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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.