Controlling 0-5V from 0-3.3V PWM pin and 5V power

Question

What I have is I think a standard problem in home electronics but I can't wrap my head around it. I'd like to control an IKEA light chain from an ESP32 board. The chain is basically a set of LED diodes connected parallelly. The power cannot be sourced from the pin itself because it exceeds board limitation of 40mA draw (the lights consume 0.72W which is ~144mA at 5V). I want to shift somewhat linearly 0-3.3V from the pin to 0-5V to the lights to turn them on/off and dim.

At first I tried tried using a BC337 NPN transistor but I'm only getting 2.6V max.

^{simulate this circuit – Schematic created using CircuitLab}

I've also tried using an L293DNE motor driver but the max voltage drops to 2.7V under load.

^{simulate this circuit}

1. What are some other ways to do it? I've read somewhere that a MOSFET transistor might work.

2. How can I find a proper transistor and calculate the emitted voltage based on values from its specification.

3. Is there a standard name for what I'm trying to achieve? I'm basically looking for keywords here so I can find relevant information.

Answer 1

This means that with a 5 volt drive level on GPI)18 the max voltage on the emitter is about 4.3 volts.

I want to shift somewhat linearly 0-3.3V from the pin to 0-5V to the lights

Of course, if your supply voltage is 3.3 volts then, the maximum voltage you'll get on the emitter is 2.6 volts.

You should try using an op-amp and BJT circuit to adjust the current through the LED: -

Image from here. The LED would connect between the supply and the drain of the MOSFET above. There's also this version that uses a MOSFET: -

Image from here.

You should also be aware that most LEDs will fail if given too much voltage and that controlling the LED current is a more reliable method.

Answer 2

1. Use PWM.
2. Connect the transistor in a common emitter configuration, with emitter connected to ground, base connected via a 4k7 resistor to GPIO, and collector collected to the cathode (-) of the LED light.

PWM is inherently linear no matter what the peak voltage is. Here you’re using a 3.3V PWM signal to control a switch that toggles a 5V load. 100% PWM will be equivalent to full brightness, 50% will be approximately half brightness, etc.

This will work as long as the LED light doesn’t include some fancy active drive circuitry, like a boost power converter. If it only included an active current source then the chances are it will work fine with PWM, perhaps only in a limited frequency range. You’ll have to experiment. Start with 1kHz PWM and go up if it misbehaves.