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 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.


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.

Solar Powered Cell Phone Charger

The simple circuit below regulates the voltage from a 6v solar panel to a fixed +5v.  That voltage can be fed to any cell phone or USB connected portable device, to charge its battery.  The circuit uses a Reg113-5 voltage regulator from Texas Instruments. The tiny device can handle up to 400ma of charging current.  I would suggest using a solar panel rated for less than 2 watts and one which contains 12 solar cells.  One such panel is shown below.

Simplest FM Radio

C1 1 Tuning Capacitor
C2 1 0.1uf Disc Capacitor
R1 1 10 Meg 1/4 W Resistor
D1 1 1N34 Germanium Diode
U1 1 741 Or Similar Op Amp
L1 1 "Loopstick" Antenna
MISC 1 IC Socket, Crystal Earphone, Wire, Antenna

1 - The antenna can be a piece of wire or a telescoping antenna. 18 inches is a good length for in the city.
2 - The tuning capacitor is a regular broadcast band tuning capacitor. I got mine from a junked AM radio. I got the loopstick antenna from that same radio.
3 - You can change L1 and C1 to recieve different bands (eg. Shortwave). To recieve shortwave, try this: Make L1 30 turns of 30 gauge wire wound on a film can and make C1 a 10-365pf capacitor.
4 - Combine this circuit and the 8 Watt Audio Amp for a really neat radio.