Add battery wiring diagram

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 # Battery-Powered 28BYJ-48 Stepper Controller
 This project uses one 18650 cell, a TP4056 charger module, an MT3608 boost
 converter, an Arduino Nano, and a ULN2003 driver board to power a 28BYJ-48
 stepper motor.
 A DPST switch controls the battery-to-boost path and independently disconnects
 the motor supply. The Arduino Nano clone used here has **no USB VBUS Schottky
 diode**, so the USB and battery-powered 5 V supply must not be connected to the
 Nano at the same time.
 > This document describes wiring and power only. It does not include Arduino
 > code.
 ---
 ## 1. Components
 | Component | Notes |
 |---|---|
 | 18650 Li-ion cell | Single cell, suitable holder or secure battery connection |
 | TP4056 charger module | Use a protected module with separate `B+`, `B−`, `OUT+`, and `OUT−` terminals |
 | MT3608 boost converter | Adjustable; set its output to 5.0 V before connecting the loads |
 | DPST on/off switch | Both poles operate together |
 | Arduino Nano clone | This clone has no VBUS Schottky diode |
 | 28BYJ-48 stepper motor | Use the 5 V version |
 | ULN2003 driver board | Matches the motor’s 5-pin plug |
 | Optional fuse | Near TP4056 `OUT+`; rated above normal load current and below safe limits of the cell, protection board, and wiring |
 | Optional capacitor | 100–470 µF across the 5 V rail; observe capacitor polarity and voltage rating |
 Check the labels on the actual modules: board layouts and terminal names vary.
 Some TP4056 boards do not have a protection circuit or separate `OUT` terminals.
 This diagram requires a protected module with separate load terminals. Do not
 assume an unprotected TP4056 board provides battery over-discharge or
 over-current protection, and do not use its `B+`/`B−` pads as though they were
 protected `OUT+`/`OUT−` pads.
 ---
 ## 2. Power flow
 ```text
 18650 ──► TP4056 protected battery/load terminals
                     │
                     └── OUT+ ──► DPST Pole A ──► MT3608 ──► 5 V rail
                                                               ├── Nano 5V
                                                               └── DPST Pole B ──► ULN2003 VCC
 ```
 The TP4056 is a **charger**, not a 5 V power supply. Its `IN+`/`IN−` terminals
 are the USB charging input. The `B+`/`B−` terminals connect to the battery.
 The protected module’s `OUT+`/`OUT−` terminals provide the battery-voltage load
 connection.
 The MT3608 boosts the cell voltage (approximately 3.0–4.2 V) to 5.0 V.
 ---
 ## 3. Wiring schematic
 ```text
 Battery:
  18650 (+) ──► TP4056 B+
  18650 (−) ──► TP4056 B−
 Charging input:
  USB 5 V (+) ──► TP4056 IN+
  USB GND     ──► TP4056 IN−
 Battery-powered load path:
  TP4056 OUT+ ──[optional fuse]──► DPST Pole A common
  DPST Pole A switched terminal ──► MT3608 IN+
  TP4056 OUT− ──► common system GND
  MT3608 IN−   ──► common system GND
  MT3608 OUT−  ──► common system GND
  Nano GND     ──► common system GND
  ULN2003 GND  ──► common system GND
 5 V and motor supply:
  MT3608 OUT+ ──► 5 V rail ──┬──► Arduino Nano 5V
                              └──► DPST Pole B common
  DPST Pole B switched terminal ──► ULN2003 VCC
 Control and motor:
  Arduino Nano D9  ──► ULN2003 IN1
  Arduino Nano D10 ──► ULN2003 IN2
  Arduino Nano D11 ──► ULN2003 IN3
  Arduino Nano D12 ──► ULN2003 IN4
  28BYJ-48 5-pin plug ──► ULN2003 motor connector
 ```
 ### Important TP4056 ground detail
 Connect the cell’s negative terminal to **`B−` only**. Connect system ground to
 **`OUT−`**. Do **not** add an external wire directly joining `B−` to `OUT−`;
 that can bypass the protection circuit on a protected TP4056 module.
 The battery negative and system ground may be connected internally through the
 module’s protection circuitry. Use the module’s labeled load terminals for the
 load.
 ---
 ## 4. DPST switch operation
 The DPST switch has two electrically separate poles operated by the same
 handle:
 - **Pole A:** connects TP4056 `OUT+` to MT3608 `IN+`.
 - **Pole B:** connects the 5 V rail to ULN2003 `VCC`.
 | Switch state | Pole A | Pole B | Result |
 |---|---|---|---|
 | **ON** | Closed | Closed | Battery powers the MT3608; Nano and motor receive 5 V |
 | **OFF** | Open | Open | Battery is disconnected from the boost input; motor supply is disconnected |
 With Pole B open, the motor’s supply is physically disconnected even if the
 Nano is powered separately. The Nano itself can still be powered by USB when
 the switch is off.
