Resistors, Capacitors and Passives

The BOM has a handful of passives. Each has a specific job; substitution is fine within reason, but know what the value does.

Resistors

Ref	Value	Role
R1, R2	330 Ω	LED current limiters
R3	100 Ω	Audio output series
R3a, R3b	100 kΩ	Battery voltage divider (halves VBAT for the ADC)

• The divider ratio is what matters: 100k/100k halves the voltage. Using 10k/10k keeps the ratio but raises the current drain from 21 µA to 210 µA - still fine, but 100k is the design value.
• LED resistors: 330 Ω gives ~6-8 mA. 220-470 Ω all work; the LED just gets brighter or dimmer.

Capacitors

Ref	Value	Role
C1	100 µF electrolytic	Bulk decoupling on 3.3 V rail
C2	100 nF ceramic (x2)	Local decoupling near the ESP32 and radio
C3	10 µF electrolytic	AC-coupling on the audio path

• The 100 µF bulk cap smooths the PA’s current bursts. A smaller value (47 µF) still works in practice.
• The 100 nF ceramics are standard bypass; value is not critical (10-100 nF all fine).
• The 10 µF audio cap sets the low-frequency cutoff with the 100 Ω (about 160 Hz) - fine for Morse tones (440-2200 Hz).

Cold weather substitution

Below -10 C, electrolytic capacitors stiffen and lose capacitance. Replace C1 with a 47 µF X7R ceramic for alpine use. This is documented in the BOM notes.

Sizing

The BOM specifies 0805 SMD or through-hole; either works. Through-hole is easier to solder by hand. Tolerance: 5% resistors, 10-20% capacitors - all fine here.

Testing passives

• Resistors: measure with the multimeter; cold solder joints on the divider are a top cause of wrong battery readings.
• Capacitors: a shorted cap reads near 0 Ω; a dead-open cap reads open. Replace any suspect cap before debugging further.