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I built a simple magnetic inductance coil using:

For transmitter:

28 awg magnet wire in a "pancake coil" (flat spiral) npn transistor (TIP31C)

2 resistors in parallel

battery (tried a 3 volt battery, 2 3-volts hooked together, and 9-V. 9-V got too hot too quickly)

For receiver:

28 awg magnet wire in tight coil LED

My question:

How can I stop my LED from burning out each time I get it too close to the 9-V battery?

The whole point of this project is to allow someone to basically "play" with the receiver coil and make the LED go to a maximum or minimum brightness at their leisure. If I use the 9-V battery, I get the distance I want, but heat build-up is terrible and the LED burns out if it gets too close to the transmitter.

So I guess a second question:

How do I keep the LED from burning out AND how do I keep my 4 inch minimum receiver distance (for decent light from LED, about half the possible brightness from about 3-4 inches away) and stop my transmitter from getting too hot? (I plan on the transmitter/receiver running for at least a few hours and be reuseable)

P.S. The receiver needs to be as simple as possible, (2 or 3 small parts maximum if possible). And the transmitter can be no larger than the back of the average person's hand. (I was thinking "stacking" the pancake coils together and using 2 6-V power sources to make the magnetic fields addative, but that would increase the weight of the transmitter and I'd like to keep that to a minimum).

If knowing what it is for would be helpful then:

I plan on putting the transmitter in the back of a leather glove, growing borax or alum crystals over the tranceiver, and making the crystals "glow" via magnetic induction for a few wizard/witch costumes for halloween (hence the weird size/weight restrictions). Also I'm worried about people who may try to "play" with the gloves (with their hands not being in the gloves) and put the crystal with the LED too close to the transmitter, thus burning out the LED (and this would be a major issue as the LED is currently inside a crystal and cannot be replaced).

Anyway, thoughts/suggestions for my 2 problems would be helpful. Thank you.

Schematics that I found online, only difference is that I used the magnet wire for the transmitter pancake coil as well as the receiver.

schematic

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    \$\begingroup\$ Can you give us some schematics? \$\endgroup\$
    – Hearth
    Oct 14, 2018 at 14:15
  • \$\begingroup\$ just updated with schematics. Only difference between the schematic and mine was that I used the 28 awg magnet wire for the transmitter as well as the receiver (not sure if that is the reason for my short range, doubt it though). \$\endgroup\$ Oct 14, 2018 at 14:26
  • \$\begingroup\$ Adding a resistor in series with the LED wouldn't hurt \$\endgroup\$ Oct 14, 2018 at 14:27
  • \$\begingroup\$ that won't mess up my range? (its the main reason I didnt use it to begin with) \$\endgroup\$ Oct 14, 2018 at 14:49
  • \$\begingroup\$ Which LED’s do you want? I might suggest some \$\endgroup\$ Oct 14, 2018 at 14:59

1 Answer 1

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Solution for the receiver: You can use a Zener diode for „eating up“ too much power.

Place a small resistor in series with your LED. This will increase the voltage linearly with the LED current. Once this voltage reaches the break through voltage of the zener, any additional power will be absorbed by the diode and the LED current stays at its maximum.

This circuit will also have the advantage of protecting your LED from too much reverse voltage during negative half wave. Of course, your zener must be able to handle that power.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ ok. I'm a complete beginner. Do I put the zener diode and the small transistor in series, both in the receiver, and that will let me: keep my range without dimming the LED, and prevent LED blow-out from the LED getting too close to the transmitter coil? (again, I'm a complete beginner (so please treat me like I'm stupid, it will be helpful, not insulting) and this is my first real project) \$\endgroup\$ Oct 14, 2018 at 14:36
  • \$\begingroup\$ No transistor! Place a resistor in series with the LED. Then place a zener parallel to that and the receiving coil. You will have to specify the resistor value and the zener voltage. (I have no circuit drawing tool here, so I can‘t provide you a drawing) \$\endgroup\$ Oct 14, 2018 at 14:41
  • \$\begingroup\$ place a zener parallel to what? The LED? they are two separate entities (the whole point is that the LED should appear to light up "like magic" when brought close to the glove). Again, I'm sorry for asking so many questions but I am a complete beginner. \$\endgroup\$ Oct 14, 2018 at 14:51
  • \$\begingroup\$ The zener is parallel to LED AND resistor. This corcuit does limit the maximum current through the LED. \$\endgroup\$ Oct 14, 2018 at 14:52
  • \$\begingroup\$ Ok. So both go in the receiving coil. (that's what I wasnt sure of). Thank you for all of your help! \$\endgroup\$ Oct 14, 2018 at 14:54

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