Driving 3.3 V LEDs with an Arduino and a shift register

Question

I'm planning to build a simple LED matrix with an Arduino and some 74HC595 shift registers. However my Ardiuno outputs 5 V to the registers which will pass that 5 V on. How and where along the line do I reduce this voltage to 3.3 V?

Edit: Found this after a while of Googling. Is a resistor in front of every LED all i need (which i need anyway)?

(5–3.3) / 0.02 = 85 ohm for a 20 mA 3.3 V LED?

Edit 2: Can i provide 3 V from a different source to the Vcc pin of the shift register, but send 5 V logic on the serial pins? Im guessing that won't work but just wanted to double check.

Answer 1

There are a number of options, most of them discussed in this post. Since the communication is one way you can use:

• Voltage dividers: cheap and easey: 2 resistors. Cannot provide stiff voltage sources unless you lose a lot of power through them
• A diode with a resistor circuit. Same as previous
• Level shifter (something like a 74LVC245)

There are shift registers that do run on 3.3V: depending on how many lines you need, an easier solution might be to shift the Arduino output to 3.3V, and feed that to a 3.3V shift register. This will likely (again, depending on your setup) in fewer lines to be level-shifted.

EDIT: Yes, you will need current-limiting resistors for your LEDs, however, voltage divider != a single resistor. Voltage divider looks like this:

^{simulate this circuit – Schematic created using CircuitLab}

EDIT2: Usually not. You should read the datasheet for the shift register you are planning to use about its various maximum voltages. P.S. When you say "serial pins" you don't mean the Serial TX and Serial RX, right? Usually, one drives shift registers using general purpuse digital IO pins. You can use serial pins for two of those, but usually you need a more pins to drive a shift register (usually at least three in the case of 74HC595). The point is 74HC595 is driven via SPI protocol, not RS-232

Answer 2

Like pretty much every IC without specially designed over-voltage tolerances, the 74HC595's input voltage level has a maximum of VCC or VCC + 0.3v. Powering the 74HC595 from 3v is okay, but directly connecting the Arduino's 5v outputs to the 74HC595 at 3v will not be good. A level shift as @angelatlarge has shown would work.

That said, the other thing is that the IC does not pass along the voltage from the inputs to the outputs. While it's logical diagram does not show any transistor/mosfet used, it does have buffered outputs. Outputs are referenced to the 74HC595's VCC and GND. For example, at 4.5v VCC, the Logic Level High only needs to be 2.4v (typical) to be a level high, but the output will still be 4.32v Typical.

So, you can power the 74HC595 from 3v, use a voltage shifter to bring the inputs to the 3v level, and connect your leds that way. But you would still want resistors on the leds to control any current.

In fact, using the resistors will make powering the 74HC595 at 3v AND using the voltage divider moot. The resistors should be calculated the same way you would if you were directly connecting a led + resistor to a battery.

Note this schematic:

Powered from 5v, considering each led, as most standard LEDs go, will be anywhere between 2 to 3.5 Volts at 20mA forward current, means there will be 3 to 1.5v in excess. The resistors will, when calculated for a (5v Source Voltage - LED Forward Voltage Drop) / LED Current, will "Take up" the remanding voltage, while setting that current. Same applies for LED Matrix on the 74HC595 as well, except the location of the resistor would vary and all. You haven't told us what type of matrix you want.