Squidwrench Vending Machine: OEM Wiring Diagram

An old vending machine in need of rebooting may provide fodder for some electronics tutorials at Squidwrench. To that end, here’s the OEM wiring diagram pasted inside the door:

SqWr Vending Machine - OEM Wiring Diagram

SqWr Vending Machine – OEM Wiring Diagram

That’s endured a perspective transformation and a bit of contrast stretching; it looks awful, but being able to view it without squatting inside the machine makes it much easier to read…

Each selector and motor cycle switch pair interact with the motor thusly:

Vending Machine - Switches and Motor Doodle

Vending Machine – Switches and Motor Doodle

All of the motors have one side connected directly to the 24 VAC power transformer. The wiring diagram shows a pair of transformers in parallel, which seems odd.

The Selector switches (an array of 30 on the front panel, with one broken that will surely call for some 3D printing) are in series, so the lowest-numbered one wins; the NO terminal of each Selector switch  goes directly to the control box. Pressing a switch connects the Red·Orange wire on the C terminal of the first switch to the control box on the same wire as the corresponding motor lead.

Assuming the Motor Cycle switch parks in the NC position, it will disconnect the Orange wire from the Orange·Black wire and connect it to the lower motor lead and the Select switch (which may or may not be pressed by then), although we don’t know the timing. There’s surely a cam on the motor shaft.

Some possibly relevant patents, found after a brief search with the obvious keywords:


  1. #1 by RL on 2015-12-14 - 10:24

    As to the paralleled transformers… The low voltage control world usually consists of Class 2 circuits which are “power limited,” meaning they can’t provide more than 100VA of power. If you look at the transformers, their labels might specify this. Sometimes higher power is needed and it’s not uncommon to find two transformers in parallel, rather than a single, larger transformer.

    • #2 by Ed on 2015-12-14 - 13:24

      rather than a single, larger transformer

      Saves them from having another part number, too, which probably counts as much as anything else.

      • #3 by Mike on 2015-12-15 - 00:51

        Especially when one manufacturers product line might have several different sizes of machines, all of which use the same basic control system.

  2. #4 by hexley ball on 2015-12-14 - 12:56

    “An old vending machine in need of rebooting may provide fodder for some electronics tutorials at Squidwrench.”

    If you want to go all-in on the reboot, how about bringing the machine into the network age? Processors everywhere, communicating internally via the MultiDropBus and exporting machine status via the Data EXchange protocol. The good folks at are here to help — see for a starting point.

    And what could possibly make more sense than putting Linux in a vending machine?

    • #5 by Ed on 2015-12-14 - 13:23

      If you want to go all-in on the reboot

      The Smart Folks will be doing all that. I’ll run alongside and (try to) show how to measure voltages & currents & waveforms: the stuff that doesn’t show up in apps and programming frameworks.

      It ate $0.85 from my wallet, so I’m thinking it’s payback time…