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CubeSat Simulator · Volume 4

The Schematics — Vol 4

The Schematics — Vol 4

Introduction

This volume reads the v2.1 board schematics circuit by circuit. The three PDFs are reproduced here as figures and annotated; the goal is that you can follow every functional block — compute, RF, sensing, power — from net to net. The source files are the project’s own EAGLE/CAD exports in the GitHub hardware/v2.1 folder.1

License / attribution. Each board’s title block reads ”© CC-BY-NC-SA+ 2019–2024 AMSAT — KU2Y / KI6ZUM / WD6DRI — CubeSatSim.org.” These schematics are reproduced under CC-BY-NC-SA: attribution required, non-commercial, share-alike. Credit AMSAT in any reuse.

How to read these. The CubeSatSim schematics use named nets rather than drawing every wire — e.g. PI_RF, FM_RF, FILTER_OUT, PICO_SDA/SCL, BATT_SW, AUDIO_OUT, PTT, PD. A signal appears wherever its label appears. The walk below follows those names.

1. Main board (STEM Payload Board)

The full Main board schematic sheet (PDF) is in the Schematics ▾ launcher at the top of this reader. Schematic: AMSAT CubeSatSim v2.1 (CC-BY-NC-SA), alanbjohnston/CubeSatSim. The PCB top render below cross-references it.

Figure 1 — Main board v2.1 PCB top render, for cross-reference to the schematic.
Figure 1 — Main board v2.1 PCB top render, for cross-reference to the schematic. — Image: AMSAT CubeSatSim v2.1 (CC-BY-NC-SA), alanbjohnston/CubeSatSim.

The Main board is the whole avionics suite on one PCB. Five functional blocks:

1a. Compute

  • U1 — Raspberry Pi Pico (RP2040). The payload MCU. Its GPIO carries the I²C sensor bus (PICO_SDA/PICO_SCL), a second I²C (PICO_SDA2/SCL2, via the optional U4 SparkFun Qwiic adapter), the FM-module serial (TX2/RX2), PTT, PD (power-down), AUDIO_OUT, the push-button PB, and the status-LED drives.
  • Raspberry Pi Zero (2) mounts on the 20×2 stacking header J1. Shared nets include PI_RF, PI_TXD/PI_RXD, PI_LED1/2, PI_3V3. A nice detail: the Pico senses 3.3 V on a Pi pin to detect whether a Pi Zero is present — so the same board runs either as a Pi-hosted satellite or as a standalone Pico STEM payload.

1b. RF chain — the downlink

This is the heart of the board. Signal flow: audio in → FM module → attenuator → low-pass filter → SMA antenna.

  • Transmitter module — U6 (SR105U), with U7 (DRA818/SA818) as a legacy alternative. The board has footprints for both, but the as-built radio is U6, an SR_FRS-family 0.5 W UHF FM module marked SR105U (“U” = UHF; the pin-compatible SR105V is the VHF sibling AMSAT uses for APRS balloon payloads). The current build instructions consistently mount “the SR_FRS board U6,” and the firmware self-identifies it (a 1 kΩ in R3/R4 ⇒ SR105U fitted). U7 is the older DRA818/SA818-class walkie-talkie module — in the v2.1 BOM it carries a “check availability” note (it became hard to source, which is why the SR_FRS is now default). Either module exposes the same control pins: PTT, PD, MIC_IN (audio), H/L (high/low power select), VBAT, ANT, and RXD/TXD for serial frequency/config. (One trap: the U7 schematic symbol is labeled “DRA818V”, but the board is a UHF design — “UHF low-pass filters”, “1k in R1/R2 ⇒ UHF LPF present.” If you ever populate U7 for the 434.9 MHz downlink you need the UHF variant, DRA818U/SA818U — the “V” is just the generic symbol/value, not an endorsement of the VHF part.)
  • Audio generation. Telemetry/SSTV audio is produced by PWM on the Pico/Pi, then smoothed by an RC low-pass filter (the design is credited in-schematic to a hackster.io “PWM audio on Pi Pico” technique). The filtered AUDIO_OUT → FILTER_OUT feeds the module’s MIC_IN. This is the bridge from digital bits to an FM-modulated tone stream (AFSK/FSK/SSTV).
  • Output attenuator. A 17 dB pi-attenuator drops the module’s ~500 mW to about 10 mW for safe, legal low-power demonstration (the schematic even cites a pi-attenuator calculator). Bridge JP2 to bypass it; for high power (~500 mW), cut JP11 and bridge JP2.
  • Output filtering + antennas. Each of the two SMA female jacks (X2 = Antenna 1, X3 = Antenna 2) sits behind a UHF low-pass filter built from L1/L2 = 13 nH and C1=18 pF, C2=16 pF, C3=18 pF, C4=16 pF (LPF design credited to Randy Standke, KQ6RS). Solder-jumpers select one antenna or both. This LPF is the “integrated 433 MHz filter” the AMSAT papers refer to — it cleans up the FM module’s harmonics so the downlink is spectrally clean.

1c. Sensors (telemetry payload)

  • U2 = MPU6050 — 9-DOF IMU (accelerometer + gyroscope) with on-chip temperature, on header J4, I²C.
  • U3 = BME280 — temperature / barometric pressure / humidity, on header J5, I²C.
  • D3 = 1N4148 — a diode used as a simple temperature sensor (net TEMPERATURE); a forward-biased junction’s voltage is a clean temperature proxy.

