Design modification status: Audit/test documentation only. This plan does not modify the schematic, PCB, BOM, or layout.

• Use a current-limited bench supply for all powered tests.
• Start at 12V with a low current limit before testing 24V.
• Do not connect the dispenser machine until all bench tests pass.
• Treat RELAY_OUT as a dry-contact output only. It must not generate voltage by itself.
• Because the ESP32 DevKit can include onboard USB power, avoid connecting USB and external PCB power at the same time unless the specific back-power safety procedure below is followed.
• If any “Do not proceed” condition occurs, stop testing and investigate before continuing.

• Bench DC power supply, adjustable 0–30V, current limit capable.
• Digital multimeter with continuity, resistance, and DC voltage modes.
• Oscilloscope with 10x probe and short ground spring/lead.
• Optional: electronic load or power resistors for loading the 5V rail.
• USB power meter or current-limited USB data isolator/test adapter, if available.
• Thermometer, IR camera, or thermocouple probe.
• Computer with ESP32 serial monitor/programming environment.
• Test leads, insulated clips, and a known-good external low-voltage continuity/load circuit for dry-contact testing.
• Dispenser/coin-trigger interface simulator or actual dispenser input for final integration.

• Magnifier or microscope.
• Good lighting.
• Assembly drawing/BOM/schematic reference.

• Board unpowered.
• No USB connected to ESP32 DevKit.
• No external machine wiring connected.

1. Confirm correct polarity/orientation of J1, F2/PPTC, Q2 PMOS, D3 TVS, D6 zener, U1 LM2596, D2 Schottky, L1, C1/C2 electrolytic or polarized capacitors if applicable, U6 INA219, Q1 transistor, D1 flyback diode, K1 relay, and ESP32 DevKit headers.
2. Inspect all solder joints for bridges, tombstoned passives, unsoldered pins, cold joints, lifted pads, or cracked components.
3. Confirm field terminals are mechanically secure.
4. Confirm silkscreen labels for input polarity and relay output are readable.

• All components fitted in correct locations and orientations.
• No visible solder bridges, damaged parts, or assembly errors.

• Any polarity-sensitive part appears reversed.
• Any solder bridge or missing/incorrect part is found.
• Terminal polarity/labeling cannot be confidently identified.

• Suspected short, reversed IC/diode/capacitor, wrong relay, wrong regulator, wrong MOSFET, or unclear J1 polarity.

• Digital multimeter.

• Board unpowered.
• No USB connected.
• No external machine wiring connected.
• Capacitors discharged.

1. Measure resistance/continuity from J1 P1 positive input to J1 P2/GND.
2. Measure resistance/continuity from 5V rail to GND.
3. Measure resistance/continuity from 3V3 rail to GND.
4. Confirm J1 P2 is common with board GND.
5. Confirm relay dry contacts are isolated from GND, 5V, 3V3, VIN nets, and ESP32 pins.
6. Confirm RELAY_OUT P1/P2 are not shorted when relay is off, unless the relay contact arrangement intentionally uses normally-closed contacts. For this design, expected output is COM/NO dry contact, so off-state should be open.

• No hard short from input positive to GND.
• No hard short from 5V to GND.
• No hard short from 3V3 to GND.
• J1 P2 has continuity to GND.
• RELAY_OUT contacts are isolated from powered PCB nets.

• Near-zero ohms or continuity beep between any power rail and GND.
• RELAY_OUT connected to any powered board net.
• RELAY_OUT contacts shorted in relay-off state.

• Any power rail appears shorted to GND.
• RELAY_OUT is not isolated from PCB power.

• Digital multimeter.
• Optional insulation tester only if appropriate for relay/contact ratings and board design; do not exceed component ratings.

• Board unpowered.
• No external machine wiring connected.

1. Measure resistance from RELAY_OUT P1 to GND, 5V, 3V3, VIN_RAW, VIN_FUSED, VIN_PROTECTED, and VIN_FILTERED.
2. Measure resistance from RELAY_OUT P2 to the same powered nets.
3. Measure resistance between RELAY_OUT P1 and P2 with relay off.

• RELAY_OUT P1 and P2 show open circuit to all powered PCB nets.
• RELAY_OUT P1 to P2 is open when relay is off.

• Any measurable low-resistance path from relay contacts to board power/logic nets.
• RELAY_OUT P1/P2 shorted while relay is off.

• RELAY_OUT is electrically connected to PCB-generated voltage or logic ground unexpectedly.

• Current-limited bench DC supply.
• Digital multimeter.

• No USB connected.
• No external machine wiring connected.
• Bench supply current limit set very low, e.g. 50–100mA for initial test.
• Apply reverse polarity deliberately: supply positive to J1 P2 and supply negative to J1 P1.

