I mentioned finding a great photodiode in a previous post. I also found a Digitron Light Engine. You can follow the link here but don’t expect to find much. They don’t seem to have anything more then pictures and marketing stuff at their site. I have a white LED model as shown below. This thing is bright and operates from 8-30V. It will work to lower voltages but at about 7V the output is fixed and regulated.
The 5-pin SMT device, shown near the yellow wire, is a ZXLD-1350. 30V 350mA LED Driver.
The LED Driver has an ADJ pin which can be used for PWM brightness control. Unfortunately this signal is not brought out to a connection point. There is a pad set there for an optional component. I soldered the yellow wire you see in the picture above to that pad set. Note that this connection would be very fragile. I taped the yellow lead to the power leads to provide some stress relief.
Sure enough, pull the ADJ pin to ground and the light goes out. I watched the current used and it goes to a mA or less, I was not looking for currents under a mA.
For some applications being able to switch these LEDs on and off quickly would be important so I wanted to test the on/off delay of the ADJ input. The data sheet recommends that this pin is driven from an open collector transistor. I scrounged up a NPN for this.
Here is my test setup. Not pretty but works for me. From left to right, a tiny Arduino compatible board, the photodiode mentioned before (upper), the LED engine shining at a low voltage (hurts eyes, and camera), the NPN transistor is a little black spot near the center, the red yellow black wires from the LED engine on a header plugged into the breadboard and finally some stray wires and my scope probe.
The transistor wiring is simple, a 1K resistor from a digital I/O pin to the base. Emitter grounded. Collector to the yellow wire, ADJ pin of the LED controller.
I rigged up the photodiode to show light levels on my oscilloscope and tested it’s response with an LED driven directly from the I/O signal. The photodiode is fast, spec sheet shows something like 5MHz so I’m calling it good for measuring the on/off time of anything.
The Arduino is running a small sketch that just toggles the I/O pin constantly so I can measure things.
Here are the scope traces. First is turn off. Blue trace is signal the transistor, going high to turn on the transistor, which comes on to turn off the LED. You can see it takes about 400uS before the yellow trace goes all the way up, off signal on the detector. I’m sure all these signal phases are making this complicated…
Here is the turn on timing.. The blue trace goes down, turning off the turn off transistor (LOL). The yellow trace goes down to show the light level. It’s faster at turn on, about 110uS to full on.
The ZXLD-1350 LED controller switches fine using an open collector drive to it’s ADJ pin. It switches on fully in about 110uS and off fully in about 400uS. For my experiments I will use 200uS for on the on time and 500uS for off.