1d. Power & charge

  • Inputs: USB-C via J8 (SparkFun USB-C breakout) or J9 (direct USB-C); battery via the BATT/BATT_SW nets.
  • RBF (Remove-Before-Flight) switch gates the battery into the system — pulling it “launches” the sim. Bridge if not used.
  • F1 = RHEF100-2 PTC resettable fuse protects the battery path (it “replaces D4 in earlier versions”; bridge JP10 if omitted).
  • Charging is Pico-managed off the USB-C 5 V rail — bridge JP1 to enable the Pico to power and charge the battery. (There is no dedicated charger IC such as the CN3058 used in older revisions.)
  • Steering/protection: D1 = BYW27-400, D2/D10 = 1N5817 Schottky, optional 5V1 1 W Zener (DNI).

1e. User I/O

  • S1 = the mode push-button (PB net) — short presses cycle transmit modes.
  • Five status LEDs — LED1 White, LED2 Yellow, LED3 Green, LED4 Blue, LED5 Red — driven by Pi/Pico to show power/charge/transmit state and the selected mode. (The exact color/blink-to-mode mapping is operational detail covered in Vol 5.)

2. Battery board

The full Battery board schematic sheet (PDF) is in the Schematics ▾ launcher at the top of this reader. Schematic: AMSAT CubeSatSim v2.1 (CC-BY-NC-SA), alanbjohnston/CubeSatSim. The PCB top render below cross-references it.

Figure 2 — Battery board v2.1 PCB top render.
Figure 2 — Battery board v2.1 PCB top render. — Image: AMSAT CubeSatSim v2.1 (CC-BY-NC-SA), alanbjohnston/CubeSatSim.

A deliberately simple board: hold the cells, measure them, protect them.

  • Cells: 3× AA NiMH — two single-AA holders (BT1) or one dual holder (BT2); JP13/JP14 + a BH3AAPC holder as a fallback. Three NiMH cells ≈ 3.6 V nominal, ~4.0–4.2 V freshly charged. NiMH (not lithium) keeps the kit classroom-safe.
  • Telemetry: U2 = INA219AIDR — a high-side voltage + current monitor (the circuit follows Adafruit’s INA219 reference design) reading battery voltage and current through a 0.1 Ω shunt (R1); R2/R3 = 10 kΩ are I²C pull-ups. Nets BATT_IN/BATT_OUT.
  • I²C addressing: the primary battery is at 0x44, a secondary at 0x45 (bridge JP11) — so multiple battery boards can be chained and individually monitored.
  • Protection: F1 = PTC fuse (cut JP5 if used); JP4 bridges out the RBF if the switch lives only on the Main board.

3. Solar board

The full Solar board schematic sheet (PDF) is in the Schematics ▾ launcher at the top of this reader. Schematic: AMSAT CubeSatSim v2.1 (CC-BY-NC-SA), alanbjohnston/CubeSatSim. The PCB top render below cross-references it.

Figure 3 — Solar board v2.1 PCB top render.
Figure 3 — Solar board v2.1 PCB top render. — Image: AMSAT CubeSatSim v2.1 (CC-BY-NC-SA), alanbjohnston/CubeSatSim.

The solar board is a panel combiner with per-panel instrumentation:

  • Per-panel current sensing: ~12× INA219 monitors (a mix of through-hole “purple board” INA219 modules and INA219AIDR chips), each reading one solar input through its own 0.1 Ω shunt (R100). This is what produces the satellite’s multi-channel current telemetry — you can watch each face’s contribution change as you shade the cube.
  • Blocking diodes: Schottky D1–D6 = 1N5817 and D7–D12 = BAT20J (1 A / 23 V) OR the panels together and prevent reverse current into a shaded or cold panel — the same diode-OR a real satellite uses across its solar strings.
  • Panel connectors: a bank of Micro-JST 2-pin jacks (JP1–JP26) for the panel leads — matching the kit’s 10 JST-terminated panels.
  • Bus: 4.7 kΩ I²C pull-ups; JP9 = VDD-EN enable.

4. Inter-board interconnect

Two buses tie the stack together:

  • The 20×2 GPIO stacking header (J1) — present on every board — carries power (PI_3V3, BATT), the I²C telemetry bus (SDA/SCL), the FM/audio nets (AUDIO_IN/OUT, PTT, PD, FM, LPF, SQUELCH), serial (PI_TXD/RXD), and the LED/button lines (PI_LED1/2, PB). Boards literally stack and inherit these nets.
  • Micro-JST jumpers — carry battery and solar power/sense between the Battery, Solar, and Main boards (BATT_IN/BATT_OUT and the panel feeds).

The result: the Pi/Pico read all V/I/temperature over one I²C bus, drive the FM module over serial + a PWM audio line, and the power flows panel → solar board → battery board → Main board → 5 V to the Pi, exactly the architecture Vol 1 sketched and Vol 2 explained.

References

Footnotes

  1. CubeSatSim hardware v2.1 — schematics (cubesatsim-{main,battery,solar}-v2.1.0_schematic.pdf) + BOMs — https://github.com/alanbjohnston/CubeSatSim/tree/master/hardware/v2.1. Local copies in 02-inputs/research/schematics/. Board files © CC-BY-NC-SA AMSAT. ↩