Status of Paul's (Extra) Refrigerator at Mon May 12 18:05:55 EDT 2008

I haven't started using the new one yet.

• The refrigerator is 32 degrees Fahrenheit, 0 degrees Celsius.
• The freezer compartment is 32 degrees Fahrenheit, 0 degrees Celsius.
• The Diet Coke can is 32 degrees Fahrenheit, 0 degrees Celsius.
• The light is on.
• The door is closed.

The most frequently asked question

Answer The refrigerator temperature is not uniform. It is below freezing up by the coils at the top of the compartment, and warm by the gap in the door seal where the wires enter the refrigerator.

I've checked the sensors with a laboratory thermometer. They are usually within a degree Celsius of the actual temperature. The refrigerator itself is not a laboratory instrument.

How This Network Refrigerator Works

The parts:

1. The Miniboard was $82 (assembled) a few years ago. I got it from The Progressive Solution. I won't put the contact information here until I check that he is still making them.
2. 3 thermistors, Radio Shack catalog number 271-110 ($1.99 each).
3. Cadmium-Sulfide Photocell, Radio Shack catalog number 276-1657 (Package of 5, $2.29).
4. I used to use a Sun 3/50, bought used for $75 from a former employer. Yes, you read that correctly, the workstation cost less than the Miniboard.
   I now use a 386DX40 w/16 Mbytes. Russ loaned me the machine in exchange for space on the web. So far Russ hasn't put anything in the web space. I added a UPS, multiport serial card, and 28.8 modem.
5. Phone cords and phone connectors to connect the Miniboard to the Sun 3/50. ($.99 each)
6. General Electric full size refrigerator-freezer. It came with the house. We bought the house as a duplex, it still has two kitchens. We don't use the upstairs kitchen for much, so I can do weird things to the refrigerator. My food is in the downstairs refrigerator and it doesn't have any sensors in it.
7. 12 ounce can of Diet Coke ($2.99 + $1.20 deposit for a 12 pack).
8. Misc. parts: resistors, reed switches, wire, 9 volt power supply for miniboard, electrical tape, etc... I just had them lying around.

The Miniboard uses a 6811 processor with 256 bytes of RAM and 4K bytes of EEPROM, 8 analog inputs, 8 digital inputs, 4 motor outputs and piezo electric speaker.

I'm only using the analog inputs and the speaker. The analog inputs are used to sense the refrigerator. The speaker plays a different tune for each kind of access, finger, http or email.

The analog to digital convertors (A to D) in the 6811 measure voltages between 0.0 and the supply voltage, which is close to 5.0 volts. They are linear with 8 bits of precision. That means they assign values between 0 and 255 to voltages between 0.0 and 5.0. From the sensor's view of the world, there are 256 20 millivolt steps between 0.0 and 5.0 volts. 0 indicates a reading of 10 millivolts or below and 255 indicates a reading of 4.990 volts or above. A voltage too far below 0.0 or above 5.0 will damage the A to D.

There is a 47Kohm pullup resistor on each analog input. This lets me use each sensor to make a simple voltage divider.

I actually keep most of my drinks in the downstairs refrigerator, however, I must keep up with the coke machines on the net. So I have instrumented soda can storage. I can tell if the can is present, and if present, if it is cold. See my Instrumented Soda Platform Construction Notes if you want to build your own.

Analog to Digital Convertor Results

• A to D 0: , Short, no connection
• A to D 1: , Short, Freezer thermistor
  
• A to D 2: , Short, no connection
• A to D 3: , Short, no connection
• A to D 4: , Short, pop can thermistor (in refrigerator)
  If it is about the same as the refrigerator thermistor, than the pop can is either not there, or is as cold as the refrigerator.
• A to D 5: , Short, Refrigerator thermistor
  Bigger numbers are colder.
• A to D 6: , Short, Pop can switch, door open switch.
  The two switches are in series. There is a 27Kohm (5%) resistor in parallel with the door switch. There is a 10Kohm (5%) resistor in parallel with the pop can switch. My Micronta 22-167 Multimeter says:
  1. 36.4Kohms, A to D reads 111, both switches open
  2. 26.5Kohms, A to D reads 91, door closed, no pop can.
  3. 9.99Kohms, A to D reads 44, pop can switch open, door closed.
  4. 1.3 ohms, A to D reads 0, both switches closed.
• A to D 7: , Short, Refrigerator photocell
  The photocell is aimed at the light in the fridge. A value under 10 means the light inside the refrigerator is turned on. A value above 10 but below 200 means the door is open, but the inside light is turned off. A value above 200 means that it is dark in there. A value of 255 means that the sensor wire is broken or loose.

This page gave accurate, but useless information for 3 years. It is still useless, so I haven't felt inspired to connect up the new refrigerator.

If you're bored, wave to the cats.