This post is part of the Rack box project series.

Controls an electric heater, and has a simple 15 minutes timer. Uses an AVR AT90S2313 microcontroller.

Table of contents

Details

  • The module turns off the heat in the living room when the porch door has been open for 20 seconds. 90 seconds after the door has been closed; the heat is turned back on. The heat can also be disabled using an external switch.
  • A short signal pulse from another module or a switch, will activate the output for 15 minutes. Used for turning on the test voltage on the Rack status panel. A new pulse within these 15 minutes turns the output off.

Leakage current

The solid state relay used for the heater, leaks a small amount of current, about 10mA. This can be eliminated using a resistor in parallel over the oven supply after the relay. A 22K, 5W resistor should be the job.

I/O

Inputs

  1. Porch door, from Multi-purpose module 1
  2. Manual oven disengage
  3. Signal activate

Outputs

  1. Oven relay N.O
  2. Oven relay N.C
  3. Oven LED
  4. Oven local LED
  5. Signal output
  6. Life-signal (to Module stability monitoring unit 2)

D-Sub 9-Pin

  • Red: 5v
  • Black: 0v
  1. Input: Porch door
  2. Input: Manual oven disengage
  3. Input: Signal activate
  4. Output: Oven relay N.O
  5. Output: Oven relay N.C
  6. Output: Oven LED
  7. Output: Signal
  8. Output: Life-signal

Source code

Photos

Started assembly
Circuit board fitted inside enclosure
AVR microcontroller installed and everything wired up
Testing the I/O logic
Mounted in the rack box
Inside the solid state relay enclosure
Relay used for controlling the heater
DIN rail brackets on the back of the module
AVR microcontroller on circuit board
Inside the module

Schematic drawing

Schematics for the heater and timer module

Parts list

  • 1 × AVR AT90S2313-10PI, DIL-20, 10 MHz, 15 I/Os
  • 1 × Capacitor, aluminium electrolytic, 10 µF, 25V
  • 2 × Capacitor, ceramic, 22 pF, 100V
  • 1 × Capacitor, ceramic, 1 nF, 100V
  • 1 × D-sub soldering cups, 9 pin male
  • 1 × DIL socket, 20-pin, 7.62mm
  • 3 × Diode, small signal, 1N4148/Ph
  • 1 × Enclosure, plastic (1591), 100x50x25mm
  • 1 × LED 5mm coloured clear, Green, 2.1V, 20mA, 30mcd, 10°
  • 1 × LED 5mm coloured clear, Red, 2.1V, 20mA, 8mcd, 10°
  • 2 × LED holder 5mm, RTC51, black plastic
  • 2 × Mounting bracket, DIN rail, Plastic
  • 1 × Quartz crystal oscillator, 4 MHz
  • 5 × Resistor, carbon film, 0.25W, 330 Ω, 5%
  • 6 × Resistor, carbon film, 0.25W, 4.7 kΩ, 5%
  • 3 × Resistor, carbon film, 0.25W, 10 kΩ, 5%
  • 1 × Switch, toggle, 1-pole, micro, on-on
  • 5 × Transistor, NPN, 100 mA, 45V, 0.5W, BC547B

Last commit 2023-02-05, with message: Add series for the rack box project.