Rachel's techniques for sewable circuitry: seems somehow soldering SMB parts to make them sewable. Related idea is flexible printed circuit boards.
(i believe that) Meredith asked for a Theremin-like means for turning water on and off: the water source could be a bottle with a siphon hose and clamp or inverted bottle with hose and clamp; clamp could be controlled by some kind of motor, maybe stepper motor, which would be controlled by the Theremin-like circuit.
Use MOSFETS instead of bipolar diodes: the voltage drop across a standard diode is about 0.7 Volts. It seems it's possible to use a MOSFET (metal oxide over semiconductor field effect transistor) as a diode in drain follower mode. The voltage drop is much less than across a standard diode. as more amperage goes through the diode, more power is wasted. also for circuits with very low voltage power sources, because there is very little voltage drop, more voltage is available for the circuit load. I'd like to build a circuit or two to test this. Applications include remote, low-power systems. http://electronicdesign.com/Articles/ArticleID/19871/19871.html
"Ultimate Continuity Tester", per Electronic Design, October 2, 2008: The article claims normal continuity testers trigger through connections of as much as a few hundred Ohms, which sometimes is too crude. Sometimes what's needed is a continuity detector that will not trigger through as little as ten Ohms. The circuit described is cheap, robust, and very sensitive. I'd like to try to build this. http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=19768&bypass=1
Amplifier for function generator: it's nice to have an amplifier and speaker combo around for testing. There are various ICs that are pretty good audio amps, given a power supply and a few components.
Audio mixer: it'd be a fun hack to mix the outputs of function generators and other audio range signal generators.
Audio cube: the idea is for eight independent audio amplifier-speaker combos in each corner of a cube with a mixer that could send signals to any combination. the control for the mixer seems tricky: a multiple pot comes to mind as does a cubic container with semi-conductive liquid with terminals in each corner of the cube and a stick with an end that presents a signal source to the liquid. There's always the CPU method of spitting bit slices to each corner amp per user-interactive controlled algorithms (the GUI or CLI user interface seems problematic in the same way as the physical interface, though the user could set things up programmatically--though this is reminiscent of punch card programming).
Power test resistors: a few 100 Watt resistors of various sizes could be useful. If i find some, I'll bring them in.
Multiple voltage power supply: a single device (i.e. a single 120VAC power plug) that provides various positive and negative voltages, probably most limited to less than one Amp. These are available, but in limited combinations: the hack proposed would allow any kind of extensibility.
Anything with 12VDC, low voltage, low power, off-the-grid....
Is it possible to make a signal generator that drives an antenna at the frequency of visible light? Here's the tho't: we hear multiple octaves, about ten or so (piano-type keyboard devices generally support several octaves, a guitar supports four or five octaves...). But we see just a little less than one octave (we see a low frequency red through orange, yellow, green, blue, upper frequency violet; below red is infrared, above violet is ultraviolet, an octave is just below red to just above violet). I think it's possible, conceivable anyway, that we can train ourselves to expand our visual perception: to recognize a little below and a little above what we're used to and possibly stretch enough to see the octave: an infrared color that is half the frequency of an ultraviolet color. Singers exercise their voices to extend their ranges both downward and upward. It may be possible. If so, for the person who achieves this, their sense of color harmony will change, for in recognizing an octave in light, they can make better sense of harmonic relations that are apparent in music (the fifth and fourth degrees of a scale, major and minor thirds and sixths, augmented and diminished chord...). But the trick is training, and for that it seems is needed an antenna that generates light and can be modulated to produce the entire visible spectrum and beyond, both above and below.
Jonathan Foote writes: see my speculations about this at http://www.rotorbrain.com/blog/2008/10/on-harmony-of-light.html