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Electric heater and timer controller

From CaveLab
This page is kept for historical interest. It may document projects or methods that are obsolete and/or no longer relevant. The information here is not kept up-to-date.

This module was part of the rack box project — which was terminated when I moved out of my first apartment.
OSU installed in the Rack box

Controlling the electric heater, and give alarm if server stops responding. Uses an AVR AT90S2313 microcontroller.


  • The module turns off the heat in the living room after the porch door has been open for 20 seconds. 90 seconds after the door has been closed the heat is turned back on. It can also be controlled 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 leaks a small amount of current to the oven, about 10mA. This current can be eliminated using a resistor in parallel over the oven supply after the relay. A 22K, 5W resistor should be the job.



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


  1. Oven relay N.O
  2. Oven relay N.C
  3. Oven LED
  4. Oven local LED
  5. Signal output
  6. Lifesignal (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: Lifesignal

Source code

Bascom-AVR source is available in a git repository:


Schematic drawing


Parts list

Qty. Part
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