signal wakes the relay up. Below is the pinout of the ESP-01/ESP-01S module. Note that on some modules VCC is labelled as 3V3 and CH_PD as EN. Similarly, GPIO0 may be marked as IO0 and GPIO1 as IO1.

The circuit is designed to be constructed in a small module structure with an onboard electromagnetic relay as the primary switching device ... comparator inputs tied together and biased at 1/2 Vcc ...