ESP32 Sensor Module

GitHub: Carputer-project/esp32_sensor_module

Firmware for the sensor ESP32 on the Carputer PCB. Designed for OBD1 vehicles (pre-1996, all makes) — reads engine ECU sensor voltages, switch signals, and diagnostic pulses directly. No CAN bus required. CAN/OBD2 is an easy add — just a firmware update and an external CAN module (e.g. MCP2515 via SPI).

Currently configured for a Toyota 5S-FE as reference, but the voltage divider values and input pins work with any OBD1 ECU. Streams data over UDP to the carputer head unit.

Role

Connects to ECU as WiFi STA at 192.168.4.20, broadcasts JSON sensor data to 192.168.4.255:5001 (subnet broadcast so both carputer app and cluster display receive data). Also handles DTC (diagnostic trouble code) reading via the check-engine light blink pattern.

ECU Signal Wiring (5.6K + 10K Voltage Divider)

All ECU signals go through a voltage divider: R1 = 5.6KΩ (series from ECU), R2 = 10KΩ (to GND). Ratio = 0.641x. Max output: 5V × 0.641 = 3.21V (safe under 3.3V).

ECU Signal ──[5.6KΩ]──┬── ESP32 GPIO
│
[10KΩ]
│
GND

Analog Signals

GPIO	ECU Signal	ECU Pin Name	ADC Channel	Unit	Notes
GPIO 32	PIM	MAP (Manifold Absolute Pressure)	ADC1_CH4	kPa	0-5V → 0-3.21V
GPIO 33	THW	ECT (Engine Coolant Temp)	ADC1_CH5	°F	NTC via ECU 2.2K pullup to 5V, reversed through divider
GPIO 34	VTA	TPS (Throttle Position Sensor)	ADC1_CH6	%	Input-only pin

Digital Pulse Signals

GPIO	ECU Signal	ECU Pin Name	Edge	Calculation
GPIO 25	SPD	Speed Sensor	FALLING	1.985 meters/pulse
GPIO 26	IGf	Ignition Feedback (RPM)	RISING	RPM = (pulses × 1000 / elapsed_ms × 60) / 2 (4-cyl)

DTC Reader

GPIO	Function	Wiring	Notes
GPIO 19	T/VF read	5.6K+10K divider from ECU T pin	MIL blink pattern, normally 5V
GPIO 22	TE1 control	1KΩ → base of 2N2222 NPN, collector → E1 (ECU GND), emitter → GND	Pulled HIGH = TE1 → E1
GPIO 23	TE2 control	1KΩ → base of 2N2222 NPN, collector → E1 (ECU GND), emitter → GND	Pulled HIGH = TE2 → E1

Additional Analog Inputs

GPIO	Sensor	Circuit	Range	Notes
GPIO 35	Fuel Level	120Ω pullup to 3.3V, sender to GND	3Ω (full) to 110Ω (empty)	ADC1_CH7, inverted logic
GPIO 36	Oil Pressure	120Ω pullup to 3.3V, sender to GND	3Ω (0 psi) to 100Ω (100 psi)	ADC1_CH0
GPIO 39	Narrowband O2	Direct 0-1V, no divider	0.1V (rich 10:1) to 0.9V (lean 17:1)	~450mV = stoich 14.7:1

Door Switches (GND-Switched)

Wired with 10KΩ series resistor from switch to GPIO. Uses INPUT_PULLUP.

GPIO	Sensor	Logic
GPIO 13	Driver Door	LOW = open, HIGH = closed
GPIO 14	Passenger Door	LOW = open, HIGH = closed
GPIO 27	Trunk	LOW = open, HIGH = closed
GPIO 21	Hood	LOW = open, HIGH = closed

HC-05 Bluetooth Module (UART2 Serial Bridge)

ESP32 Pin	HC-05 Pin	Function
GPIO 16 (RX2)	TXD	Receives data FROM HC-05
GPIO 17 (TX2)	RXD	Sends data TO HC-05
GPIO 4	STATE	HIGH = BT connected, LOW = idle
5V	VCC	Power
GND	GND	Ground