1. Set supply to 12V and low current limit.
2. Apply reverse polarity briefly.
3. Observe supply current.
4. Measure 5V rail relative to board GND.
5. Measure 3V3 rail relative to board GND if accessible.
6. Remove power immediately after confirming behavior.

• Downstream 5V rail remains off, approximately 0V.
• 3V3 rail remains off, approximately 0V.
• Current remains limited to leakage or protection behavior, with no overheating.
• No component heats rapidly.

• 5V or 3V3 rail becomes energized under reverse polarity.
• Bench supply enters current limit hard or excessive current flows.
• Any component heats, smells, smokes, or discolors.

• Any downstream rail powers up during reverse-polarity input.
• Any input protection component heats significantly.

• Current-limited bench DC supply.
• Digital multimeter.
• Optional oscilloscope.

• Correct polarity: J1 P1 = positive, J1 P2 = GND.
• No USB connected.
• No relay output wiring connected.
• Set supply to 12V.
• Start with conservative current limit, e.g. 200–300mA, then raise only if needed.

1. Power the board from 12V.
2. Observe input current.
3. Measure protected/filtered VIN if accessible.
4. Measure 5V rail.
5. Measure 3V3 rail on ESP32 DevKit if accessible.
6. Observe for abnormal heating.

• Input current is reasonable for idle board/ESP32 state.
• 5V rail is near 5.0V.
• 3V3 rail is near 3.3V.
• No component overheats.

• Supply hits current limit unexpectedly.
• 5V rail missing, too high, or unstable.
• 3V3 rail missing, too high, or unstable.
• Any component heats rapidly.

• 5V exceeds safe downstream limits.
• 3V3 exceeds safe ESP32/INA219 limits.
• Input current is unexpectedly high or unstable.

• Current-limited bench DC supply.
• Digital multimeter.
• Oscilloscope recommended.
• Thermometer/IR camera recommended.

• Board already passed 12V power-up.
• Correct polarity at J1.
• No USB connected.
• No external machine wiring connected.
• Set supply current limit conservatively, then increase as needed.

1. Set input to 24V.
2. Apply power.
3. Measure input current.
4. Measure 5V rail.
5. Measure 3V3 rail.
6. Observe U1 LM2596, D2, L1, input protection parts, and capacitors for heating.

• 5V rail remains regulated near 5.0V.
• 3V3 rail remains regulated near 3.3V.
• No abnormal heating.
• Input current remains stable.

• 5V rail droops, overshoots, oscillates, or disappears.
• Excessive heating at LM2596, D2, L1, Q2, F2, D3, or capacitors.
• Supply current is unexpectedly high.

• Any voltage rail is out of safe range.
• Any switching regulator component becomes too hot to touch quickly.

• Oscilloscope with 10x probe.
• Short ground spring or very short ground connection.
• Digital multimeter.
• Optional electronic load or resistor load.

• Power from 12V first, then repeat at 24V.
• No USB connected.
• Probe 5V to GND close to the LM2596 output capacitor and again near ESP32 5V input.

1. Measure DC 5V with multimeter.
2. Measure ripple on oscilloscope using AC coupling or suitable scale.
3. Repeat at idle and during relay activation.
4. Repeat during ESP32 Wi-Fi activity if firmware enables Wi-Fi.
5. Repeat at 24V input.

• 5V rail remains close to 5.0V under idle, relay activation, and ESP32 activity.
• Ripple and transient dips do not cause ESP32 reset or relay dropout.

• 5V rail dips enough to reset ESP32 or INA219.
• Excessive ripple, oscillation, or large switching spikes are observed.
• 5V rail overshoots above safe downstream limits.

• ESP32 resets during relay or Wi-Fi current pulses.
• 5V rail is unstable or has large unexplained ripple.

• Computer with serial monitor.
• USB cable for DevKit.
• Bench supply.

• Preferred safe method: power the PCB from bench supply and use USB only if the ESP32 DevKit/back-power risk is controlled per Test 13.
• If there is no jumper/isolator/back-power protection, avoid simultaneous external 5V and USB until the USB/back-power procedure is followed.

1. Power the board from 12V input.
2. Confirm 5V and 3V3 rails are valid.
3. Observe ESP32 boot indicator/serial output if connected safely.
4. Confirm firmware starts and does not reset repeatedly.

• ESP32 boots reliably.
• Serial output or firmware status confirms normal startup.
• No brownout/reset loop.

• ESP32 does not boot.
• ESP32 repeatedly resets.
• 3V3 rail unstable.

• Back-power path is uncontrolled and USB must be connected for debugging.
• ESP32 brownout messages or repeated boot loops occur.

