19 May
2009
19 May
'09
4:45 a.m.
Chuck Guzis wrote:
On 18 May 2009 at 18:10, Richard wrote:
Here's a schematic of an Automatic Electric phone, the circuit a little more
recognisable as a simple emitter-follower oscillator with dual resonant circuits:
http://www3.telus.net/~bhilpert/tmp/AETTphone.gif
The resonant circuits are in parallel in this version rather than series.
(Note the 2% tolerance caps.)
I don't know what year you're thinking
about, but I was surprised to
see that SPICE was first presented in 1973.
DTMF has its roots in the 1940's network signalling and predates the
transistor, but the model 1500 desk set with 10-key keypad dates from
1963. There were other earlier trial models:
http://www.paul-f.com/weprotot.html 