ESP_Midea/Midea_ESP/SCHEMATIC_ANALYSIS.md

Schematic Analysis: Midea AC Controller with BLE Beacon

Project Overview

Title: Midea AC Controller with BLE Beacon
Date: 2025-12-28
Revision: 1.0
Purpose: ESP32-based controller for Midea AC units with Bluetooth Low Energy beacon functionality

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1. Power Supply System

Power Input

• Source: +5V external supply (via J1 connector)
• Regulator: AMS1117-3.3 (U1)
  • Input: +5V (Pin 3 - VI)
  • Output: +3.3V (Pin 2 - VO)
  • Ground: Pin 1 (GND)

Power Distribution

• +5V Rail:

  • Powers TXB0104PWR level shifter VCCB (Pin 11)
  • Powers AC connector J1 (Pin 1)
  • Powers +5V power symbols

• +3.3V Rail:

  • Powers ESP32-WROOM-32E VDD (Pin 2)
  • Powers TXB0104PWR level shifter VCCA (Pin 12)
  • Powers TXB0104PWR OE (Pin 10) - always enabled
  • Powers pull-up resistors (R3, R4)
  • Powers LED circuits

Decoupling Capacitors

• C1: 5V input filtering (26.67mm, 39.37mm)
• C2: 3.3V output filtering (63.5mm, 39.37mm)
• C3: 3.3V input filtering (38.1mm, 39.37mm)
• C4: 3.3V output filtering (74.93mm, 39.37mm)
• C5: ESP32 VDD decoupling (52.07mm, 81.28mm)
• C6: ESP32 VDD decoupling (52.07mm, 88.9mm)

⚠️ Potential Issue: Missing decoupling capacitors directly on TXB0104PWR power pins (VCCA and VCCB). Datasheet recommends 100nF ceramic capacitors on each power pin.

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2. ESP32-WROOM-32E Module (U3)

Power Connections

• VDD (Pin 2): +3.3V (with decoupling capacitors C5, C6)
• GND (Pin 1, 15, 38, 39): Ground plane

UART Communication

• GPIO17 (Pin 25 - TXD0/IO1): → ESP32_TX label → UART_TX → TXB0104PWR A1 (Pin 1)
• GPIO16 (Pin 27 - IO16): → ESP32_RX label → UART_RX → TXB0104PWR A2 (Pin 2)

Note: The schematic shows GPIO17 as TX and GPIO16 as RX, but these may need to be configured in software depending on which UART peripheral is used.

Control Buttons

• SW1 (Reset Button):

  • Connected to EN pin (Pin 3)
  • Pull-up resistor R3 to +3.3V
  • When pressed: EN goes LOW, ESP32 resets

• SW2 (Boot Button):

  • Connected to GPIO0 (Pin 25 - IO0)
  • Pull-up resistor R4 to +3.3V
  • When pressed: GPIO0 goes LOW, ESP32 enters download mode

Status LEDs

• D1: Connected via R1 (current limiting resistor)
• D2: Connected via R2 (current limiting resistor)
• Note: LED anodes connect to resistors, cathodes to GND

Unused Pins

Many ESP32 pins are marked as "NC" (No Connect):

• Pins 17, 18, 19, 20, 21, 22, 32 (NC pins)
• Pins 4, 5 (SENSOR_VP, SENSOR_VN) - unused
• Pins 6, 7 (IO34, IO35) - input-only pins, unused

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3. Level Shifter: TXB0104PWR (U2)

Purpose

Bidirectional voltage level translation between:

• A Port (Low Voltage): 3.3V side (ESP32)
• B Port (High Voltage): 5V side (AC Controller)

Power Connections

• VCCA (Pin 12): +3.3V (low voltage side)
• VCCB (Pin 11): +5V (high voltage side)
• GND (Pins 5, 13, 14): Ground
• OE (Pin 10): +3.3V (always enabled)

⚠️ Issue: OE is connected directly to +3.3V. While this works, datasheet recommends a pulldown resistor for power-up safety. However, since both supplies come from the same source, this is acceptable.

