Audible end of file
Don't tie up your phone line for longer
than you have to. This simple project
can be easily fitted to most modems and
sounds a buzzer at the end of a file
transmission.
By GREG SWAIN
They say that necessity is the
mother of invention. This simple
project was born out of necessity at
the SILICON CHIP office. It's fitted to
one of our own modems and has
proven to be a real timesaver.
At SILICON CHIP, we use a modem
to send text files from our computers directly to a typesetting terminal some 30km away. This highly
efficient practice virtually eliminates keystrokes at the typesetting
end and gives us fast and accurate
turnaround on our text and drawing labels.
This system works extremely
well, saving us both time and
money. But we did strike one pro-
blem. Because we are transmitting
data down the line at just 300 baud,
some of the longer files can take 1O
minutes or more to send. So unless
someone is actually watching the
computer monitor or the transmit
LED on the modem, it's all too easy
to miss the fact that transmission
has ended. As a result, both the
telephone and the computer can be
tied up for much longer than is actually necessary.
We were particularly keen to
solve this problem because, initially, we didn't want the phone line
tied up for longer than necessary.
At the same time, the person who
was sending the data didn't wish to
be tied to the computer, waiting for
the monitor to indicate that
transmission had finished.
No doubt a whiz at programming
could come up with a simple routine
to sound a tone from the computer
when transmission had ceased but
we wanted a more general solution.
What was needed was a simple
circuit that would sound a buzzer
when the transmission was completed. It also had to be cheap and
easy to install. The answer is the
circuit presented here. It is suitable
for use in all modems that use the
AM7910 world modem chip, which
means virtually all modems which
work at 300 baud duplex.
Circuit details
Fig.1 shows the circuit. It is
essentially a missing pulse detector
that monitors the TD (transmit
data) pin of the AM7910.
The circuit uses just two active
components: a 555 timer (IC1) and a
PNP transistor (Ql). Normally, the
base of Ql is held high by the 47kQ
resistor. This means that Ql is off,
the 2.2µF capacitor on pins 2 and 6
of IC1 is charged, and pin 3 is low.
Thus, if Sl were closed, the buzzer
would sound continuously.
In practice, Sl is normally left
open so that the buzzer is off. The
switch is closed only after data
transmission commences.
R
tJll\:\
·;:>< ~'
lII_TI
S1
The unit is built on a small PC board that mounts on the rear panel, inside the
modem. Flying leads connect the indicator to the modem circuitry.
46
SILICON CHIP
Fig.2: parts layout and wiring
diagram for the PCB version. The
buzzer should be a 6V type from
Tandy.
indicator for modems
+5v---------------------.
PARTS LIST
+
2.2
16VWJ
8
4
470k
47k
5V
BUZZER
ICl
555
...
B
ELJc
VIEWED FROM
BELOW
MODEM "END OF FILE" INDICATOR
SC7-1-288
Fig.1: the circuit uses a 555 timer IC wired as a missing pulse detector. During
transmission, Qt pulses on and off and discharges the 2.2µF capacitor.
Now consider what happens during data transmission. Qt is rapidly
switched on and off by data pulses
from pin 10 of the AM7910 modem
chip. This discharges the 2.2µF
capacitor which means that pin 3 is
now high and the buzzer is off. The
circuit remains in this state for as
long as data is transmitted down
the line.
As soon as transmission ceases,
Qt remains off and the 2.2µF
capacitor charges via the 470k!l
resistor. After about one second,
the voltage on pins 2 and 6 reaches
2/3 Vee and pin 3 switches low. The
buzzer now sounds to indicate the
end of the file.
., :e . :,.· BU~ER ,: .
t
TO PIN 10
OF AM7910
✓
('"
, .,
,
.)
f ,
V
Power for the circuit is derived
from a + 5V regulated rail in the
modem. Because the AM7910 requires a + 5V rail, this voltage is
already there.
Construction
We built our End of File Indicator
onto a small PCB coded SC7-1-288
(44 x 39mm). Fig.2 shows the wiring
details. As an alternative, the circuit can be built on a small piece of
Veroboard measuring 50 x 39mm
- see Fig.3.
You can mount the parts on the
board in any order you like, but
take care with the orientation of the
IC, transistor, electrolytic cap-
r.SC7-1-288
1 PCB, code SC?-1-288, 44 x
39mm (or Veroboard, 50 x
39mm)
1 6V DC buzzer (Tandy Cat.
273-054)
1 SPOT toggle switch
1 555 timer
1 BC55 7 PNP transistor
2 2 .2µF 16VW electrolytic
capacitor
1 4 70k!l 0.25W resistor
1 47k!l 0.25W resistor
Miscellaneous
Hookup wire, solder, screws,
nuts etc .
acitor and buzzer. Be sure to use a
6V buzzer for this project, as sold
by Tandy Electronics (Cat.
273-054). Don't use a 12V buzzer most will not operate reliably at 5V.
We built our End of File Indicator
into a commercial modem which
has a relatively large case but the
PCB is small enough to fit inside virtually any modem, including the
Avtek Multi-Modem and the
Beemodem. In our case, we
mounted the PCB inside the rear
panel and secured it using the
buzzer mounting screws. Flying
leads were then run from the PCB
to the switch, power supply and to
pin 10 of the AM7910.
~
'
GNU
Fig.3: parts layout and wiring for the
Veroboard version of the indicator.
Fig.4: here is actual size
reproduction of the printed
circuit artwork.
Close up view of tht! PCB version.
Note the orientation of the IC and the
two electrolytic capacitors.
FEBRUARY1988
47
The indicator is small enough to fit inside virtually any modem, including the Avtek Multi-Modem and the Beemodem.
The flying leads connect to the power supply, front panel switch, and pin 10 of the AM7910.
With some modems, it may be
easier to make these connections to
the underside of the board rather
than to the component side. If your
modem has a transmit (TXD) LED,
you can connect the base of Ql to
the LED cathode instead of to pin 10
of the modem chip.
The on/off toggle switch (S1} is
mounted on the front panel of the
modem.
The + 5V supply can be picked
up at pin 2 of the AM7910 or at the
output of the + 5V regulator in your
modem. Similarly, the ground connection can be obtained from the
ground terminal of the regulator.
Finally, the End of File Indicator
has no effect on the way in which
you use the ·modem. You simply
send data in the usual manner. All
you have to do is switch S1 on as
soon as data transmission begins so
that the circuit is activated.
't:
48
SILICON CHIP
A small toggle switch fitted to the front panel (top right) is the only external
change necessary. The circuit could also be built into a separate case.