• Baud rate: 38400 (AT command mode)
• Serial bridge UDP port: 5004
• Used for ELM327 Bluetooth OBD2 adapter communication

Network Configuration

Parameter	Value
WiFi SSID	Carputer_ECU
WiFi Password	12345678
WiFi Mode	STA (station)
Static IP	192.168.4.20
Gateway	192.168.4.1
Subnet	255.255.255.0
UDP send	192.168.4.255:5001 (subnet broadcast)
UDP listen (commands)	5002
UDP DTC results	5003
UDP serial bridge	5004 (HC-05/ELM327)
Sensor update rate	500ms
Speed/RPM calc rate	100ms

Protocol

Sensor Data (UDP 5001, broadcast)

{
"event": "sensors",
"data": {
"coolant": 185,
"oil": 212,
"ambient": 72,
"intake": 68,
"rpm": 2450,
"speed": 45,
"tps": 23,
"map": 65,
"fuel": 42,
"oilPress": 38,
"afr": 14.7,
"driverDoor": 0,
"passDoor": 0,
"trunk": 0,
"hood": 0,
"obd2FuelTrim": 0,
"obd2Load": 0,
"obd2Coolant": 0,
"obd2Rpm": 0,
"obd2Speed": 0,
"obd2FuelPress": 0,
"obd2DistDtc": 0
}
}

Commands (UDP 5002)

{"cmd": "query"}

DTC (UDP 5003)

• Send query → reads CEL blink pattern via T/VF pin
• Returns parsed DTC codes

Complete Pin Allocation

GPIO	Function	Type	Notes
2	LED indicator	OUTPUT	Build status
4	HC-05 STATE	INPUT	BT connection status
12	Door switch (old)	INPUT_PULLUP	Reassigned — now used in body controller
13	Driver Door	INPUT_PULLUP	10K series resistor
14	Passenger Door	INPUT_PULLUP	10K series resistor
16	UART2 RX (HC-05 TXD)	Serial RX2	38400 baud
17	UART2 TX (HC-05 RXD)	Serial TX2	38400 baud
19	DTC T/VF read	INPUT	5.6K+10K divider
21	Hood switch	INPUT_PULLUP	10K series resistor
22	TE1 control	OUTPUT	1K → 2N2222 base
23	TE2 control	OUTPUT	1K → 2N2222 base
25	Speed (SPD)	Digital interrupt	FALLING edge
26	RPM (IGf)	Digital interrupt	RISING edge
27	Trunk switch	INPUT_PULLUP	10K series resistor
32	MAP (PIM)	ADC1_CH4	5.6K+10K divider
33	Coolant (THW)	ADC1_CH5	5.6K+10K divider
34	TPS (VTA)	ADC1_CH6	5.6K+10K divider (input-only)
35	Fuel level	ADC1_CH7	120Ω pullup
36	Oil pressure	ADC1_CH0	120Ω pullup
39	Narrowband O2	ADC1_CH3	Direct 0-1V, no divider

Parts List

Component	Value	Quantity	Purpose
Resistor	5.6KΩ 1/4W	6	Voltage divider R1 (ECU signals + T/VF)
Resistor	10KΩ 1/4W	11	Voltage divider R2 (6) + door pullups (4) + GPIO39 fix (1)
Resistor	1KΩ 1/4W	6	Series protection (4 doors + 2 TE switch bases)
Resistor	120Ω 1/4W	2	Fuel level + oil pressure sender pullups
2N2222 NPN transistor	—	2	TE1/TE2 open-drain switches to ECU E1
DC-DC converter	Adjustable → 3.3V	1	Powers ESP32 from 12V
ESP32 DevKit	—	1	Sensor module MCU

Building & Flashing

cd esp32_sensor_module
pio run # Build
pio run --target upload # Upload via USB
pio device monitor -b 115200 # Serial monitor

Related Repositories

Repo	Role
carputer	Qt5 head unit — receives UDP sensor data
carputer-pcb	Physical PCB with J5 socket for this ESP32
carputerandroid	Android head unit — receives same UDP data
esp32_body_controller	Creates Carputer_ECU WiFi AP, forwards sensor data
esp32_cluster	TFT display — receives broadcast sensor data on UDP 5001