Reading a sensor's output.

01First, pick the output.

You choose a sensor's output when you buy it (on Apogee, the model number's suffix says which; Atlas's EZO circuits speak I²C/UART). For a first build, a smart I²C/UART circuit or an amplified 0–2.5 V output are the friendliest.

02Smart sensor circuits (I²C / UART).

The easiest of all, and increasingly common. Instead of a raw voltage, a small circuit sits next to the probe, does the analog work and the calibration, and gives the ESP32 a finished number over I²C or UART, the two buses the chip speaks natively. No ADC, no sense resistor, no divider. Atlas Scientific's EZO circuits are the worked example: a pH, EC, dissolved-oxygen, ORP, or RTD-temperature probe plugs into an EZO circuit, and your sketch just asks it for a reading. I²C lets many circuits share two wires (each at its own address); UART is one circuit per serial port. The cleanest path for water chemistry, the readings a hydroponic or aquaponic grower lives by.

03Analog voltage.

The most accessible. A 0–2.5 V signal sits inside the ESP32's ADC range, so you can read it on a pin. But the on-chip ADC is noisy and non-linear, so for a research-grade reading pair the ESP32 with an ADS1115, a 16-bit I²C ADC that's clean, linear, and can amplify the tiny self-powered millivolt sensors too. A 0–5 V output needs a simple 2:1 resistor divider first. Convert the voltage to the measured value with the sensor's calibration factor (from its datasheet).

044–20 mA.

An industrial current loop, reliable over long cable runs and immune to voltage drop, which is why it's everywhere in the field. The trick: put a precision resistor across the loop and measure the voltage it produces. A 150 Ω resistor turns 4–20 mA into about 0.6–3.0 V, right for an ADS1115. 4 mA is the bottom of the range and 20 mA the top; map linearly to the value. (The loop usually needs 12–24 V of supply, that's the loop power, separate from the ESP32.)

05SDI-12.

A digital serial protocol built for environmental sensors: one data wire, addressable, so many sensors share a bus over long distances. The ESP32 asks the sensor to take a measurement and reads the value(s) back as text. Use the Arduino SDI-12 library; SDI-12 idles at 5 V, so add a small level interface to the 3.3 V ESP32. Best when you have several sensors or long runs. (Apogee's SQ-521 is an SDI-12 quantum sensor.)

06Modbus RTU.

The industrial workhorse, RS-485 wiring, multi-drop, very reliable over long, noisy runs. Add an RS-485 transceiver (a MAX485 or an auto-direction module) to one of the ESP32's UARTs, and a Modbus library reads the sensor's registers. Reach for it when you're tying into industrial equipment or running many devices on one long bus. (Apogee's SQ-522 is a Modbus quantum sensor.)

07Turn it into oat-ods.

However you read it, a voltage, a current, an SDI-12 string, a Modbus register, you end up with a number and a unit. Fill an oat-ods reading and send it. The device emits the same message a wireless thermometer does, so the endpoint can't tell a $600 PAR sensor from a $20 probe, it's all one stream of owned data.

oat::Reading r;
r.stream_id   = "canopy-par";       // the place, the primary key
r.measurement = "par";              // what it is
r.unit        = "umol/m2/s";        // PPFD
r.value       = ppfd;               // from the ADC + the sensor's calibration
r.physical_id = "apogee-sq-512";    // the instrument (swappable)
String body; oat::encodeNative(dev, r, body);   // → POST, or MQTT

That's the same oat_ods module the BLE listener uses. These readers are becoming their own sketches, an analog/ADS1115 reader, an SDI-12 reader, a Modbus reader, each pouring into the one envelope. Watch for them in the Software Library.