ESP-01 blink

// Blink: the module's own LED on GPIO2, and an LED you wire yourself.
//
// The LED on an ESP-12 module is GPIO2, which the NodeMCU silkscreen calls D4,
// and it is active low: LOW lights it. On a NodeMCU or a D1 mini the wired LED
// goes on D1 (GPIO5) with a resistor to ground, the ordinary way round, so the
// two are always opposite.
//
// An ESP-01S has no D1 pin. There the wired LED goes from 3V3 through a
// resistor to GPIO2, the same way round as the module's own blue one, so the
// two light together and the D1 lines below drive nothing you can see.

const int kLed = LED_BUILTIN; // GPIO2, D4, active low
const int kWired = D1;        // GPIO5, an LED and a resistor to ground (not on an ESP-01S)

void setup()
{
    pinMode(kLed, OUTPUT);
    pinMode(kWired, OUTPUT);
}

void loop()
{
    digitalWrite(kLed, LOW); // lit
    digitalWrite(kWired, HIGH);
    delay(500);
    digitalWrite(kLed, HIGH); // dark
    digitalWrite(kWired, LOW);
    delay(500);
}