ADC2 channel cannot be used when WiFi is in use
ADC2 is used with WiFi. Therefor analogRead() on ADC2 pins fails. ADC1 channels are save to use ⇒ GPIO32, 33, 34, 35, 36, 39.
This list is an extract of the ESP-WROOM-32 pin definition (Source) and might help to find ADC1 and ADC2 pins:
| Name | Function |
| SENSOR_VP | GPIO36, SENSOR_VP, ADC_H, ADC1_CH0, RTC_GPIO0 |
| SENSOR_VN | GPIO39, SENSOR_VN, ADC1_CH3, ADC_H, RTC_GPIO3 |
| IO34 | GPIO34, ADC1_CH6, RTC_GPIO4 |
| IO35 | GPIO35, ADC1_CH7, RTC_GPIO5 |
| IO32 | GPIO32, XTAL_32K_P (32.768 kHz crystal oscillator input), ADC1_CH4, TOUCH9, RTC_GPIO9 |
| IO33 | GPIO33, XTAL_32K_N (32.768 kHz crystal oscillator output), ADC1_CH5, TOUCH8, RTC_GPIO8 |
| IO25 | GPIO25, DAC_1, ADC2_CH8, RTC_GPIO6, EMAC_RXD0 |
| IO26 | GPIO26, DAC_2, ADC2_CH9, RTC_GPIO7, EMAC_RXD1 |
| IO27 | GPIO27, ADC2_CH7, TOUCH7, RTC_GPIO17, EMAC_RX_DV |
| IO14 | GPIO14, ADC2_CH6, TOUCH6, RTC_GPIO16, MTMS, HSPICLK, HS2_CLK, SD_CLK, EMAC_TXD2 |
| IO12 | GPIO12, ADC2_CH5, TOUCH5, RTC_GPIO15, MTDI, HSPIQ, HS2_DATA2, SD_DATA2, EMAC_TXD3 |
| IO13 | GPIO13, ADC2_CH4, TOUCH4, RTC_GPIO14, MTCK, HSPID, HS2_DATA3, SD_DATA3, EMAC_RX_ER |
| IO15 | GPIO15, ADC2_CH3, TOUCH3, MTDO, HSPICS0, RTC_GPIO13, HS2_CMD, SD_CMD, EMAC_RXD3 |
| IO2 | GPIO2, ADC2_CH2, TOUCH2, RTC_GPIO12, HSPIWP, HS2_DATA0, SD_DATA0 |
| IO0 | GPIO0, ADC2_CH1, TOUCH1, RTC_GPIO11, CLK_OUT1, EMAC_TX_CLK |
| IO4 | GPIO4, ADC2_CH0, TOUCH0, RTC_GPIO10, HSPIHD, HS2_DATA1, SD_DATA1, EMAC_TX_ER |