Turn an external device on and off using a modem. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Turn an external device on and off using a modem. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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When it becomes necessary to control an external device from a computer, there are usually many options for connecting it to LPT, COM, USB ports, which, however, require special adapters and software.

In addition, LPT and COM ports are usually absent on modern computers (especially laptops). The USB bus remains, but only a very qualified specialist can pair anything with it on their own.

If the task is very simple and does not require the transfer of large amounts of information and the issuance of complex commands (for example, you just need to turn something on or off), the USB option becomes completely unprofitable.

I propose a simple and interesting, in my opinion, solution. A telephone modem, external or built-in, is found in almost all modern computers, although with the development of ADSL technology and high-speed Internet it is practically not used. It can be easily adapted to solve a simple "on-off" task.

Turning an external device on and off using a modem
Ris.1

For example, let's connect an LED to the modem's "LINE" jack, as shown in fig. 1 Now, using any terminal program, such as the HyperTerminal included with Windows, you can turn this LED on and off.

To launch HyperTerminal, follow the path "Start-Programs-Accessories-Communications->HyperTerminal". First of all, the program will ask you to specify the name of the new connection. Set any and click "OK". In the next step, indicate to the program the COM port of the computer to which the modem is connected, and click "OK" again. If the modem is built-in, in the computer it is usually connected to a COM port that physically exists, but does not have an external connector.

Turning an external device on and off using a modem
Fig. 2

The figure shown in Fig. will help you determine this port. 2 window, which is opened by following the path Start-Settings-Control Panel-Phone and Modem-Modems.

When further setting up the connection, refuse the offer to change the port settings, leaving them as default, however, some Hyper Terminal program modes need to be changed. In its main window, press the screen button &, in the window that opens, go to the "Parameters" tab. Set their values according to fig. 3 and click on the "OK" button

Turning an external device on and off using a modem
Fig. 3

Now you can start controlling the LED. We give the modem the command "Hook up", for which we type on the keyboard ATH1 (in Latin letters) and press the Enter key. The response should be an OK message. The modem's line circuit will close and the LED will turn on. To turn it off, you need to give the command ATNO - "Hang up the phone". On fig. Figure 4 shows what the exchange of information with the modem looks like in the main window of the terminal program.

Turning an external device on and off using a modem
Fig. 4

It is not necessary to issue commands to the AT modem by typing them manually. The automatic execution of the operations necessary for this can be included in a program of our own design, as is done in the demo program attached to the article.

The demo program can be downloaded hence.

Author: Yu. Shomnikov, village of Verkhovye, Oryol region; Publication: radioradar.net

See other articles Section Clocks, timers, relays, load switches.

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