How to pass electric through 1 mm of air? (induction?)

[liquidics]

I am attempting to pass electric through the air to light an LED mounted on a sliding part. There is a 1 mm air gap between the sliding part and the static surface under it (plus space to mount parts if needed) and the LED only needs to light up when the sliding part is at its end position. I do not want to add any moving resistance to the sliding part.

I am thinking induction is the way to go but am not sure how to make it work. I am open to suggestions and especially grateful for schematics.

The existing circuit (that does other things) is powered by a 9v battery and has a 7805 providing 5v regulated.

 

[kybert]

Im not an expert in induction, but i *think* you need 2 coils and an AC supply.

Energise one coil with an AC signal, and it should radiate to the 2nd coil. You will need more windings on the second coil to account for losses.

Not sure if this is right, try searching google for "induction charger" etc. Its a well known technology since its now used in all electronic toothbrushes etc...

 

[evandude]

kybert has the right idea. it's easiest just to experiment with the coil designs you need... but essentially, a FET driving a coil from an oscillator (such as a 555) can drive the first coil.

Depending on how much current the LED needs, it should be easy. I've driven a microcontroller circuit consuming about 3mA @ 5 volts with this type of setup, and I got it to work at a range of about 1.5 inches... and that's with a small TO-92 mosfet driving the first coil at 12 volts, no high-power stuff needed. 1mm should be a piece of cake.

 

[liquidics]

evandude, could you be so kind and supply me a schematic?

 

[ljcox]

I feel it would be more easilyu done using capacitive coupling.

Make the slider such that the capacitance between 2 plates changes as the slider is moved.

Connect the output of a squarewave oscillator to one plate and a detector to the other.

The detector drives the LED.

---------- slider in position 1
---------- ----- -----
Gnd, plates 1 and 2

When the slider is in position 1, it is above the gnd plate so there is minimal coupling between plates 1 & 2.

Note that ----- in the diagram represents a metal plate.

When the slider is moved to the right so it is above plates 1 & 2, there in maximal capacitance between these plates.

 

[evandude]

I'm not sure how feasible that is... It may be a bit off, but my back-of-the-envelope equation gave me something like 1 square meter of capacitance plate area to drive something like an LED with 5v at 100KHz. based on the values I chose it could probably be quite a bit smaller but would still almost certainly be extremely large... with inductors it could be done in a very small space quite easily, especially if he uses some ferrite cores to help concentrate the field.

perhaps I am misunderstanding you ljcox... are you talking about just detecting the presence of the sliding part? it sounds like he wanted to transfer power TO the part to light an LED that was mounted on it... if he just wanted to detect the sliding part, using an opto-interruptor or even a magnet and a reed switch would be quite a bit simpler than capacitive coupling...

 

[liquidics]

Evandude has it right. I need to transfer power to the part, there is no power source available on the sliding part. I do not need to sense its position.

How do I wire the TO-92 in?

PWM -> TO-92 Base
5v DC -> TO-92 Collector
TO-92 Emitter -> coil -> DC gnd

Is that right? Don't I need a DC-blocking capacitor somewhere?

On the LED side:

Just wire both the LED terminals straight to the coil ends as in:

LED Anode -> Coil -> 470ohm resistor -> LED Cathode

 

[evandude]

no, you'd drive the square wave to the MOSFET gate (no cap), source to ground, and coil between drain and your +5v (or whatever voltage) supply rail.

LED setup should be OK, but you may not need the resistor on the LED, you're only going to get limited current from the coil arrangement so as long as you dont design your coils for insane output power you shouldn't need to worry about protecting the LED.

as I mentioned, you could probably get a little better performance if you put ferrous cores in the coils, such as winding each coil around a piece of iron/steel rod, and make it so the rods line up on top of each other when you want the LED to light. that helps direct the magnetic field, so more power gets to your slider LED.

 

[ljcox]

I'm not sure how feasible that is... It may be a bit off, but my back-of-the-envelope equation gave me something like 1 square meter of capacitance plate area to drive something like an LED with 5v at 100KHz. based on the values I chose it could probably be quite a bit smaller but would still almost certainly be extremely large... with inductors it could be done in a very small space quite easily, especially if he uses some ferrite cores to help concentrate the field.

perhaps I am misunderstanding you ljcox... are you talking about just detecting the presence of the sliding part? it sounds like he wanted to transfer power TO the part to light an LED that was mounted on it... if he just wanted to detect the sliding part, using an opto-interruptor or even a magnet and a reed switch would be quite a bit simpler than capacitive coupling...

I assumed that he wants a contactless switch. I had not meant to imply that a LED could be driven directly via the capacitive coupling.

 

[liquidics]

Thanks! I got proof of concept working but I think I am doing something wrong.

How did you get 3ma @ 1.5 inches using a TO-92-sized mosfet?? I am using an IRF540 power mosfet and two 220uH cylinder-shaped chokes as coils and I can only get the LED to be at about half-luminance when the coils are held together side by side. A mililmeter apart and the LED is just barely lit. End to end so the coils are one on top of another produces no light at all.

I am pulsing the line on and off with a 17 microsecond delay between cycles for a roughly 30khz frequency (running from a 20mhz PIC.)

 

[ljcox]

I would include a full wave rectifier bridge so that energy is supplied to the LED on both half cycles.

Alternatively, you could use a dual colour LED that has 2 LEDs inside, eg. red and green, connected anode to cathode, cathode to anode. The 2 wire version, not the 3 wire.

I think it will appear to be yellow since the red and green LEDs will be on alternatively but the light will be integrated by the eye.

 

[evandude]

Well first off, i'm driving the first coil with 12v. The coils are also not equal... the transmitting coil is about 41uH and the output coil is about 2.89mH.
making the first coil a smaller inductance lets you drive more current through it at a particular supply voltage and operating frequency.

Also, it's not a bad idea to tune your transmitting coil to resonance at the frequency you are driving it at, by placing a capacitor in parallel with it, value determined by:

f=1/[2*pi*sqrt(L*C)]