From the article in the October issue at HomeToys titled No cost / Low cost Web based monitoring system using your computer, and a free web based service from BroadBandSentry — a serial port on your computer can be used to create a very cost effective monitoring system.

Now we have added to the remote client the ability to modify the state of the control lines in that serial port.



Data Carrier Detect



Receive Data

RD or RX


Transmitted Data

TX or TD


Data Terminal Ready



Signal Ground



Data Set Ready



Request To Send



Clear To Send



Ring Indicator


DTR and RTS are controllable pins on the RS-232 port, we have added the ability to control them remote using The Web based interface now allows for the setting or clearing of the state of DTR and RTS, as well requesting a pulsing of these two lines.

RS-232 uses a +12 volt to ?12 volt range at it?s maximum swing, and will change states in a narrow range +5 volts to ? 5 volts. There are numerous web pages with all the details, a web search with your favorite search engine should supply all the details you might require. But for now we will assume we have a serial port that is working from the +12 to ?12 range. Which on my computers actually turns out to be a around -11.3 to ?11.3 volts. We have this DC voltage and live in a AC world, lights, fans, pump motors, etc. often are 110 volt AC, or 220 AC, so how to control them? Quite simply go wireless! Several companies offer wireless AC control modules with remote controls much like a car door opener. offer Wireless AC Outlet Controllers, and offers similar products. These products consists of a small remote, much like you would have for your car or garage door opener, and a module that plugs into an AC outlet, providing a controllable outlet.

The remotes uses a 12 volt battery for it?s power source, conveniently close to the voltage found on a RS-232 RTS or DTR pin. To remotely control the remote, we can simply remove the battery, place a diode between the RTS/DTR pin and the plus side of the battery input to keep the supplied voltage from going negative, remember that RS-232 swings from +12 to ?12 volts. Connect the negative side of the battery connection to the ground on the RS-232 connector (pin 5 on a 9 pin RS-232 connector). The remote is now controllable from the serial port. Using the remote client from the DTR/RTS pin can be toggled, the on time can be set for this pulse.

The remote used above does not have a selectable transmission code as you would find on garage door opener, and should be considered very insecure, fortunately offers a line of wireless remote products that offer a user selectable operation code. These products allow for more then one receiver be controlled as well. In a single receiver application the technique used above can be used, solder across the switch forcing it to be allows depressed, then modify the connections to the battery. If your application requires two receivers, then the power source will remain the battery and relays are used to act as the switches, please see for details.

These wireless devices are part of the F.C.C. part 15, which allows for user to individually modify a device with out requiring certification. To understand the details of the regulations the particular device you are experimenting with the link below can be used to lookup the F.C.C. ID number.

The complete text of F.C.C. regulations can be found at:

Please make a point of researching what portion of the F.C.C. rules the device you are experiment with fit, the amount of time you are allowed to transmit maybe restricted. For some of the devices used in this article the transmit time is very limited and must be taken into account in your implementation.

As with all devices that can start automatically remember to include in your planning interlocks to prevent unexpected operation due to any cause. Having an object move because a motor turned on can cause injury if safety is not properly considered!!!!!!!

These remote devices offer a solution for remotely sensing a event of interest. Want to sense a door (garage door for example) opening, window, gate but it’s not convenient to run ethernet or other wires to it. Then use the remote and a wall module that supports a relay connection. offers a module that is sold as a replacement remote garage door opener. This wall module opens and closes a relay as commanded by the remote.

The connection to the relay, the small open on the side of the wall module, can be used to open or close one of the status pins on your serial port. One side of the connection is to the GND (5) pin the other side can be connected to any of the status pins. With the client software from the configuration is set on the web to sense a change to the status pin chosen. Messages, control signals to other wireless modules can be configured.

For more details on the Web based service by please visit our web site and our previous articles.

Home Monitoring Network – December – 2004
Web based monitoring system – October – 2004
Web Enabled Monitoring System – August – 2004
Internet based Monitoring – June ? 2004