 Pole B disconnects only the ULN2003 `VCC`; it does not disconnect the driver’s
 `IN1`–`IN4` signal wires or its ground. If the Nano is powered over USB while
 Pole B is open, its outputs can still drive the ULN2003 inputs and draw input
 current. For USB debugging in this state, hold the Nano in reset or disconnect
 the `IN1`–`IN4` signal wires. The motor supply remains disconnected because
 ULN2003 `VCC` is open.
 ---
 ## 5. MT3608 setup
 The MT3608 is adjustable; it is not a fixed 5 V supply.
 1. Turn the DPST switch **OFF**.
 2. Leave the Nano and ULN2003 disconnected.
 3. Connect the cell and measure MT3608 `OUT+` to `OUT−`.
 4. Adjust the module’s potentiometer until the output is **5.0 V**.
 5. Power-cycle and recheck the output.
 6. Connect the Nano and ULN2003 only after confirming the voltage.
 The module’s current capability depends on its components, input voltage, and
 cooling. The MT3608 IC’s “2 A” switch-current rating does not mean every module
 can deliver 2 A at 5 V.
 ---
 ## 6. Operating modes
 ### Run from battery
 - DPST: **ON**
 - Nano USB: **unplugged**
 - TP4056 charging USB: **unplugged**
 The Nano and motor are powered from the MT3608’s 5 V output.
 ### Charge the battery
 - DPST: **OFF**
 - TP4056 USB input: connected to a 5 V charger
 - Nano USB: **unplugged**
 With Pole A open, the battery-powered load path is disconnected. Keep the Nano
 USB unplugged during charging as the conservative procedure.
 ### Debug over USB serial
 - DPST: **OFF**
 - TP4056 charging USB: **unplugged**
 - Nano USB: connected to the computer
 - Disconnect the MT3608 `OUT+` wire from the 5 V rail while debugging.
 The Nano clone has no VBUS Schottky diode. Disconnecting the boost output during
 USB debugging avoids applying USB 5 V to the MT3608 output. Pole B remains open,
 so the ULN2003 motor supply is isolated.
 **Never connect Nano USB while the DPST is ON.** In that state, the MT3608
 supplies the Nano’s 5 V pin, which could back-feed the computer’s USB port.
 With the DPST OFF during USB debugging, the signal pins remain connected to
 ULN2003 `IN1`–`IN4`; hold the Nano in reset or disconnect those wires, as
 described above.
 ---
 ## 7. First power-on checklist
 1. Confirm the cell goes to TP4056 `B+` and `B−`.
 2. Confirm the circuit ground goes to TP4056 `OUT−`; do not bridge `B−` to
   `OUT−`.
 3. With the switch OFF and loads disconnected, set the MT3608 output to 5.0 V.
 4. Recheck polarity and wiring.
 5. Connect the Nano and ULN2003.
 6. Run from battery only with the switch ON and Nano USB unplugged.
 ---
 ## 8. Design notes
 - Use the Nano’s **`5V`** pin for the regulated 5 V rail. Do not use `VIN` for
  this 5 V supply.
 - The 28BYJ-48 plugs directly into the ULN2003 board; individual coil wires
  should not need to be jumpered.
 - Standard 28BYJ-48 ULN2003 boards do not have a current-adjustment trimmer.
  Leave their bypass capacitors installed.
 - An optional 100–470 µF capacitor can be placed across the 5 V rail and GND
  near the loads to help with supply dips. Observe its polarity.
 - Keep the Nano, MT3608, and ULN2003 grounds connected to the TP4056
  `OUT−`-referenced system ground.
 - Pole B isolates the ULN2003 motor supply only. It does not isolate the control
  inputs or ground, so it is not a complete electrical disconnection of the
  driver board.
 - Do not charge and run the load simultaneously with this simple TP4056
  arrangement. Supporting that requires a proper power-path/load-sharing
  design.
 ---
 ## 9. References
 These tutorials cover the stepper/driver portion of the project:
 - 28BYJ-48 + ULN2003 + Arduino walkthrough:  
  https://lastminuteengineers.com/28byj48-stepper-motor-arduino-tutorial/
 - element14 stepper wiring walkthrough:  
  https://community.element14.com/products/arduino/arduino-projects/b/blog/posts/how-to-control-28byj-48-stepper-motor-with-uln2003-driver-and-arduino
 TP4056 load-sharing background:
 - Arduino discussion of TP4056, boost converter, and Nano:  
  https://forum.arduino.cc/t/using-li-ion-battery-with-tp4056-to-power-arduino-nano/1324112
 - TP4056 load-sharing modification discussion:  
  https://forum.arduino.cc/t/tp4056-lithium-charger-module-modification-to-add-power-sharing/1008473