# 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

---

## 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.

---

## 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

---

## 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.

---

## 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)
```

---

## 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.

---

## 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 |

---

## 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
```

---

## 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  

---

## 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)

---

## 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

---

## 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)

---

## 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)

---

## 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.