many old typewriters didn't have separate characters for “l” and “1”, or “0” and “O”; the number row started at “2” and ended at “9”. Due to typewriter legacy, even today, we often use the same character “'” for “‘” and “’”, and the same character ‘"’ for ‘“’ and ‘”’, and the same character “-” as a hyphen, minus sign, em dash, and en dash. If you use the same approach here, allowing the “5” and “2” glyphs to double as “s” and “z”, then you get back trig operations and most common English words. The most common ones missing are “was with have from which we were would will what who more them some him two time my like me now”.
These glyphs do not include any arithmetic operators other than ‘-’. Some calculators use 0x05, 0x21, or 0x25 for “/” for a fraction bar.
For the more limited purpose of rendering decimal or hexadecimal digits, you want the list {0x7e, 0x30, 0x6d, 0x79, 0x33, 0x5b, 0x5f, 0x70, 0x7f, 0x7b, 0x7d, 0x1f, 0x0d, 0x3d, 0x6f, 0x47}.