Aisha, the single low-side MOSFET has a catch I’ve been bitten by. When it’s off, the bottom of the divider floats, and the top resistor pulls the ADC pin up to the full 12 V through 100k. That’s above the ESP32’s 3.3 V limit; the pin’s protection diode clamps it and current leaks into the 3.3 V rail. So you save the divider current and stress the pin instead.

The two-transistor version switches the top: a P-channel (AO3401 or similar) between the battery and the divider, gate pulled up to the battery with 100k so it’s off by default. A small N-channel (2N7000) driven by a GPIO pulls that gate down to switch it on. You need the N-channel because a 3.3 V GPIO can’t pull the gate of a P-channel sitting at 12 V high enough to turn it off. With the top switched off, the whole divider sits at 0 V, the ADC pin too.

In the firmware: switch on, wait about 10 ms, take 16 readings and average them, switch off. Use the calibrated reading (analogReadMilliVolts) and send battery_v as a number with two decimals.

For the flapping: a lead-acid battery’s voltage jumps when charging starts, so a plain threshold will always bounce. Give the rule hysteresis so it only clears well above the threshold, and two severities:

{
  "type": "battery_low",
  "title": "{{device.name}}: battery {{value}} V",
  "severities": [
    { "severity": "MAJOR", "condition": { "key": "battery_v", "op": "<", "value": 11.5 } },
    { "severity": "MINOR", "condition": { "key": "battery_v", "op": "<", "value": 11.8 } }
  ],
  "hysteresis": 0.4,
  "debounce": { "mode": "count", "raise": 2, "clear": 2 },
  "notify": { "recipients": [ { "kind": "email", "email": "field-team@example.com" } ], "minSeverity": "MINOR" },
  "enabled": true
}

It opens below 11.8 and clears only at 12.2 or above, two readings in a row. Kevin’s idea would hide the problem, not fix it. Hysteresis and debounce are explained on this page.

Rules card. I run lab ventilation schedules this way.

First give the Pi’s profile a command, e.g. heaters.set with a Yes/no param on, Basic tier. On the Pi:

from rpi_rf import RFDevice
tx = RFDevice(17); tx.enable_tx()
CODES = {True: [5592405, 5592407, 5592411, 5592413],
         False: [5592404, 5592406, 5592410, 5592412]}

def on_rpc(req_id, body):
    if body.get("method") == "heaters.set":
        for code in CODES[bool(body["params"]["on"])]:
            tx.tx_code(code, 1, 350)     # protocol 1, pulse length from the sniff
        client.publish(f"v1/rpc/res/{req_id}", '{"ok": true}', qos=1)

(Your codes and pulse length come from the sniff, mine are placeholders.)

Then on the terrace asset’s Rules card:

• Rule “Heaters on”: When Sunrise or sunset, Sunset, offset -15. Then Send a command heaters.set on = yes to the Pi.
• Rule “Heaters off”: When A time of day 23:00, every day. Then the same with on = no.

Sunset needs the place’s location, so put latitude/longitude on the terrace asset and place the Pi under it, otherwise the rule won’t save. Also, 433 MHz has no feedback, so send the command twice a minute apart if you want to be sure. Doc: Time, sunrise/sunset, dates and holidays.

Close, but it’s the other way round, and it trips everyone. The names are from the Pi’s point of view: MOSI is the Pi sending to the chip, MISO is the chip sending to the Pi. DO on your breakout means “data out of the MAX31855”, so DO goes to MISO, GPIO 9, pin 21.

The MAX31855 never receives anything, you just pull CS low and clock out 32 bits, so it has no input pin at all. Leave MOSI (pin 19) unconnected. Wiring:

VIN -> 3.3 V (pin 1)
GND -> GND (pin 6)
DO  -> GPIO 9  MISO (pin 21)
CLK -> GPIO 11 SCLK (pin 23)
CS  -> GPIO 8  CE0  (pin 24)

Enable SPI in raspi-config, then read 4 bytes with spidev at 1 MHz or below. Bits 31..18 are the thermocouple temperature in 0.25 C steps, bit 16 is the fault flag. Check that flag before you send anything, an open thermocouple otherwise shows up as a strange number on your chart.