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• Costs around $10, or you can get two free samples from Dallas Semiconductor, the manufacturer.
• 8051 archictecture.
• Also a 40-pin DIP package; just add a crystal and some capacitors.
• 33 MHz, and most instructions are executed in 1 clock cycle.
• 1k RAM, 64k Flash.
• Two serial ports, 32 pins of digital I/O, timers, counters, etc.
• Serial port programmable with just a MAX232 and a 74HC125; there's a bootloader program in ROM built in on the chip.
• Programmable in whatever, but there's a free C compiler available, which is quite easy to get started with

The answer lies in the datasheets. Many RTC chips have more than 128 bytes of NVRAM, and use a bank swapping technique to select the alternate banks. Dallas Semiconductor is one such source, although most new systems have the RTC embedded in one of the main ICs of the chipset or on the super IO chip. Here is one example, the SMC FDC37N958FR, which is used in the Dauphin Orasis v1, an SBC I am experimenting with putting LinuxBIOS on to get around certain limitations. Page 215 is the start of the RTC/NVRAM access. This particular device has 256 bytes of NVRAM, several of which are reserved for the RTC and 8051 scratchpad. Since there is only 256 bytes, there is no bank switching. The DS1251 is an RTC/NVRAM chip with 512KB of static RAM. The little bitch is expensive, too. :-) It uses a banking method where one of the normally user-available registers is now a bank selection register.

I seriously doubt many systems have more than 256 bytes of NVRAM. That is a LOT of space for settings.

I'll betcha the IR reflectivity of an eyeball is different from that of an eyelid. And cheap IR sensors and emitters are all over the place -- for which you can thank your remote control.

The tough part is getting something to process the signals (and then doing the signal processing), but I'm sure something like that could be found readymade too... maybe Dallas Semiconductor?

Take a look at the Dallas Semiconductor TINI. It's a Java runtime environment on a 72-pin SIMM, complete with ethernet, serial, I2C, parallel IO, battery up to 1 meg of NVRAM, filesystem emulated in RAM, etc, etc. You can write web or ftp services for it in a few lines of Java, thanks to the supplied classes. You develop your Java code on your PC, compile it to Java bytecode, and then FTP it up to the little TINI device. My description is not doing this hardware justice, so I'll leave some links below.

Anyways, my point is this type of device is probably easier to program than a Linux Dreamcast. It may or may not be cheaper (sub-$100). And it's a lot easier to hide, if that's the goal. I've programmed a handful of hobby projects with this board, and it's really quite amazing for the price. (Compared to trying to implement an TCP/IP stack on a PIC microcontroller, say.)

TINI hardware
TINI
TINI board resource center
more resources
DalSemi discussions

Brian Lane has made public drivers for these nice Dallas Semiconductors temperature sensors. The software can be loaded from here.

--jarkko

The most extreme, fun way is to use pyrometric cones - just wait for these cones to droop and move the joystick, and you'll find out the temperature! Here's how to use the cones when upgrading the wiring of your computer.

A much more practical way is to use the Dallas Semiconductor (now bought by Maxim, and not the magazine)

Dallas has a demo application you can use as an example - a weather station and some good application note examples. It uses the DS1820 or the DS18S20 and you can get up to 2 free samples of each. This device is digital, so no calibration is needed for the accuracy you need. They have a lot of other temperature sensors; some even have alarm outputs, so once you program it, reading only one bit will tell you if the temperature is out of limits. It has a well-written and complete datasheet. They've got software for win32, linux, beos, java, and 8051. If you write your own software or modifiy theirs, you don't really need a serial port adapter; just a wire on the parallel port will do (and it will power the device, too!!)

If anyone's interested, I can dig up some c-code that I used - it works with the parallel port under dos.

Doesn't this box have a serial port? I monitor the temperature of our computer room using Dallas Semiconductor DS-18S20 sensors and Digitemp. The sensors can be connected to your computer via the serial port and are relatively inexpensive (approx $3.00 for the sensor, $10.00 for the serial port interface)

Really, I wasn't doing a search on patent 5965809, it was the exercising a cat one, honest!

my favorite idea would be a USB dongle-type device (or "token") that could be worn or carried on a keychain

Dallas Semiconductor have (IMHO) the best of such devices, as part of their iButton product range. It's a USB fob for iButtons. Hang it on your keyring and it's an iButton fob. Plug it into a laptop for a moment and it's a USB-connected authentication device (capable of running robust JavaCards). Plug it permanently into a desktop and it's a cheap iButton interface for static machines.

This happened a while ago, but some of the avid Dallas Semi fans on /. may not have heard yet (Dallas Semiconductors products are a popular among hardware hackers). Maxim will acquire Dallas Semi (based on their product line this looks like a good match). It will be interesting to see what direction they will move after the acquisition.

It sounds like nothing more than an iButton with extra memory. iButton has been selling these things for a while, but with java and less memory. Check it out here.

Dallas is giving away their OS, apps, docs, schematics, and source for free from their site.

I'm surprised that the TINI never made it on Slashdot before. Who cares about 486-that-fits-in-my-pants-and-costs-as-much-as-a-P C systems when you can do just about the same for much less? Sure, it's neat-o and geekworthy that it could be done, but is it practical?

The Dallas Semiconductor 5002FP encrypts the address and data buses. It's an Intel 8051 compatible microprocessor, so forget about running Linux or *BSD.

I have one of these , for my final year project, and they're pretty cool. These guys have written some GPL code that lets you run servlets. Of course, you'd probably better not plan on getting too many hits - It doesn't seem to handle too many hits too well! Well, it is only 103mm by 32mm by 10mm, so what can you expect

(I know this is kinda off-topic, but, well, so what)

My point is this - I'm sure it wouldn't be too hard to build a small (and cheap - thats why we're using it :-) system that allows device control using the computer and the TINI as a bridge to the 1-Wire network.

The 1-Wire network consists of a dasiy chain of devices using standard RJ11 networking gear. So this would be relatively cheap and easy to instal. And devices can talk to it using cheap 1-Wire interface modules (2 way for approx $10-$15 US) from Point Six Inc.

The TINI can also be used to send email - so maybe even some sort of alarm. If a button is pressed an email goes out asking for help.