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2. Electronic circuit

A general diagram is like this:

--------- 0-5V  --------- ~220V  ----------------
|  PC   |===>===|Circuit|========|Coffee-Machine|
---------       ---------        ----------------

The concept is that we take a controling voltage from the computer, which drives an electrically isolated circuit with Relay or Triac.

You must choose a Relay circuit, if you have a big coffee-machine (greater than 200W or so), otherwise you can use a triac-based one.

All circuits presented are at least once tested, but it's YOURS RESPONSIBILITY the results. If you have no experience with electronics you should NOT try with these, otherwise you may get a bad one...

You should be very careful while experimenting with 220V, and there is no obsolence in using an appropriate fuse.

2.1 Driving voltage 0-5V from the computer

Here is a simple example to get a voltage 0-5V from the parallel port of the computer.

      Back View          -----    Pin 10 - ACK
      Male DB-25         |   |    Pin  9 - D7
      Connector          |   |                           Pin 2 - D0
                         v   v                           v   Pin 1 -
~Strobe
       ____________________________________________________________
      /                                                            \
      \     13  12  11  10   9   8   7   6   5   4   3   2   1     /
       \                                                          /
        \     25  24  23  22  21  20  19  18  17  16  15  14     /
         \______________________________________________________/

Pin 1 is Strobe (inverse logic)

Pins 2-9 is DATA BUS's signals, exactly what was written to the parallel port's latches with an OUTB command.

Pin 10 is the acknowledge signal (ACK), controlled by you, so that you can produce an interrupt to the CPU.

Pins 18-25 are short-circuited and this is the ground (GND).

In detail:

<= in   DB25    Cent    Name of         Reg
=> out  pin     pin     Signal          Bit     Function Notes
------  ----    ----    --------        ---     -----------------------------
=>       1       1      -Strobe         C0-     Set Low pulse >0.5 us to send
=>       2       2      Data 0          D0      Set to least significant data
=>       3       3      Data 1          D1      ...
=>       4       4      Data 2          D2      ...
=>       5       5      Data 3          D3      ...
=>       6       6      Data 4          D4      ...
=>       7       7      Data 5          D5      ...
=>       8       8      Data 6          D6      ...
=>       9       9      Data 7          D7      Set to most significant data
<=      10      10      -Ack            S6+ IRQ Low Pulse ~ 5 uS, after accept
<=      11      11      +Busy           S7-     High for Busy/Offline/Error
<=      12      12      +PaperEnd       S5+     High for out of paper
<=      13      13      +SelectIn       S4+     High for printer selected
=>      14      14      -AutoFd         C1-     Set Low to autofeed one line
<=      15      32      -Error          S3+     Low for Error/Offline/PaperEnd
=>      16      31      -Init           C2+     Set Low pulse > 50uS to init
=>      17      36      -Select         C3-     Set Low to select printer
==      18-25   19-30,  Ground

2.2 Controlling with a Relay

The simplest circuit that somebody can build is:


                             Vcc
                              |
                              +------+
                              |    __|__
                            Relay   /^\  Diode 1N4002
                             Coil  /---\
                              |      |
                              +------+
                              |
                           | / 
                 4.7K    B |/  C
parallel port >-\/\/\/\/---|        NPN Transistor: BC547A or 2N2222A
data pi                    |\  E
                           | V
                             |
parallel port >--------------+
ground pin                   |
                          Ground
Connect Vcc with the same voltage as the relay type (usually 5 or 12V). Obviously, the relay's specifications should be reasonable for your coffee-machine.

Barmen, usually, tend to put the relay AFTER the transistor, at the emitter (E) pin instead of the collector (C) pin. This is a bad practice because it biases the transistor badly, and may result in bad coffee :-). Diode 1N4002 is useful to protect the transistor from the relay's currents. If you don't use it the transistor will become darker and smelling...

2.3 Controlling with TRIAC #1

If you only want a simple circuit, you can use Motorola's triac driver MOC301012, together with a general purpose TRIAC like SC141D. This method has the advantage that you don't need an extra power supply.

For non-inductive loads this is the circuitry:


        270     1 +-------+ 6    180
  +5v -VAVAVA-----+       +----VAVAVA-----+-------------- Line Hot
                2 |  MOC  |               |
  TTL in ---------+ 3012  +nc            VA  SC141D
                  |       | 4           / |
                nc+       +------------/  |
                  +-------+               +----\/\/\/---- Line Neutral
                                                LOAD

If you are going to work with 220V, prefer a 3021. Inductive loads should be used in conjuction with bypass capacitors, see Motorola Application Note AN-780. Coffee-machines are mainly resistive loads and not inductive (like a motor), but who knows what's yours...

2.4 Controlling with TRIAC #2

+5VDC
|    180                      180            2.2k
+---/\/\/\----+-----+   +----/\/\/-+--/\/\/\---+-------> 120V
              |    1|   |6         |           |         Hot
              |    +=====+         |           | MT1
              |    | MC  | TRIAC   |          +-+
              |    | 3032| Driver  |        G | | TRIAC
              |    +=====+         |         /| |
              \    2|   |4         |        / +-+
       2N3904  |----+   |          |        |  | MT2
              /     |   +--------- | -------+  |
             V      \              |        |  |
             |      /              |        \  |
             |      \ 43    .01u  ---   10k /  |
             |      /       500V  ---       \  |
             |      |              |        /  |
             +------+              |        |  |            Neutral
             |                     +--------+--+---o    o--> 120V
             /                                      load
 >-/\/\--|  2N3904
             \
              V
              |
             ---
            ///
You should change resistors accordingly for 220V.

Circuit description:

The MC3032 is an optoisolator TRIAC driver. The 180-ohm resistor sets the current for the LED emitter in the optoisolator. Change the value of this resistor - if necessary - to get reasonable current (e.g., 15 mA).

Note that you cannot test this circuit without a load. The TRIAC will not switch unless connected to an AC voltage source, so you can't test it for simple switching w/o applying AC and a load. Note the 500V rating on the .01 cap.


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