Signal Connections

• A1 (Pin 1): ESP32 TX → UART_TX
• A2 (Pin 2): ESP32 RX → UART_RX
• B1 (Pin 9): AC RX → J1 Pin 2
• B2 (Pin 8): AC TX → J1 Pin 3

Unused Channels

• A3 (Pin 3): NC
• A4 (Pin 4): NC
• B3 (Pin 7): NC
• B4 (Pin 6): NC

Note: Only 2 of 4 channels are used. This is fine - the IC supports partial channel usage.

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4. AC Interface Connector (J1)

Connector Type

• Type: Conn_01x04_Pin (4-pin single-row connector)
• Footprint: PinHeader_1x04_P2.00mm_Vertical

Pin Assignments

• Pin 1: +5V (power to AC dongle)
• Pin 2: AC_RX (receive from AC, via level shifter B1)
• Pin 3: AC_TX (transmit to AC, via level shifter B2)
• Pin 4: GND (ground)

Connection Flow

ESP32 GPIO17 (TX) → Level Shifter A1 → Level Shifter B2 → J1 Pin 3 (AC_TX)
ESP32 GPIO16 (RX) ← Level Shifter A2 ← Level Shifter B1 ← J1 Pin 2 (AC_RX)

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5. Additional Connector (J2)

Connector Type

• Type: Conn_02x04_Counter_Clockwise (8-pin dual-row connector)
• Footprint: PinHeader_2x04_P2.00mm_Vertical

Purpose

⚠️ Unclear: J2 is present in the schematic but connections are not fully visible. This may be:

• Debug/programming connector
• Additional I/O expansion
• Reserved for future use

Recommendation: Verify J2 pin assignments and document purpose.

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6. Component Summary

Active Components

Ref	Component	Value	Purpose
U1	AMS1117-3.3	3.3V LDO	Power regulation
U2	TXB0104PWR	Level Shifter	3.3V ↔ 5V translation
U3	ESP32-WROOM-32E	MCU	Main controller

Passive Components

Ref	Component	Value	Purpose
C1-C6	Capacitors	Various	Power filtering/decoupling
R1-R4	Resistors	Various	Current limiting/pull-ups
D1, D2	LEDs	Status indicators	Visual feedback
SW1, SW2	Push buttons	Reset/Boot	Control buttons

Connectors

Ref	Type	Pins	Purpose
J1	1x04 Pin Header	4	AC Controller interface
J2	2x04 Pin Header	8	Unknown/Reserved

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7. Signal Flow Analysis

UART Communication Path

AC Controller (5V)          Level Shifter          ESP32 (3.3V)
─────────────────          ─────────────          ────────────
J1 Pin 3 (TX) ────────→ B2 ────────→ A1 ───────→ GPIO17 (TX)
J1 Pin 2 (RX) ←──────── B1 ←──────── A2 ←─────── GPIO16 (RX)

Power Flow

External 5V → J1 Pin 1 ──┬──→ U1 (AMS1117) ──→ +3.3V ──→ ESP32 VDD
                         │
                         └──→ U2 VCCB (5V side)
                         └──→ +5V rail

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8. Design Strengths

✅ Proper Power Regulation: AMS1117-3.3 provides stable 3.3V for ESP32
✅ Level Shifting: TXB0104PWR correctly handles 3.3V ↔ 5V translation
✅ Bidirectional Communication: Automatic direction sensing in level shifter
✅ Reset/Boot Controls: Proper button implementation with pull-ups
✅ Decoupling: Multiple capacitors for power supply filtering
✅ Status Indicators: LEDs for visual feedback

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9. Potential Issues & Recommendations

⚠️ Critical Issues

1. Missing Decoupling Capacitors on TXB0104PWR

   • Issue: No 100nF capacitors on VCCA (Pin 12) and VCCB (Pin 11)
   • Impact: Potential signal integrity issues, noise on power rails
   • Fix: Add C7 (100nF) from VCCA to GND, C8 (100nF) from VCCB to GND
   • Placement: As close as possible to power pins (<5mm)

