esp8266
I am developing my first PCB and I would like to know how best to power a 3.3v relay with control from ESP32 module. Because I am powering the relay with 3.3v do i even need the optoisolator in my circuit? the circuit below is currently activated with a LOW from ESP32.
The ESP32 has configurable output ports that support up to 40 mA when driving high:
See the datasheet.
All ports will sink 40 mA driving low (the absolute maximum is 80 mA). The default as shown is 20 mA for a high.
Connect the signal VCC to the Relay 5 V VCC, and the Opto diode in series with DSx ensures that the ESP32 pin cannot be pulled above about 2.5 V.
If you want to absolutely ensure the voltage can't rise then you could add say a 10k Ohm resistor from each output you use to ground.
There are many derivatives of the relay board you have chosen and many of the datasheets specify they work on 3.3 V signal levels. This then gives you an input circuit like this:
simulate this circuit – Schematic created using CircuitLab
Best Answer
The optoisolator is overkill here for most applications. You can drive T1 straight from the ESP and size R3 so that the Base-Emitter current times the transistor's gain is plenty to flip the relay on. As long as you have the flyback diode you should be fine. If you see problems from small voltage spikes when the relay turns on and off, you can keep a capacitor with low ESR close to the relay so that it pulls from the cap during transients.