I recently happened to spot some LED landscape lights at my local Menard’s home improvement store. What caught my eye was the price: just 97 cents apiece. The lights are made by Patriot Lighting (in China) with a part number of 343-0555. Part of my plan after purchasing a few of them was to install either a red, green or blue LED in place of the original white LED – mostly just to see how it looks.
The lights have four major components; the top housing which contains the silicon solar cell, battery, and white LED lamp; the clear lens with a silver LED reflector; the main body support; and finally the support stake. The top, body and stake are made of a semi-rigid black plastic.
The top housing, which has the solar cell for charging the battery, can be further disassembled, by removing two Phillips head screws, into two main components; the top half containing the solar cell and the bottom portion that contains the battery and a small circuit board containing three components. When purchased new, the top has a paper tab that is pulled out in order to activate the battery.
The battery is a replaceable and rechargeable nickel metal hydride (NiMH) type manufactured by Gossi Inc (China). The battery is AAA in size with a current rating of 300 mAh at 1.2 VDC. A holder for the battery is molded into the bottom portion of the housing.
The solar cell is mounted into a recess on the top of the housing and measures 36mm by 30mm and is protected by a clear plastic window. On the reverse side of the top housing, the solar cell wires enter into the housing and are sealed with hot melt glue.
In order to measure the electrical output of the solar cell, the cell was placed six inches beneath a 65W GE Reveal flood lamp. Under no load conditions, the voltage output measured 2.1 VDC with a current output of 2.5 mA. This is sufficient output to charge the NiMH battery.
A small fiberglass circuit board measuring 19mm by 10mm contains three electronic components; a single white LED; a small 470 uH inductor; and a YX8018 boost converter IC. The PCB was well made including a silk screen on the component side and a solder mask on the solder side. The board was cleaned of any solder flux residue.
An LED (Light Emitting Diode) is a PN (diode) junction that is optimized to emit light through the recombination of electrons and holes when current is forced through the PN junction. Before any appreciable amount of current can flow across the PN junction, a minimum forward voltage bias must first be applied. This minimum voltage is typically 1.6 VDC for red LED’s and 2.5 VDC for blue and white LED’s.
Since the output voltage from the internal NiMH battery is only 1.2 VDC, the voltage to the LED must be increased to at least 2.5 VDC in order to satisfy the LED forward voltage requirement. This is the job of the inductor and the YX8018 IC. The YX8018 seems to be in common use in many of these type of landscape lights.
A simple boost converter can be created by simply using a square wave oscillator to switch an inductor on and off at a rapid rate, typically using a MOSFET transistor. Although not necessary in AC output applications, boost converters in DC applications add a diode and a capacitor at the inductor / MOSFET junction in order to prevent the boost voltage from decaying when the transistor is in the on state. Boost converters are very efficient, allowing the input voltage to be boosted 10 times or even much more depending on the oscillator frequency and component values used.
Since data on the YX8018 is somewhat hard to come by – at least in English, I am going to assume that the YX8018 works in a similar manner. In looking at the data sheet for the YX8018, I noticed a pin labeled “CE” (chip enable?) that is connected to the black lead of the solar cell. This would presumably allow the solar cell to trigger the boost converter when the light level falls to a specific value.
The waveform pictures shown below indicate a no load oscillator output of 12.9 volts peak to peak at a frequency of 227 kHz. Adding a 1N4148 diode and a 1uF capacitor between the output and ground produced a no load DC output of about 11.5 VDC. With the LED connected, the output voltage dropped to 3.2 peak to peak. All measurements were made with a battery voltage of 1.25 VDC.
Considering that these landscape lights cost less than a dollar, I found them to be of very reasonable quality. The ability to replace the battery and the use of a commonly available battery size and type is a definite plus. Still to be determined is how well these lights work in colder weather and how many charge and discharge cycles the battery can withstand.
Replacing the supplied white LED with a hi-efficiency red, green or blue LED also worked very well and resulted in a unique look when compared to the standard white LED.
REFERENCE: BOOST CONVERTER