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I've tried to research this, but the only circuit examples I can find use an "oscillating field".

I have a project where I would like to put two differently colored LEDs in an object, and depending on where on the "base" (or controlled by an MCU) one or the other color will light up.

My idea was to use "wireless power transmission" (because I don't want to put a battery in the object) and then control the direction of the field to dictate which LED lights up (similar to a "biderectional LED"). But I can only find circuits that use AC (to be more efficient‽), either with a capacitor & transistor or just a transistor and "self oscillation".

The Rx-coil probably can't have a bigger diameter than 2 (3cm MAX), the Tx-coil could be at least Ø5cm (potentially a bit bigger).

ETA: The distance between the coils could be minimum/best case ~5mm (with PLA plastic between them)

ETA2: This for an "interactive display piece": One location in the object lights up yellow, everywhere else it lights up blue. It will be on for 8+ hours every day (which makes me hesitant to use batteries in the object).

Any suggestions? Or will it just not be possible to do it on such low power?

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  • \$\begingroup\$ Forget beaming power across the room. Or even across a desk. If that's what you were meaning. Find another way. (like Bluetooth to control which LED lights; when they already have their own power, e.g. battery) \$\endgroup\$
    – user16324
    Commented Sep 8, 2020 at 20:52
  • \$\begingroup\$ I added the distance to the Post (I forgot to add it before): But still, thought it was clear that the the distance would be short (<1cm), since I mentioned "putting the object on a base".. \$\endgroup\$
    – ChrisH
    Commented Sep 8, 2020 at 21:01
  • \$\begingroup\$ the wireless charging and location detection are separate challenges ... the base could be painted with multiple colors, with the color difference detectable only in the IR spectrum \$\endgroup\$
    – jsotola
    Commented Sep 8, 2020 at 21:02

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My idea was to use "wireless power transmission" (because I don't want to put a battery in the object) and then control the direction of the field to dictate which LED lights up (similar to a "biderectional LED"). But I can only find circuits that use AC (to be more efficient‽), either with a capacitor & transistor or just a transistor and "self oscillation".

Polarity will not work; this sort of "wireless power transmission" is really a transformer, and transformers only pass AC.

You could potentially use the orientation of the field, ie, have two solutions at right angles to each other, but a lot of contemporary solutions have the field oriented vertically, so that won't work either.

Instead, what you'd probably want to do is to modulate information in a detail of the drive signal - possibly something as simple as a small change in frequency, or a pattern of pulses and gaps.

Or you could just use some cheap radio technology.

The advantage of the latter is that you could give in and use a battery for power after all, which you're likely to find quite a bit easier to get working.

Easiest still would be a battery and some reed switches, and then put magnets or electromagnets in the base; then you don't even need to do low-power design to preserve your battery, since the reed switch functions as an actual switch.

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  • \$\begingroup\$ Ok, bummer.. Using reed switches was something I thought about*. The issue with using a battery is that the ones I could potentially fit in the object wouldn't last that long. This is for a "interactive display piece", so it would be lit up for several hours each day. (*Even using just the standard induction/"wireless power transmission", and then reed switches to control the LEDs, but that probably wouldn't work with the magnetic field, right‽) \$\endgroup\$
    – ChrisH
    Commented Sep 8, 2020 at 21:08
  • \$\begingroup\$ Polarity could be made to work : e.g. an asymmetric square wave, 20% high,80% low and vice versa from 2 transmit coils. That may give (say) + 2V and -0.5V and vice versa, which could illuminate one diode or the other. \$\endgroup\$
    – user16324
    Commented Sep 8, 2020 at 21:10
  • \$\begingroup\$ Brian's proposing that you exploit the non-linearity of the LEDs, though I suspect it might require changing the slope not simply the duration of a square pulse. Might be something interesting to play with using audio software and a computer soundcard, feeding a unitary transformer with the LEDs on the other side and only worry about the power transmission type "transfer" if you get the simple case to work. In terms of a conventional wireless charger plus something else for information I think you'll find there's not really much interference between that and a magnet and reed switches. \$\endgroup\$
    – Chris Stratton
    Commented Sep 8, 2020 at 21:30
  • \$\begingroup\$ Perhaps use electromagnetism to power the device and one or more hall effect devices to determine the orientation/position? Possibly you could use Earth's magnetic field or one or more magnets in the base so the device can determine when it's correctly placed. \$\endgroup\$
    – Frog
    Commented Feb 11, 2021 at 9:02

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