Hey! So you’re squeezing a climate monitor into a 4‑inch cube—nice challenge. Here’s a quick play‑book that should keep the cost low and the precision decent. **Microcontroller** Use an ESP32‑DevKitC or a tiny ATtiny85 if you’re all about power. ESP32 gives you Wi‑Fi or Bluetooth to ship data to your phone, while the ATtiny85 is a cheap, super‑small option but you’ll need a separate Wi‑Fi module (ESP‑01) if you want wireless. **Temperature sensor** The DS18B20 is a great choice. It’s tiny, one‑wire, and can be nailed to the glass with a bit of epoxy. It gives ±0.5 °C accuracy and can even work in a 0–90 °C range—perfect for a greenhouse. **Humidity sensor** DHT22 is the most common low‑cost sensor, but it’s a bit bulky. A better fit is the SHT31‑I2C; it’s only about 2 mm thick, gives ±2 % RH accuracy, and I2C means you only need two pins on the MCU. **Power** A 3.7 V Li‑Po cell with a small boost converter to 3.3 V keeps the enclosure tight. The ESP32 can sleep most of the time, waking every 5 minutes to take a reading. **PCB / layout** Use a 1‑layer PCB to keep it simple. Mount the DS18B20 on the edge of the board so it’s exposed to the air. Put the SHT31 right above it, shielded by the enclosure but still in contact with the greenhouse air. **Enclosure** Print a 4 cm³ case with a small opening for the sensors. Add a thin layer of silicone sealant around the edges to keep moisture out but still let the sensors breathe. **Software** Just a few lines: read DS18B20, read SHT31, send data via MQTT or Bluetooth. Add a calibration routine so you can tweak offsets if the greenhouse is a little humid. **Extras** If you want a display, drop a tiny 0.66 inch OLED on the PCB; it’s 1.5 mm thick and fits nicely. Or just log data to a cloud dashboard—no display needed. That’s it. It’ll fit, it’ll be accurate enough for a small greenhouse, and it’ll keep the budget low. Happy building—just don’t forget to let me borrow the soldering iron once you’re done!