Voltage Divider as Input to Level Shifter

Question

I need to shift a 3.3V PWM signal to 5V to power a strip of LEDs. We are using a set of JSTs to connect from an Arduino to an LED strip and so only have access to the 5V power. I am having trouble with the circuit I've created not working 100% of the time but sometimes working, so I wanted to double check that my solution to use a voltage divider to shift down the LV reference voltage of a level shifter makes sense electrically. I'm using this level shifter "4 Channels IIC I2C Logic Level Converter Bi-Directional Module 3.3V to 5V Shifter for Arduino" that is a BOB-12009 (amazon.com/dp/B07YZTW2SM). The PWM signal from the Arduino Nano is said to run at 490Hz. Here's the circuit:

^{simulate this circuit – Schematic created using CircuitLab}

The goal is to route the SDA, SLC pins from the JST which are hooked up to pins on the Arduino BLE 33 (11/12) respectively. The LED code is running on pin 12 which is connected to the pin getting routed to the LED strips D1 input i.e. the SLC pin. It works consistently on one output of one of the boards but works intermittently on every other board. I've checked for shorts and there doesn't seem to be issues there. I don't currently have access to a scope so have been debugging with a voltmeter, but figured I could check here to make sure that there wouldn't be anything wrong using a voltage divider for the level shifter.

Answer 1

You're using a divider to get 3.3 V from a 5 V source.

The top resistor is 510\$\Omega\$, and it needs to drop 1.7 V, so the current through it will be $$1.7V/510\Omega=3.333mA$$

Now what happens if whatever you're using the 3.3 V for draws a bit of current? Let's say 1 mA. You'd have $$4.33mA\times 510\Omega=2.21V$$ and instead of 3.3 V you now have 2.79 V.

This is why using a divider as a regulator is generally a bad idea, unless the change in load current is small compared to the divider string current (1/10th or less) they don't regulate very well. 3.3 V regulators are cheap and plentiful, use one.

Answer 2

If you don't have 3V3 supply available, then you need to modify the FET level shifter.

If you make a voltage divider, it must be quite strong and waste a lot of power to keep voltage stable when the 3V3 side pulls low.

You can simply remove the 3V3 pull-ups as the PWM is push-pull signal, and use somewhat reasonable kilo-ohm level resistive divider to drop 5V to 3V3 for FET gate.