Feature: Hardware design
Figure 3: Shared 3.3V I²C implementation: (a) The CHT20MEMS temperature and humidity sensor with local decoupling; (b) The ATTINY816 controller, I²C pull-ups, OLED display and display connector
not a battery-life claim. Self-discharge, converter efficiency, temperature, component tolerance, OLED behaviour and real firmware timing must later be included and measured at the battery terminals. Peak-current analysis asks a different question: how far
does the supply fall during a load step? A useful first-order estimate is:
ΔV ≈ ΔI(Rcell + Rpath + ESR) + ΔIΔt/Ceff For a 20mA step, 150mΩ total resistance, a 100µs regulator
response interval and 47µF effective capacitance, the estimated droop is about 46mV. Te calculation must use capacitance aſter DC bias and temperature derating, cell resistance at low state of charge and the worst permitted combination of loads. Te project’s power sheet makes these decisions visible:
USB-C input and protection feed a BQ24074 charging stage, while a TPS63031 generates the 3.3V system rail and a MAX809 supervises it. Each choice must be checked against quantified limits. In a linear charger, dissipation is approximately (VIN VBAT
− )ICHG . At 5V input, a 3.2V cell and 500mA charge current,
that approaches 0.9W before concurrent system load is considered. Charge current must therefore come from junction- temperature, PCB-copper, enclosure-temperature and power-path budgets rather than the cell’s maximum rating alone; see Figure 2.
Treat power, data and sensing as electrical contracts Naming a protocol is not an interface specification. An interface record should include voltage domain, signal direction, logic thresholds, reset state, pull devices, maximum frequency, expected capacitance, unpowered behaviour, ESD exposure and ownership during boot.
18 October 2026
www.electronicsworld.co.uk The project places the ATTINY816, CHT20MEMS and
OLED on a shared 3.3V I²C bus. For the standard RC rise- time approximation is:
tr = 0.8473RpCb For a 300ns rise-time requirement and an estimated 200pF
bus, the maximum pull-up resistance is approximately 1.77kΩ. If the devices must keep low-level current below 3mA with a 0.4V low-level target, the calculated minimum is about 967Ω. A 1.5kΩ candidate sits between those limits but consumes roughly 1.93mA whenever a line is low. The result is a timing-versus-energy trade-off to be verified at the farthest node with the selected OLED module and actual PCB capacitance; see Figure 3. USB-C charging has similar hidden requirements. A 5V
sink still needs correct configuration-channel termination and must respect the current advertised by the source. Connector choice alone does not define input current, ESD path, reverse-current behaviour, battery protection, temperature policy or what happens when charging and system operation compete for available power. For an environmental sensor, mechanical placement is
also an electrical requirement. The CHT20MEMS sensing aperture needs access to representative air while remaining isolated from heat produced by the charger, regulator, processor and display. An electrically correct sensor placed beside the charging path can report a stable but biased temperature. Vent geometry, splash and dust exposure, coating restrictions and calibration strategy must therefore enter the requirements before PCB placement.
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