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
AVR ATmega8 on circuit board
Voltage regulator with heat-sink
MAX232 interface IC
Inside module
Inside module
Inside module
Completed module

Schematic diagram

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