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Wireless LED
Driver
Nov.2, 2008
designed
by David A. Johnson, P.E. |
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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. |
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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 ¼ 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 ½ inch,
try using a 74C14 (CD4069) with a 12v supply. |
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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. |
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A very nice bright green LED, which
works great for this circuit, is one from Kingbright, available from Digikey, part
number 754-1089-1. |
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Click on Circuit Below to view PDF of Schematic |
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eMail David A.
Johnson, P.E. about this circuit |
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