1 Nov
2011
1 Nov
'11
1:29 p.m.
For example, most cheap clock radios have one 4-pin
DIP running the
Iasusme htat's '40 pin DIP'. The only 4 pin DIPS I've seen have been
bridge rectifiers.
whole show (and it's pretty much been the same
chip since the '80s). It
uses the 60/50 Hz zero crossing on the AC line to keep time, which gives
it very precise time (Laurent Hammond, inventor of the AC synchronized
motor which ran both home clocks and the Hammond organ, took advantage
of this and now the power companies must make sure that the power cycles
average 60/50 Hz over the day to a few ppm, if anecdotes inform
correctly) and also gives it a handy time base for multiplexing half
the LED segments. Normally, as mentioned earlier, a 60 Hz refresh
frequency might cause a headache, but I imagine there are holdup caps on
the LEDs so they're not quite so obnoxious.
An wful lot of those clocks are not multiplexed at all. Think about it.
THey are genuine 7 segment displays (no decimal points0, and the most
significant digit is either blank, 1 or 2 (or maybe blank and 1 if you
have 12 hour clock). There's easily enough pins on a 40 pin chip to
direclty drive those, along with a few other outputs (flashing colon,
alarm, turn-on-radio, etc) and inputs (setting swtiches, etc).
Many 3.5 digit meters (DMMs, etc) are direct drive too. Look at the
7106/7107 and friends. Again a 40 pin DIL pacakge with plenty of pins for
direct drive.
-tony