• Firmware or test sketch capable of toggling GPIO23.
• Bench supply.
• Multimeter or oscilloscope.

• Power board from 12V first.
• No external machine connected to RELAY_OUT.
• Optional: connect multimeter across relay coil or transistor collector/emitter nodes if accessible.

1. Load firmware/test sketch that toggles GPIO23 at a slow rate, e.g. 1 second on / 1 second off.
2. Confirm relay clicks on command.
3. Measure 5V rail during relay activation.
4. Repeat at 24V input.

• GPIO23 activation energizes relay consistently.
• Relay releases when GPIO23 is inactive.
• 5V rail remains stable.
• ESP32 does not reset.

• Relay does not activate.
• Relay chatters.
• Relay remains stuck on/off.
• ESP32 resets when relay activates.

• Relay activation causes 5V/3V3 collapse, excessive heating, or uncontrolled output behavior.

• ESP32 firmware reading INA219 over I2C.
• Serial monitor.
• Multimeter if measuring shunt voltage.
• Optional oscilloscope.

• Board powered from 12V.
• Relay activation firmware available.
• INA219 library/test code configured for the expected shunt value: RSHUNT = 0.22 ohm.

1. With relay off, read INA219 current.
2. Activate relay from GPIO23.
3. Read INA219 current while relay is energized.
4. Optionally measure shunt voltage directly across RSHUNT.
5. Repeat multiple on/off cycles.
6. Repeat at 24V input, since relay coil is still on the 5V rail and should read similar current.

• Relay-off current reading is near zero for the relay coil path.
• Relay-on current reading is plausible for the relay coil, expected around the relay coil current range.
• For a 0.22 ohm shunt, reference voltages are:
  • 70mA → 15.4mV
  • 80mA → 17.6mV
  • 100mA → 22.0mV
• Reading is stable and repeatable.

• INA219 not detected on I2C.
• Current reading does not change when relay activates.
• Reading polarity is reversed from expectation.
• Measured shunt voltage does not match reported current.

• Relay current path bypasses the shunt or INA219 readings are nonsensical.
• I2C errors cause ESP32 instability.

• Digital multimeter in continuity/resistance mode.
• Firmware/test sketch for GPIO23 relay control.

• Board powered from 12V.
• No external machine wiring connected.
• Multimeter connected across RELAY_OUT P1 and P2.

1. With relay off, measure RELAY_OUT P1 to P2.
2. Turn relay on using GPIO23.
3. Measure RELAY_OUT P1 to P2 again.
4. Repeat several times.

• Relay off: RELAY_OUT P1/P2 open circuit.
• Relay on: RELAY_OUT P1/P2 closed, low resistance/continuity.
• Behavior follows GPIO23 command.

• Contacts closed when relay is off.
• Contacts open when relay is on.
• Intermittent contact behavior.
• Contact resistance unexpectedly high.

• RELAY_OUT does not reliably follow relay state.

• Digital multimeter.
• Firmware/test sketch for relay control.

• Board powered from 12V, then repeat at 24V.
• No external voltage connected to RELAY_OUT.
• Multimeter in DC voltage mode.

1. Measure RELAY_OUT P1 to board GND with relay off.
2. Measure RELAY_OUT P2 to board GND with relay off.
3. Measure RELAY_OUT P1 to P2 with relay off.
4. Activate relay.
5. Repeat all three voltage measurements with relay on.

• Approximately 0V on RELAY_OUT relative to board GND and across P1/P2 because the relay contact is dry and no external voltage is applied.

• Any meaningful DC voltage appears on RELAY_OUT generated by the PCB.
• RELAY_OUT is referenced to 5V, 3V3, VIN, or GND unexpectedly.

• RELAY_OUT produces voltage by itself.

• USB power meter, current-limited USB source, or sacrificial/current-limited USB test adapter.
• Digital multimeter.
• Bench supply.
• Optional series Schottky diode or USB data-only adapter for safer test setup.

• This project powers the ESP32 DevKit through its 5V/EXT_5V pin. If the DevKit USB is connected while the PCB 5V rail is powered, a back-power conflict may exist unless a removable jumper, diode, ideal diode, fuse, or load switch is added.
• For the current prototype, treat simultaneous USB and external PCB 5V as unsafe unless verified with current limiting.

1. Do not connect PC USB while the board is externally powered unless necessary.
2. If programming/debugging via USB is required, first power off the external 12V/24V input.
3. Connect USB to the ESP32 DevKit only.
4. Measure PCB 5V rail. If USB back-powers the carrier PCB, note this behavior.
5. If simultaneous USB and external power must be evaluated, use a current-limited USB source or USB power meter, not a valuable PC USB port.
6. With external PCB power off and USB connected, measure whether the carrier 5V rail becomes energized.
7. With external PCB power on and USB connected through a protected/current-limited adapter, check for current flow into or out of USB 5V.

