30 Dec
2005
30 Dec
'05
5:51 p.m.
On 12/31/2005 at 12:17 AM ard at p850ug1.demon.co.uk wrote:
I thought the touch-tone keypad was 2 transistors, one
for the column
frequencies, one for the row frequencies. But I've never seen one.
No sir, it used only a single transistor. Transistors were viewed as the
least reliable part of a telephone and so usage was minimized. Here's a
schematic of the dial on one of them:
http://www.bellsystemmemorial.com/images/2526bm_schematic_2.gif
I remember learning this from a friend and being more than a little
skeptical. But 'tis true - a dual mode oscillator. There's also a
reference on this forum:
http://www.castalk.com/ftopic8083-15.html
I think WE doesn't get nearly enough credit for creating some really
bulletproof designs.
Cheers,
Chuck
