19 May
2009
19 May
'09
7:56 p.m.
On 18 May 2009 at 16:17, Dave McGuire wrote:
Doea anyone know where I could obtain one of these 1-transistor keypads?
I bought a few US 'phones in a second-hand shop over here [1]. Most of
them have rotary dials, but the pushbutton one has a keypad with na IC on
it (cryptically labelled, but it's the same pinout and functionality as
one of the standard DTMF enocders made by TI et al). I was somewhat
disapointed, I'd much prefer the 1-transistor verison.
I've not looked on E-bay, so if they're common there, feel free to tell me...
[1] One of them is a 564 (I think). It's got the row of buttons in front
of the dial to select one of severla lines, etc. Do scheamtics for such
things exist on-line anywhere?
-tony
Oh, how thoroughly bizarre. Thanks for the
explanation. I'd love
to see a SPICE simulation of that.
What's noteworthy is the stability of the thing. ISTR that DTMF
signalling relies on a tolerance of something like +/-2%. A mass-
produced LC oscillator that operates reliably close to tolerances in
a steel mill or a meat-packing plant is a marvel.