• Preferred prototype-safe operating rule: do not connect ESP32 USB while external PCB power is applied unless a removable link/back-power protection is installed or removed appropriately.
• If USB is connected with PCB power off, any carrier back-powering must be documented as a known limitation.

• USB powers the carrier PCB unexpectedly and energizes downstream loads.
• External 5V back-feeds into the USB port.
• PC or USB source reports overcurrent or disconnects.

• Any uncontrolled current flow occurs between USB 5V and PCB 5V.
• Testing requires connecting an unprotected PC USB port while external power is present.

• Use a removable jumper or solder bridge between BUCK_5V and ESP32_EXT_5V, labelled:
  “JP_ESP32_5V — install for field operation, remove when programming via DevKit USB”.

• Bench DC supply.
• Firmware/test sketch for repeated relay activation.
• Thermometer, thermocouple, or IR camera.
• Oscilloscope optional for rail monitoring.

• Board powered at 24V input for worst-case buck input voltage.
• No external machine connected initially.
• Relay output may be unloaded for first test.

1. Run relay activation at realistic duty cycle for the dispenser application.
2. Include long on-pulses representative of worst expected field use.
3. Run repeated pulses for at least 15–30 minutes during prototype validation.
4. Monitor temperatures of U1 LM2596, D2, L1, Q2, F2, D3, Q1, D1, K1, RSHUNT, and connectors.
5. Monitor 5V rail for droop or reset events.
6. Repeat with the expected external relay contact load only after dry-contact tests pass.

• No component reaches unsafe temperature.
• Relay activates consistently.
• ESP32 remains running.
• INA219 readings remain plausible.
• 5V rail remains stable.

• Any component overheats or changes behavior with temperature.
• Relay drops out, chatters, or fails to release.
• ESP32 resets during repeated pulses.
• RSHUNT, Q1, D1, buck diode, inductor, or regulator temperature rises unexpectedly.

• Thermal runaway, repeated resets, relay malfunction, visible discoloration, smell, or unstable 5V rail.

• Dispenser machine or validated coin-trigger input simulator.
• Multimeter.
• Optional oscilloscope.
• Bench supply or intended field supply.
• Firmware configured for final trigger behavior.

• Only perform after Tests 1–14 pass.
• Confirm RELAY_OUT is dry contact and generates no voltage.
• Confirm dispenser input voltage/current requirements are compatible with relay dry-contact rating.
• Connect RELAY_OUT across the dispenser coin-trigger input exactly as a switch/contact closure.
• Keep board powered from intended 12V–24V supply.

1. Power the controller normally.
2. Confirm ESP32 boots and firmware is running.
3. Trigger relay using final control logic or diagnostic command.
4. Confirm dispenser detects the coin-trigger/contact closure.
5. Confirm no unintended multiple triggers, stuck trigger, or missed trigger.
6. Repeat for multiple cycles.
7. Monitor 5V rail and board temperature during repeated operation.

• Relay contact closure reliably triggers the dispenser input.
• No voltage is sourced from the PCB into RELAY_OUT except the external machine’s own circuit through the dry contact.
• No ESP32 resets or false triggers.
• Board remains thermally stable.

• Dispenser does not detect trigger.
• Dispenser triggers unexpectedly.
• RELAY_OUT applies voltage incorrectly.
• ESP32 resets during machine operation.
• EMI/noise from machine wiring disrupts operation.

• Any unsafe interaction with machine wiring.
• Any evidence that RELAY_OUT is not isolated/dry-contact behavior.
• Repeated false triggers or uncontrolled relay operation.

• Visual inspection passes.
• No shorts are found before power.
• Reverse-polarity test does not energize downstream rails.
• 12V and 24V power-up tests pass.
• 5V and 3V3 rails remain within safe operating range.
• ESP32 boots reliably.
• GPIO23 controls the relay reliably.
• INA219 current reading changes correctly with relay activation.
• RELAY_OUT behaves as an isolated dry contact and generates no voltage.
• USB/back-power limitation is either mitigated or handled with a strict warning/procedure.
• Repeated relay pulses do not cause overheating or resets.
• Final dispenser coin-trigger integration works reliably.

• Do not connect ESP32 DevKit USB and external PCB input power at the same time unless back-power protection or a removable 5V link procedure is implemented.
• Do not connect RELAY_OUT to any circuit until dry-contact isolation and no-voltage tests pass.
• Do not proceed to 24V testing until 12V testing passes.
• Do not connect the dispenser until all bench-level relay and rail stability tests pass.