Metron

Digital "measuring tape" with OLED display and HCSR04 ultrasonic distance sensor.

Hardware

• ATtiny84
• Push-button
• 10k resistor
• 22k resistor
• 2x 100nF ceramic capacitor
• 128x32 I2C OLED display
• HCSR-04 ultrasonic distance sensor module
• 10k NTC thermistor (B = 3435, see below)
• 1k resistor
• 2N3904 transistor
• 5v boost converter module
• AAA battery and holder

Software

• Arduino 1.8.9 (or later) with ATTinyCore
• Tiny4kOLED library
• SimpleTimer library
• uC-Makefile (optional)

Thermistor

Assuming operating temperature is in the range [0, 50) degrees C, we generate an array of resistances as follows:

(defn steinhart-hart [R0 B]
  (let [T0 298.15
        rinf (/ R0 (Math/exp (/ B T0)))]
    (fn [T] (Math/round (* rinf (Math/exp (/ B (+ T 273.15)))))))

(map (steinhart-hart 10000 3435) (range 0 50))

This is for an NTC part with R0=10k and B=3435, see the description of the Steinhart-Hart equation.

(Conveniently all of the values in this range fit in 16-bits so the entire array takes up only 100 bytes of flash.)