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  • If you still want to use this string of LED @emma.makes I can extend the answer to describe a possible approach. But it would involve soldering and adding resistors and using a voltage / current meter. It would be simpler to buy loose LEDs and resistors. But that wouldn't be as fun. Commented Jun 27, 2021 at 13:54
  • so you say the LED string in Question will no light with 3 AA batteries? but that is a working product not a DIY Commented Jun 27, 2021 at 14:41
  • @st2000 Thank you for your elaborate answer! I have no problems with soldering and adding resistors! I could also measure the current drawn by the LEDs when powered by the three AA batteries. One alternative I see, is to use the 3.3V power pin from the ESP32 and control power with a transistor via GPIO. Commented Jun 27, 2021 at 14:50
  • @Juraj, I have no doubt that the LED string in the question lights up using 3 x 1.5 Volt batteries. But I don't know how the string of LEDs are connected. Guessing, I might assume they are connected in what BigClive called a Z arrangement. Possibly 50 parallel groups of 2 in series LEDs. That might give us about 4 Volts at about 150mA (given that (again, this is a guess) each LED is rated 2 Volts at 3 mA). Such a load is reasonable for 3 in series AA 1.5 Volt batteries. Commented Jun 27, 2021 at 15:17
  • @emma.makes, that's a great idea to control the string of 100 LED using a transistor. But the ESP32 usually runs on 3.3 Volts. Before going down this road consider testing if the string of 100 LEDs will light up using a 3.3 Volt power supply. And while you are doing that consider testing the current to find the necessary capacity of the transistor you will be using to control the string of 100 LEDs. (Can you switch voltages higher then 3.3 Volts such as the 3 x AA 1.5 volt batteries? Yes. You just need to take care how you design the circuit.) Commented Jun 27, 2021 at 15:24