Here’s the scenario: you’re going to be traveling somewhere and you’ll be charged roaming fees if you use your cellphone. But there is free WiFi available in this place. You can save yourself money by leaving your SIM card at home and using a GSM-to-Skype bridge to take calls on your phone via WiFi.
[Trax] is using a USB GSM modem to take cellphone calls on a PC. He leaves his sim card in this modem so that it can make and receive calls and text messages through your normal telephone number. For some reason, the USB connection only provides control of this modem and doesn’t pass bi-directional audio. To make this happen, he built an audio interface cable using two transformers and a few passive components to connect the modem to the computer’s audio card.
On the software side of things, an application written in Delphi 7 manages the modem, the audio stream, and the Skype application. When a call is incoming it sets up a Skype connection with your handset via the Internet, passing along the caller ID data in the process. If you choose to answer the Skype session the application will pick up the GSM call and you’ll be connected. It works the same way when placing an outgoing call.
This seems easier to manage than a rig that physically pushes a cellphone’s buttons via the Internet.
Hack-a-Day forum member [Necromant] was recently working on a router when he made a terrible mistake. He connected the wrong power brick to the router, causing a 2.5v over-voltage. The router itself was just fine, as it contained a good stepdown converter, but the HSDPA modem connected to the router’s USB port was not so lucky. It seems that the USB host is powered directly from the router’s power supply without any conversion – this meant his modem got a nice 7.5v zap when he used the wrong plug.
He assumed the modem was dead, so he figured there was no harm in disassembling it. He examined the modem’s circuit board and found that when plugged in, the onboard stepdown converter supplied 0 volts to the rest of the PCB. He couldn’t find any documentation for the converter online, so he employed a little bit of
trial and error clever investigation to determine what sort of voltage the stepdown provided before being cooked.
After a bit of testing with his home-built low dropout regulator, he determined that the damaged stepdown provided 3.3v to the rest of the modem (that’s a 4.2v over-voltage for those of you who are keeping track). He added a linear voltage regulator to the board in place of the old stepdown, which worked for about 15 seconds before overheating.
In the end, he decided to add a pretty hefty 3.3v stepdown converter to the modem, throwing aesthetics to the curb. The result is one ugly, but quite functional HSDPA modem.
If your boss is like [Michael Scott] you probably find yourself in constant need of plausible reasons for your action or inaction. Now you won’t have to waste away the workday coming up with those ideas yourself because this little box will always provide you with an excuse. It’s actually a 14.4 Kbps modem, which brings back memories of the early ISP days when you’ve find banks of these in the corner to service incoming calls. [Alex] altered the circuit board to map out an ICSP port for the PIC 16F690 that controls the system. Just use your key to unlock the Emergency Excuse Generator and press the button to spit out a doozy. The 8,000 word memory on the microprocessor stores all of the excuses which can be combined a number of different ways based on how the rules files is built. This rule file is by far the most interesting part of the build and worth looking over.
We think this would be a nice addition to the other office electronics you built.
[via Dangerous Prototypes]
You can add a huge measure of extensibiltiy to a project by using a cellular connection. Anywhere the device can get service you can interact with it. In the past this has been a pretty deep slog through datasheets to get everything working, but this tutorial will show the basics of interacting with phone calls and text messages. It’s the 26th installment of what is becoming and mammoth Arduino series, and the first one in a set that works with the SM5100B cellular shield.
We love the words of warning at the top of the article which mention that a bit of bad code in your sketch could end up sending out a barrage of text messages, potentially costing you a bundle. But there’s plenty of details and if you follow along each step of the way we think you’ll come out fairly confident that you know what you’re doing. Just promise us that you won’t go out and steal SIM cards to use with your next project. Find part two of the tutorial here and keep your eyes open for future installments.
This alarm system senses motion and then alerts you by phone. [Oscar] had an old external modem sitting around and, with some wise hardware choices, he came up with a simple circuit to use it. First up is the PIC 16F628A chosen because it doesn’t require an external crystal. This connects with the modem via a DS275 RS232 transceiver because it requires no external parts for connection. The final portion of the puzzle is a PIR sensor that triggers a pin interrupt in the sleeping PIC, which then dials your number to alert you. It doesn’t look like anything happens other than your phone ringing, but that’s enough for a simple system. We’re just happy to see how easy it was to use that modem… time to go hunting for one in dreaded junk trunk. Don’t miss the clip after the break.
Continue reading “Modem used in an alarm system”
[Travers Buda] is giving new life to his abandoned childhood toys. He cracked open a set of Family Radio Services radios he had received for a birthday which work up to 2 kilometers apart. With just a bit of extra circuitry he was able to get them to act as wireless modems. The system functions but it looks like it would benefit from some more refinement, including error correction. In the end [Travers] manages to send and receive ASCII based messages at a whopping baud rate of 10.