Hello! I'm trying to determine what type of transistor I need in order to pulse an LED strip I have. I have attached a simplified schematic (omitting resistors) as a visual aid to show basic intended operation. (I have very little knowledge about schematic creation, so I apologize—especially for the LED strip and USB power parts. I hope it is helpful.)
Here is some information about my components:
I did not receive a datasheet with the LED strip or identifying part information to locate one. I know the following:
- It contains two different colored LEDs, alternating down the strip.
- It has one positive terminal and two negative terminals (one for each color LED).
- It requires 24 VDC.
- Multimeter tests show that each negative terminal reads 0.5 A, for a total draw of 1.0 A.
Additionally, here is a photograph of one section of the strip (it can be cut across each section of terminals—the intensity of the flash in the photo appears to shift the colors). Also the back has a prepared adhesive applied.
Output: 24 VDC, 5 A (Product page)
Particle Photon microcontroller (Datasheet)
BC517 Darlington NPN 30 V, 1 A (Datasheet)
My goal is to dim each of the two colored LED arrays using PWM from two of the µcontroller's +3.3 V I/O ports. The µcontroller hardware/software is not an issue for me—however, I am having this issue: the LED is always on in a less-than-full-brightness state when this circuit is closed. Even if I disconnect the MCU from the transistor base, and connect a floating wire in its place, the result is the same—but if I disconnect the base completely, the LEDs turn off.
Is this an issue with the transistors I have—and if so, which transistors should I use instead? Whichever transistors I use must be able to switch states faster than 200 µs and ideally faster than 150 µs.
Thanks for the taking the time to read!