Top (motor drive) NO ---------------
NC --------- ----
Middle (bypass) NO ---- -------
Bottom (cam hold) NO ---- -------
Purchase Hold NO --------- -------
1 2 3 4 5 6
The top signals in the diagram represent the state of the three
switches controlled by the cams attached to the dispenser motors
on the coke machine. The bottom signal represents the power
being applied to one side of the purchase relay.
The bottom two switches (Bypass and Cam Hold) are mechanically
coupled. All three switches are controlled by a single rotating cam
attached to the soda-dispensing motor.
We begin by assuming that the user has entered 40 cents and
the money counter has just tripped the purchase relay to put
the machine in a state where pressing a button will cause a
soda to drop.
- The machine is in a purchase-ready state (the purchase relay is
tripped, that is, it is in its NO position) but the user has not
yet selected a frosty beverage. In this state, AC-common is being
provided to all of the coke-selector switches from line Y10 on
the purchase relay through the entire collection of cam-hold
switches (bottom switch on the wiring diagram). The top switch
(motor drive switch) provides power to hold the purchase relay in
the "purchase enabled" state.
- The user presses a button, thus routing AC-common power to the
corresponding bin-empty switches and, assuming the bin in
question is not empty, the power then flows to what in the
diagram is the left side of the motor. The right side of the
motor is always AC-hot, so the motor begins to turn.
- The lower two switches flip from their NO positions to NC. This
redirects AC-common away from the soda-select buttons on the
front of the machine such that no other bin can be selected and
results in the bottom (cam hold) switch providing AC-common to
the motor. The middle switch (bypass switch) does nothing in
this state and the top switch (motor drive) still holds the
- The top (motor drive) switch flips to its NO position so that now
both it and the bottom (cam hold) switch are providing power to
the motor. In this position, the top switch no longer holds the
purchase relay, so it flips back to its NC state.
- Now that the purchase relay is disabled, and there is another
source of power for the motor (top switch) the cam hold relay
can re-enable the soda-select buttons, so it flips back to NO
along with the bypass switch which then re-enables the
hold line on the purchase relay (but the relay is in its NC
state, so the hold line is not actually doing anything).
- The top relay (motor drive) flips back to NC so that the top
two switches are powering the (currently unused) relay hold
line. There is no more power to the motor, so activity stops.
This is the original state except that the purchase enable
relay is in the NC position instead of NO.