The light dimmers that have a rotary knob to control the light level had a rheostat (variable resistor) but it was in the control circuit to the "SCR" and didn't directly control the light.BornThisWay wrote:Maybe I'm too wordy or boring for people for people to carefully read my posts.
They use to make dimmer switches with Rheostats.
As someone else pointed out, a light dimmer switches on, full for a period and then shuts off for a period. This works well on a resistive load, such as an old fashion light bulb. It does NOT work well on a reactive load, like a coil. In fact, a pure reactive load would probably kill a light dimmer very quickly.
I don't recommend making a DIY release mechanism based on electricity if you don't have a good understanding of it. Using a variable resistor (rheostat) to control the voltage/power to an electromagnet is a very bad idea. Any circuit that gets hot in operation is a bad idea.
As someone else stated, you can reduce the current that an electromagnet requires by simply adding more turns of wire. The magnetic strength is a factor of "ampere turns" and the magnetic qualities of the core. If you currently have a magnet that has, say, 100 turns of wire and requires 1 amp to hold your key you can get the same result by having 200 turns of wire and only 1/2 an amp. When you double the turns of wire you more than double the length of the wire (because the outer windings take more wire per turn) so the required voltage might increase a slight amount.
if you desire an increase in voltage (so you don't have to reduce it from your source) then the easy solution is to use more turns of a smaller diameter wire. A smaller wire has more ohms per foot, plus it fits better on a coil.
I noticed in your pictures that you were using a full insulated wire for your windings. This is unnecessary and, actually, not advised. If the coil gets warm the plastic insulation will melt (and at a relatively low temperature) and once the plastic gets soft all bets are off. "Enamel" wire is what you wind a coil with. The insulation if very thin so it doesn't take up much space, which allows you to get more winds of wire in a given space. The enamel will also withstand higher temperatures than the typical plastic insulation.
The biggest "bang for the buck" is in the choice of the core for your magnet. Hard steel typically has very poor magnetic qualities. Good old soft iron is a whole lot better! You can get two or three times the magnetic strength from the core, even with the same ampere turns. There are more exotic materials that can be even better, but probably not what a DIY person would have access to.
Why don't you just buy an electro-magnet? Get something like this:
http://www.ebay.com/itm/DC-12V-Open-Fra ... 4d18af3ab2
You may need to make a slight modification so that the plunger will fall all the way out, but it should be easy. The solenoid should easily hold a ring of keys.
Then get a 12 volt power supply, rated at something more than 1 amp. Easy to come buy, and not expensive. You can control the power supply at the AC mains side (lamp timer) or on the DC side (relays, switches, etc.) Cheap, easy, reliable.
Ron