(C) 2008 G. Forrest Cook
This project can be used with a CirKits solar circuit kit.
This is an ultra-simple LED spot lamp that consists of two LEDs and a current regulator circuit. It is designed for use in 12V solar powered lighting applications and can be used in cars, boats, RVs and even fountains. Only seven components are required. The circuit produces non-flickering regulated light with an input of 11 to 20 volts. If the lamp is potted in Silicone caulk, it will be completely waterproof. The circuit will support either 20mA, 25mA and 30mA LEDs by changing the value of one resistor.
I have built more than ten of these lamps and they are all going strong after many years of daily operation. The lamps have been used in several solar-powered night-lighting systems that use an SCC3 charge controller, a lead acid battery and a DAS1 dark activated switch.
The circuit works with both white and colored LEDs, a lamp that uses two different colors can produce nice accent lighting. Good color combinations include emerald green/ultra-bright blue, emerald green/aqua, white/emerald green and white/amber. Red, amber and yellow LEDs operate at a lower forward voltage, the circuit can be used to regulate three LEDs at 12V on this side of the spectrum. A lamp made with deep red, orange and ultra-bright yellow LEDs complements the emerald green and blue lamp nicely. See the photos above for examples, the LEDs will be much more visible in a dark setting.
Nominal Operating Voltage: 12V DC Regulated Light Voltage Range: 11-20V Operating Current: 20ma or 25ma
The input power is filtered through an R-C filter consisting of two 100nF capacitors and a 22 ohm resistor, this isolates power line voltage spikes from the rest of the circuitry and can prevent damage to the solid-state components from nearby lightning strikes. The LM317 and a fixed resistor act as a fixed-value current regulator. Use a 68 ohm resistor for 20mA LEDs, use a 56 ohm resistor for 25mA LEDs and use a 47 ohm resistor for 30mA LEDs.
A small circuit board was made using press-n-peel blue film, the board was sized to fit inside of a 1/2" PVC pipe connector. The parts were soldered into the circuit board and a length of two conductor speaker wire was soldered to the board for the power lead. The circuit can also be hand-wired onto a piece of perforated circuit board material.
A knot was tied in the power cable to act as a strain relief. The power cable was fed through a hole in the PVC connector. The bottom of the PVC connector was coated with packing tape to keep the wet Silicone in place. Be sure to test the circuit before potting in Silicone. The PVC connector was filled with clear GE Silicone II caulk and left to dry. After drying, it may be necessary to trim off the excess PVC around the LEDs using some wire snippers.
Be sure to let the silicone dry in a warm place for several days before applying power. The acetic acid that is released when the silicone cures is conductive and the circuit will corrode and fail if power is applied too soon. An alternative method is to use electronic RTV (room temperature vulcanizing) silicone that does not contain acetic acid.
If you want to have a screw for mounting the LED lamp, drill a small hole in the PVC connector near the wire entrance and insert a screw through the hole. The matching nut can go on the outside to secure the screw. For a mounting bracket, I like to use a short piece of perforated metal plumbing strap since it can be easily bent to any shape.
Connect this circuit to a 12V battery or power supply, if it does not light, reverse the polarity of the wires. The LEDs should light up brightly, the light output should not vary if the input voltage changes within the 11-20V range. This light can be used for a night light, a flash light, automotive interior lights and background house lighting. The low current draw allows the lamp to run for many hours on battery power.
2x white LEDs, T1-3/4 size 1x 47, 56 or 68 ohm 1/4 W resistor, choose for LED current 1x 22 ohm 1/4 W resistor 2x 100nF, 50V capacitors 1x LM317L adjustable voltage regulator 1x 1/2" Schedule 40 PVC pipe junction or end cap GE Silicone II caulk Two conductor wire
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