I'm fairly inexperienced in electrical engineer so forgive the simple question. Using a microcontroller (Netduino) and two 8bit shift register (serial in, parallel out) I'm controlling 16 LEDs.
The logic works fine, I was expecting the shift register and lights not to work when I don't plug in the auxiliary power source and just fired up the microcontroller. Right now, the microcontroller is powering all the LEDs and I don't want that.
Even though the schematic isn't perfect, here is my circuit:
The resistances are 220 ohms +/- 10%. The shift registers are SN74HC595s.
I know I could add NPN transistors on each of the shift register outputs, but that would require quite some transistors, and most importantly, the microcontroller would still power the shift register.
I did try to use transistors on Pins 1-3 from the microcontroller but the LEDs were still powering on without the external source of power.
As a secondary point, I know I should add one/some capacitors, but I'm not exactly sure where beside maybe a large one around the power source.
Edit: Here is the adapted schematic, in which the outputs triggers a transistor for each LEDs. I didn't replicate the transistor for every LEDs, but it's assumed that from Qa to Qh on both shift register, it's the same:
Best Answer
If the microcontroller is running with 3v and the shift registers are at 5v that could be a problem. Per the spec the shift register chips want to see at least a 3.15v min high logic when their supply voltage is at 4.5. (A 5v supply would require even higher logic levels). If you power the shift registers with 3v that should make both compatible. Is there a reason you cannot use the same microcontroller power for the shift register chips?