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OPTOCWPLER TESTER
by Saa Cross
Bodwin Distributing
North Little Rock, AR
The circuit shown in Figure 1 is a circuit
scheaatic of an optocoupler tester. This
circuit, which will not damage the
sensitive optocoupler device, tests its
proper functioning when a game which uses
the optocoupler as a player input cor,troi
is not available,
It can be constructed on a section of
standard breadboard and is set up to use
+5VDC available from any source. If the
board is layed-out in the form of Figure
2 1 this will allow the checking of any
freestanding opto's and also individual
late IIOdel Atari mini trackball assemblies
and individual late 110del Atari
optocoupler boards.
Of course you must find the proper plug
type which fits the Atari trackball
assembly, and if you convert any games and
have spare wire harr,esses around you wi 11
probably locate one. The type of plug
which fits the whole Atari trackball
asse11bly is a 11ale plug with female insert
pins, It has six pins ari
this plug is shown in Figure 1,
The individual Atari opto boards have an
easily available 4 pin connector which
again can be found from an old wiring
harness, Again, the wiring of this plug is
shown in Figure 1,
The way I have this board wired is that
when the detector transistor "sees" light
from the infrared LED, the correspor~ing
LED on the tester board will be glowing.
When the detector doesn't see light, the
corresponding LED or, the opto tester board
won't be on.
NOTE: A rule of operation for using this
board; don't have ar,y other opto hc,oked up
than the one that you are testing.
As I said before, this tester can test all
the optocouplers I found from different
Manufacturers, and this includes
Bally/Midway, Sega, and of course Atari.
All optocouplers function upon the sarae
principle. That is, they use a infrared
LED, Milich eaits light invisible to the
eye. In an optocoupler assembly, which is
also called an interrupter assembly, this
LED is fixed on one side of the assembly
and a photodetecter transistor, which is
sensitive to infrared light, is fixed on
the other side. Betweer, them, a toothed
wheel 11akes and breaks the light shining
on the detector. When light is shir,ing or,
the base of the detector, it provides a
corresponding base current which turr,s the
detector •on", allowing current to flow
through the C-E tenainals.
Additional logic provided by the
manufacturer will detect the "or,• state of
the transistor and act upon it as
necessary. fllore thar, one set of emitter
detector pairs, most always two pairs, may
be 110lded ir,to one asse11bly, as is the
case with late model Atari ga111es.
Optocouplers may fail in two ways. One,
the optocoupler detector will not provide
enough current any longer for exterr,al
circuitry to detect an "on• state, or two,
the LED wi 11 not emit light ar,y longer. A
third possibility is that a piece of
foreign material can lodge in the
asse■bly, blocking light permanently.
When I built this tester, it was
discovered that the current limiting
resistor used in the detector side could
be increased almost 6 to 10 ti111es its
original value used by the game
11anufacturers, and still the optocoupler
functioned norraally. Also, the currer,t
limiting resistor in the LED sect ior, could
be increased to twice its stated value and