2. OE Pin Connection

   • Current: OE directly connected to +3.3V
   • Issue: No pulldown resistor for power-up safety
   • Impact: Minor - may cause brief high-impedance during power-up
   • Fix (Optional): Add 10kΩ pulldown resistor from OE to GND

⚠️ Design Considerations

3. J2 Connector Purpose

   • Issue: J2 connections not clearly defined
   • Action: Document purpose and pin assignments

4. ESP32 UART Pin Selection

   • Current: GPIO17 (TX), GPIO16 (RX)
   • Note: Verify these are the correct UART peripheral pins
   • Check: ESP32-WROOM-32E pinout for UART0/UART1/UART2 assignments

5. LED Current Limiting

   • Issue: Resistor values (R1, R2) not specified
   • Recommendation: Calculate based on LED forward voltage and desired current
   • Typical: 220Ω-1kΩ for 3.3V supply with standard LEDs

6. Power Supply Sequencing

   • Current: 5V → 3.3V (via regulator)
   • Note: TXB0104PWR requires VCCA ≤ VCCB (satisfied: 3.3V ≤ 5V)
   • Status: Acceptable, but ensure proper power-up sequence

💡 Recommendations

7. Add Test Points

   • Consider adding test points for:
     • +5V rail
     • +3.3V rail
     • UART_TX/RX signals
     • GND

8. Add Fuse/Protection

   • Consider adding:
     • Input fuse for +5V protection
     • TVS diodes on UART lines for ESD protection

9. Component Values

   • Document all component values:
     • Capacitor values (C1-C6)
     • Resistor values (R1-R4)
     • LED specifications (D1, D2)

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10. PCB Layout Recommendations

Critical Placement

1. TXB0104PWR Decoupling:

   • Place 100nF capacitors within 5mm of VCCA and VCCB pins
   • Use short, wide traces

2. ESP32 Decoupling:

   • C5, C6 should be as close as possible to VDD pin
   • Use ground plane for return path

3. Level Shifter Placement:

   • Place TXB0104PWR between ESP32 and J1 connector
   • Minimize trace lengths for UART signals

4. Power Traces:

   • Use adequate trace width for current capacity
   • ESP32 can draw up to 500mA peak
   • Minimum 0.5mm trace width recommended

5. Ground Plane:

   • Use continuous ground plane
   • Connect all GND pins to ground plane
   • Avoid ground loops

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11. Testing Checklist

Power Supply

• Verify +5V input voltage
• Verify +3.3V output from regulator (within ±5%)
• Check for ripple on power rails
• Verify power consumption

Level Shifter

• Verify VCCA = 3.3V, VCCB = 5V
• Test bidirectional signal translation
• Verify OE pin is HIGH (enabled)
• Check signal integrity on UART lines

ESP32

• Verify power-on and boot sequence
• Test reset button functionality
• Test boot button (download mode)
• Verify UART communication
• Test LED indicators

AC Interface

• Verify J1 connector pinout
• Test UART communication with AC controller
• Verify signal levels (3.3V ↔ 5V translation)

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12. Bill of Materials (BOM) Summary

Required Components

• 1x ESP32-WROOM-32E module
• 1x TXB0104PWR (TSSOP-14)
• 1x AMS1117-3.3 (SOT-223)
• 6x Capacitors (various values)
• 4x Resistors (various values)
• 2x LEDs
• 2x Push buttons
• 2x Connectors (J1, J2)

Missing Components (Recommended)

• 2x 100nF capacitors (TXB0104PWR decoupling)
• 1x 10kΩ resistor (OE pulldown - optional)

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Conclusion

The schematic shows a well-designed circuit for an ESP32-based AC controller with proper level shifting for 5V communication. The main areas for improvement are:

1. Add decoupling capacitors on TXB0104PWR power pins
2. Document component values (resistors, capacitors)
3. Clarify J2 connector purpose and connections
4. Verify ESP32 UART pin assignments match software configuration

Overall, the design is sound and should work correctly with the recommended additions.