* LED on 16 pin
  * press boot before upload

<code cpp>
#include <Arduino.h>

void setup() {
  Serial.begin(115200); 
  pinMode(16, OUTPUT);
}

void loop() {
  static int zzz = 0;
  Serial.println(zzz++);

  digitalWrite(16, HIGH);
  delay(1000);
  digitalWrite(16, LOW);
  delay(1000);      // wait for a second
}
</code>

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{{:esp32:pasted:20190527-143143.png}}

https://www.instructables.com/id/ESP32-With-Battery-Holder/