![]() Any errors from the opamp’s input (PNP) bias currents should be small, and should be more on the negative than positive, so hopefully any tiny error will tend to reduce the LED current, not increase. After putting this together, I decided to try a different approach, sensing on the emitter side, and no current sensing path to add to the LED current! I also included 4.7K resistors on the feedback looks, and the positive inputs see about a 1K impedance. Of course, the opamp circuit isn’t perfect. ![]() The power for everything comes from this simple little power supply, which is just (approx) 24 volts unregulated, and a 5 volt regulated output from the pot, which is from a 7805 regulator. I guess I didn’t care about an extra 5 mA. I still used a 1K pot anyways, though a higher value would have consumed a little less current. Because it’s driving only the inputs to opamps, it doesn’t have any substantial load. The 0 to 5 volt signal just comes from this potentiometer on the front panel. It still have a tiny bit of the lowest part of the range where the LEDs won’t completely turn off. ![]() As you can see in the schematic, I change the values to about as high as I reasonably could. The idea was any small change in ground potential between the board 0-5V control signal wouldn’t matter, if I ran separate signal and power ground lines.īut I didn’t consider the current draw though those resistors around the upper opamp. At the time, it seemed like a good idea to sense the current using a resistor between the NPN transistor’s collector and the LEDs. I wanted to make sure the current was perfectly constant since the camera might choose a quick shutter time. It takes a 0 to 5 volt input and regulates a 0 to 20 mA output current to a string of 5 LEDs. This little board is a constant current regulator. This is the type of shadow I’m hoping just a little bit of under side lighting will eliminate. It’s a soft shadow due to the light tent casting light from many directions, but it’s still very present. All the PCBs mount with double sticky mounting tape.Īs you can see in this LED photo, there’s a bit of shadow. The Cree LED is a surface mount part, but fortunately Lean’s PCB group order made it very easy to convert to something I can solder wires onto. Maybe that matters, maybe not, but it seemed like a good idea. The LEDs are Cree CLM3C-WKW-CWBYA453, which are supposedly the same 5500K color temperature as the CFL lights outside the tent. This view is inside a cheap 2 foot sized light tent I purchased from some ebay vendor, and a couple bright lights outside the tent on both side. The build used boards from OSH Park (then “Laen’s PCB group order”), materials from TAP Plastics, 15 white LEDs and parts I mostly had laying around. Most of the white background photos you see on the PJRC site are shot with this light table. Years ago, in my slow quest for better photography of electronic projects, I built a light table to eliminate shadows. ![]() Beatmaster2k – Sequencer Using Novation Launchpad.Magnetic Railgun Paper Airplane Launcher.Light Table For Web White Background Photos ![]()
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