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High Power Flashing LED   (February 27, 2010)

Wily got an unusual request from a caller.  The company wanted to use an off-the-shelf solar powered LED path light but instead of a steady state light, they wanted the thing to flash.  The company did not want to go into detail about the ultimate application but they wanted the light to flash at a certain rate, a specific duration and be very bright.  The kicker was that instead of flashing a visible light, they wanted this device to flash invisible infrared light.  Wily suspected that the device was going to be used for some kind of military application but he didnít push the company for more details.

The company had already selected the path light then wanted to use.  The device contained two AA size rechargeable NiMH
cells, which were charged by a solar panel on top of the fixture.  The original LED path light used a yellow LED, but they wanted to flash an infrared LED with a wavelength of 850nm.  Could Wily do it? 
Wily gave the company a rough estimate for designing and building a couple prototype units.  They company agreed and shipped two path light assemblies to Wily to modify.

The first thing that Wily had to do was look at the voltage and current requirement of an 850nm infrared power LED.  The SFH4232 LED from OSRAM Opto looked like it should do the trick.  This part had a forward voltage drop of 1.5v with a current of 1 Amp. However, the data sheet indicated that with a low duty cycle and a short pulse duration, as much as 2 Amps could be pumped into the part.  At 2 Amps the voltage drop was about 1.7v.   This told Wily that a precision constant current drive would not be needed.  He could pick a current a bit above 1 Amp and let the drop in voltage, as the battery was depleted, serve as a crude current limiter.

OSRAM 850nm Infrared Power LED

The client company specified that they wanted the unit to flash once every two seconds for 25ms.  If Wily set the peak LED current at 1 Amp, that meant that the average current would be 0.5Hz x 0.025s x 1A = 0.0125A.  So, if the thing flashed for even 48 hours, with no help from the solar panel, the Amp-hour capacity of the battery would have to be about 0.6 Amp-hours.  Since a fresh NiMH
cell might have an Amp-hour capacity of 2.5 Amp-hours, and the device would be turned off during the daylight hours, Wily was satisfied that there would be plenty of power to do the job.
8 Solar Cell Panel     AA NiMH
Each NiMH
cell puts out 1.2v and would plunge to perhaps 1.0v toward the end of its charge condition.  So, with only two cells, the available voltage could get down to 2.0v. This should be just enough voltage to drive the LED, provided Wily was able to come up with a scheme to control the peak current with very little wasted power.  Wily had a few ideas on how to do this and began drawing up the schematic for the flashing circuit.  Wily also inspected the rest of the path light circuit.
As in the original path light circuit, Wily could use the voltage developed across the 8 cell solar panel as a nice daylight sensor. He would suppress the LED flashing circuit during the daylight hours.  He noticed that the path lightís solar panel was linked to the two battery cells using a single diode.  There was no fancy charge control circuit.  The limited current and voltage from the solar panel and the fact that the light would be flashing every night, should keep the battery from being overcharged.  The diode prevented current from leaking back into the solar panel from the batteries, during the night.  Since this was a proof of concept project, Wily figured that a better battery and charge circuit would not be needed. Perhaps in the future, he might suggest that the company consider a lithium ion battery with a smart charger circuit.
Wily completed the path light inspection and removed the main circuit board from each.  He took some mechanical measurements and was satisfied that his circuit could fit into same slot as the original board.  Wily purchased the parts he would need for the prototypes.  His purchase included one green power LED, which used the identical package as the infrared part.  When Wily received the parts, he first inserted the green LED in a position inside the path light assembly, where he figured it would produce a wide even illumination pattern.  When connected to a DC power supply, the light from the green LED did indeed generate a nice pattern of light.  With the placement of the LED tested, Wily shifted his focus to the high pulse current circuit.

Original Path Light Circuit Board

The circuit that Wily came up with is shown below.  A tiny dual Schmitt trigger inverter IC formed the pulse generator and the daylight sensor circuit.  The circuit produced a positive pulse, which fed the constant current LED driver circuit.  With each pulse, a heavy 1 Amp current pulse was sent through the infrared LED. A second Schmitt trigger was used in the day/night sensor.  The voltage produced by the solar panel when illuminated by sunlight was used to suppress the pulser oscillator during daylight
The LED driver circuit used a quality n-channel MOSFET with a low channel resistance and a simple current regulator.  The regulator relied the voltage developed across a resistor.  When the voltage reached about 0.6v it was enough to begin turning on a NPN transistor.  When turned on, the transistor reduced the drive voltage on the MOSFET gate. This negative feedback kept the current set close to 1 Amp.  To aid in maintaining current regulation, by keeping the battery voltage from plunging, Wily also installed a 4.7 farad supercapacitor across the battery. 

March 2010     Issue 7

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