driving 12 transistors with few I/O's - is it possible?

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hi,

i've build waveshield (http://www.ladyada.net/make/waveshield/)
and have 12 push buttons (with 12v lamps inside) split out on a0, a1 , a2, each own resistor ladder.

in addition this, able switch specific lamp on when button pushed. initial thought use 12 transistors , turn them on/off digitalwrite.

apparently wasn't going straightforward thought, because have 10 available pins (a3, a4, a5, 0, 1, 6, 7, 8, 9, 12).

so either can try making longer resistor ladder , see if can have buttons on 1 analogin-pin, or else know if there kind of smart way drive 12 transistors with, say, 2 or 4 i/o's? kind of matrix or.. don't know.

well if have @ least 4 pins available, can use 4 - 16 demux here.


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