I rebuilt an existing interface module — to be a stand-alone temperature and humidity sensor, with serial RS-232 interface. Uses an AVR ATmega8 microcontroller.

Table of contents

About

This module was originally installed in the rack box, as an I/O module. When the rack box project was terminated, I instead repurposed the unit to become a stand-alone temperature and humidity sensor.

It has two sensors, LM35 and HIH 4000, connected to ADC inputs on the AVR microcontroller. When requested; the device returns the ADC value. This must be converted into a proper unit by the receiver.

I/O

Inputs

  • PC0 Temperature sensor (analog)
  • PC1 Humidity sensor (analog)

Calculations

Temperature

ADC × 5
------- ≈ °C ± 0.5
  10

Humidity

ADC × 5 - 826
------------- ≈ %RH ± 3.5
    31.5

Outputs

  • PB0 Life signal
  • PB1 Link activity

Serial communication

  • PD0 Rx
  • PD1 Tx

D-sub 15-pin

  1. 13.8V
  2. 0V

Communication

Interfacing is done with RS-232, using MAX232 or MAX202 and the SIOS protocol.

Serial settings

  • Baud: 38400
  • Data bits: 8
  • Parity: None
  • Stop bits: 1

Source code

Photos

Sensors LM35 and HIH 4000 poking out
Sensors LM35 and HIH 4000 poking out
AVR ATmega8 on circuit board
AVR ATmega8 on circuit board
Voltage regulator with heat-sink
Voltage regulator with heat-sink
MAX232 interface IC
MAX232 interface IC
Inside module
Inside module
Inside module
Inside module
Inside module
Inside module
Completed module
Completed module

Schematic diagram

Schematic drawing for temperature and humidity sensor
Schematic drawing for temperature and humidity sensor

Parts used

  • 1 × AVR ATmega8-16PU, DIL-28, 16 MHz, 23 I/Os
  • 5 × Capacitor, aluminium electrolytic, 1 µF, 50V
  • 1 × Capacitor, aluminium electrolytic, 10 µF, 25V
  • 2 × Capacitor, aluminium electrolytic, 47 µF, 25V
  • 1 × Capacitor, ceramic, 1 nF, 100V
  • 3 × Capacitor, metallized polyester foil, 10 nF, (0.01 µF)
  • 1 × D-sub soldering cups, 15 pin male
  • 1 × D-sub soldering cups, 9 pin female
  • 1 × DIL socket, 16-pin, 7.62mm
  • 1 × DIL socket, 28-pin, 7.62mm
  • 1 × Enclosure, plastic (1591 FL), 120x65x40mm, flange
  • 1 × Fuse 5x20 mm, 200 mA, fast-acting
  • 1 × Fuse holder, open, PCB, 5x20mm
  • 1 × Fuse holder, open, PCB, Protective cover
  • 1 × Heatsink, 27.3K/W, 19mm, attachable, TO220
  • 1 × Humidity sensor, HIH 4000, 0 - 100 %, +4 to +5.8 V
  • 1 × LED 5mm clear, Yellow, 2.0V, 20mA, 250mcd, 6°
  • 1 × LED 5mm coloured clear, Green, 2.1V, 20mA, 30mcd, 10°
  • 2 × LED holder 5mm, RTC51, black plastic
  • 64 cm2 PCB, stripboard, 100x160mm, 160cm2
  • 1 × Power jack, panel, 2.1mm, plastic housing
  • 2 × Resistor, carbon film, 0.25W, 330 Ω, 5%
  • 2 × Resistor, carbon film, 0.25W, 4.7 kΩ, 5%
  • 1 × RS232 interface, MAX232CPE, dual
  • 25 × Straight pin header, female, Single row, 2.54mm
  • 25 × Straight pin header, male, Single row, 2.54mm
  • 1 × Temperature sensor LM35DZ, 0 to +100 °C, +-0,6 °C
  • 1 × Transistor, NPN, 100 mA, 45V, 0.5W, BC547B
  • 1 × Voltage regulator +5V, 2 A, L78S05CV