I'm Making a little mini greenhouse for a single Venus Flytrap that has a temperature sensor and a heater that will turn on when the temp drops to low. I originaly thought of using a little light bulb as the heater, but was thinking of making a little heater element/coil instead.
What is a good way to make a small heater? Would I need a wire with a particular resistance to make heat or maybe overpowering a resistor would work?
Any ideas out there?
Depends how small and how warm you want the greenhouse. Wirewound resistors spring to mind :
A spring from a clicker pen and a 9V battery does a good job. Smells bad for a bit and you might blow up the battery and/or burn out the spring. 
You may want something like this.
@Digimike: THat's horrible, hehe, smoked flower in a pot, covered in battery acid ;D
David
I'd go with the small light bulb.
Incandescent light bulbs have the advantage of easy availability and relatively high efficiency in turning electricity into heat (which, of course, is why it sucks so much to use them for lighting...).
Ideally, what you want to do is get as much of the expended energy into the air as possible. So you want to minimize the mass of the heating element, and maximize heat transfer to the surrounding air. This saves energy, and improves control: a big lump of ceramic or "sandstone" resistor will store some heat energy, and continue releasing it for a while after you've switched off the power.
So my suggestion is to shop the surplus market for something similar to the above-mentioned heating element, and couple it to a heatsink with lots of fins to improve heat transfer to the air. You might want to include a small fan (really small, like one of the little 1-inch ones they use for external hard drive enclosures) to help circulate more air past the heatsink and speed up warming.
If you can't find the heating element, I think resistors like these:
would provide the best coupling to a heatsink (though they have more mass than you'd really like).
What about a 7805 with a heat sink, fed by a 12-15 V DC wall adapter, using the 5 V DC to light up a 6 V / 0.6 W light bulb ?
Well the proper way to proceed, I think, is to quantify how much heat you want to be able to dissipate continuously in BTU or watts. Heating requirements once known, will help pick the easiest most economical solution. One watt = 3.415 BTU/hour.
Lefty
Somewhat interesting that the only ones left using BRITISH thermal units are the Americans. Heating tends to be kW and kWh here these days 
Heating tends to be kW and kWh here these days
Undoubtedly due to the differences of opinion on global warming solutions between the two countries.
Lefty
Perhaps you can look into a peltier device? They work pretty well for cooling things, and equally well for heating :).
Perhaps you can look into a peltier device? They work pretty well for cooling things, and equally well for heating .
With perhaps the worst electrical efficiency possible.
Lefty
how can you make a heater with terrible efficiency (yes i suppose you could but you'd probably have to be doing it on purpose).
how can you make a heater with terrible efficiency
Unfortunately, it's not hard: the "central" heater in my house is located at one end. The gas furnace loses way too much of its heat up the chimney, and, by the time the air gets to the opposite end, it's like (as my friend Dave used to say of the heater in his Lotus Super 7) a little old lady breathing on your ankles. I've actually closed off the registers at that end of the house, rather than waste energy raising the temperature only a few degrees.
The lesson for gigatropolis' project being that one should endeavor to transfer as much heat as possible to what one wants warmed.
I 'm going to control a wire I ripped out of an electric blanket. it's a great heater element, and you can wrap it around stuff. I'm also using Scott Dixon's Temperature Control board:
http://dorkbotpdx.org/blog/scott_d/temperature_controller_board_final_design
just find the cheapest 12V DC solder iron you can find that is rated 15W and use it with a transistor and a 12V DC powersupply and pwm pin on the arduino
You can use PID (PID arduino library) to get a great control of the temperature, if Solder iron is too big, just buy 12V cigarette lighter for a car (spare ones are not hard to find) and solder leads to the right places. (the handle is al least not hard to fix to some sort of a surface)
This is probably the easyest solutions, short of buying the heating element itself (nicrome wire, sold at electonic components shop) and make a small coil of a heater)
DAvid