In this project we designed and built a temperature+humidity sensor that could be locally accessed with buttons and remotely accessed via a web interface. The purpose of the sensor is to monitor the temperature and humidity of a solar system's battery pack. The sensor maintains 24 hour history (minimum and maximum) for temperature and relative humidity. The design features the following hardware components:
- Combined temperature and humidity sensor
- OLED screen to display temperature and humidity
- PIR sensor to enable/disable display
- Buttons to display history and reset history
- LEDs to indicate operational status
- Jumper to disable LEDs
- Web interface over WiFi
Software Design
The system runs on the Parallax Propeller chip. The Propeller chip is an 8 core, no interrupts, shared memory cpu. The project software is written in C (other languages supported by the Propeller include Spin, assembler, and BlocklyProp). The software design takes advantage of multiple cores to accomplish tasks concurrently. The functionality is divided among the cores as follows:
- Initialization, core management, and watchdog
- LEDs, buttons, jumper, and history
- Temperature and humidity sensor
- OLED display
- PIR sensor
- Web interface via 802.11 b/g/n
- Timer
The temperature and humidity is sampled every 2 seconds. The current temperature and humidity as well as the 24 hour maximum and minimum temperature and humidity are displayed on both the OLED and via a web page. Since the design does not include a RTC, we use a cog as a software timer/clock for managing the 24 hour history (we divide the system oscillator which is running at 80 MHz). Buttons on the board are used to reset the 24 hour history, display the temperature history, and display the humidity history. LEDs indicate when the temperature is updated, humidity is updated, when a history reset is requested, and when the sensor is active (heartbeat). The PIR sensor is used to determine when the display should be on or sleeping (screen saver). The 802.11 b/g/n device is configured as an access point with a RFC 1918 address. All of the temperature and humidity data is available on a web page served over WiFi.
Printed Circuit Board (pcb)
Front of pcb (revision A)
Back of pcb (revision A)
Assembled board
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| Front of completed temperature humidity sensor | Back of completed temperature humidity sensor |
Component datasheets & Schematic
- CPU - Parallax Propeller
- CM2302 Digital temperature and humidity sensor
- OLED display
- ESP8266 WiFi module
- PIR sensor
- Schematic, Revision A
Bill of Materials
| Item | Description |
|---|---|
| CPU | Parallax Propeller, 40 pin QFP |
| Motion sensor | Panasonic PIR sensor, 5M, Digital |
| Resistor | 47K, 1%, 1/4W |
| Temperature/Humidity | CM2302 Digital temperature/humidity sensor |
| Resistor | 4.7K, 1%, 1/4W |
| Display | SSD1331 OLED, 95x64 |
| Oscillator (clock) | 5 MHz 20pf crystal, PTH | Non-volatile memory | 256Kb I2C EEPROM |
| Resistors | 10K resistor, 1%, 1/2W (5) |
| OLED connector | Molex 8 pin, right angle connector |
| Temp/Humid connector | Molex 4 pin, right angle connector |
| WiFi module | ESP8266 |
| Power supply | 7.5 VDC, 2A, 15W wall wart |
| Power jack | DC power barrel connector |
| Power regulation | LDO Voltage Regulator, 5V, 1A |
| Power regulation | LDO Voltage Regulator, 3.3V, 1A |
| Power regulation | 4.7 uf, 25V, 10%, MLCC (4) |
| Push button | 6mm button, PTH (4) |
| Resistors | 150 Ohm, 1%, 1/2W (4) |
| Status indicators | LED, SMD orange (4) |
| Resistors | 100 Ohm, 1%, 1/2W (4) |