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More  LED circuits    Driver circuits

Wireless LED Driver  Nov. 2, 2008
designed by David A. Johnson, P.E.

There are times when you want to control a LED indicator light through the side of a plastic box, without wires and without drilling a hole in the box.  One example where this may be needed is in data collection systems.  These are often used out of doors in harsh environments and have to be hermetically sealed. Holes drilled in the side of the box for panel mounted LEDs or light pipes can often leak.
The circuit below solves this problem by sending power to the LED through the plastic, using a magnetic coupling technique. The circuit below can route power through plastic enclosures as thick as inch.  The circuit will not work through metal boxes.

An expensive inductor, driven by a series resonant mode 125KHz oscillator, forms the power transmitter.  A similar inductor, wired as a 125KHz parallel resonant circuit, forms the power receiver.  A voltage doubler circuit at the receiver efficiently converts the collected AC into DC.  The circuit will operate over a wide 3v to 6v supply range.  

With a 5v supply, the circuit draws about 25ma of current.  However, by gating the oscillator on for a brief 20ms period, with a 0.5Hz rate, the average power can be reduced to about 250 microamps.  If you want to extend the range of operation out to inch, try using a 74C14 (CD4069) with a 12v supply.

Using surface mounted components; the complete LED assembly can be encapsulated and glued to the outside surface of the box.  Tiny unshielded surface mounted inductors can be used to reduce the size of the transmitter and receiver.  However, smaller parts will reduce the power transfer range to perhaps only a 1/8 inch separation.
A very nice bright green LED, which works great for this circuit, is one from Kingbright, available from Digikey, part number 754-1089-1.

Click on Drawing Below to view PDF version of Schematic

Circuit - Wireless LED Driver


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