12 October 2016
If you follow the current flow through the diodes on
each half cycle, you will see a path where no matter what
the polarity of the transformer is, only one polarity ends
up at the load (see Figure 5). Let’s say that we remove
that transformer and replace it with a battery. Connecting
the battery one way is like the transformer giving us the
positive half of the sine wave; connecting it the other way
is like the negative half. What the load sees is always a
In the case of your microphone, I’m guessing there
is a full-wave bridge rectifier between the battery and the
circuit that amplifies and transmits the signal. You can use
this same technique to make other projects do the same
thing. So, why don’t we see this more often?
My best guess is that there are two reasons. The first
is that a full-wave bridge rectifier is more expensive than
a single reverse polarity protection diode; it’s four instead
of one of something. Sometimes there isn’t even a single
diode for protection, so if you put the battery in backwards
the circuit is damaged. The second reason is voltage drop.
Silicon diodes must have an approximately 0.6 volt drop
across them before they will begin to conduct. For the
full-wave bridge, that drop doubles to 1.2 volts. Even if
you’re using a 9V battery, that’s a 13.3% drop, leaving you
with 7.8V. For a pair of AAA batteries, almost one batterys’
worth of voltage is lost.
Not all circuits are designed to be able to function at
lower voltages, but obviously this microphone can do it. Of
course, you could build the bridge out of Schottky diodes
and take advantage of their lower forward voltage drop,
but they are more expensive.
Help with a Set of Archer Catalog
QI found a set of ICs when I moved into my new house. They are Archer catalog number 276-1783, and have the words: “VOICE SYNTHESIZER” on the package. The first chip
(a 16-pin DIP) is a GI8351, with part number SPR016-117
and the bottom is stamped CC4. The second chip (a 28-pin
DIP) is a GI8349, and on the bottom there is 32100-017.
I have tried to look all over the Web to find out how to
make it into a working voice synthesizer. If you can help
out with this, I would be so happy.
SSG E6 U.S. Army 1963 to 1972
Disabled Vet George Rowe
AThis was a difficult chipset to track down, but I think I’ve found the original part number for the set. There are several variations on these chips that were made by General Instruments
and this is the SP0245-17 set, I believe. It looks like the
first device (SPR016-117) is a 16K mask ROM (a Read
Only Memory programmed by actually producing a
semiconductor lithography mask with the bits embedded
in it) containing pre-programmed words that can be
connected to the 24-pin SP0256-AL2 voice synthesizer
chip. There is a scan of the datasheet at www.speechchips.
The second page of the datasheet shows two
variations for connecting the devices, with the first being a
stand-alone configuration, and the second making use of
n FIGURE 5.
n FIGURE 4.