I recently came into ownership of a (sort of) arcade machine and I'm looking for a little help to figure out if it's possible to modify it.

Long story short, the coin-op uses a simple circuit composed of a 555-timer/coil relay to cut power off to the game console, and a johnson counter/dip-switches to control the number of tokens required to play (except for switch 9, which I have no idea what it does).

The problem I've found is, inserting additional tokens during play does not reset the timer to extend play. What I'm trying to figure out is if there's an easy way to modify the circuit to have the coin-op switch (inserting a token) reset the 555 timer without the output/relay shutting off (killing power to the console).

traced out circuit circuit pcb

I've spent some time tracing out the circuit as best I could. Please forgive the spaghetti of wiring, I laid out components in the schematic how they were arranged on the PCB to make following the wires easier. I'm sure it's not perfect, there were a lot of annoying vias hidden underneath the ICs, so take it with a grain of salt and absolutely question anything that doesn't make sense so I can check/correct it.

Would anyone have an idea on how to have the coin-op switch cause the 555 timer to reset without the relay shutting off?

For anyone interested, the long of it is the machine is actually a store demo kiosk, basically just an arcade machine-shaped metal cabinet that had the plain home console inside it. Usually they were used for advertising purposes to let people try the console before buying it (or to demo new games) in toy stores, movie rental stores, and the like. They were built to order, and this specific one is a bit of an oddity in that it sports the uncommon pedestal arcade coin-op mechanism (presumably it was ordered for use in an arcade, but it's been through a few owners hands by now so the origins are unknown).

  • \$\begingroup\$ There are a few bugs in your schematic: The capacitor placed in series with the relay coil is extremely unlikely. Having the coin-op switch provide the only connection between supply ground and the rest of the circuit can't possibly be correct. And all the missing power connections you already know about. \$\endgroup\$
    – brhans
    Commented Aug 22, 2021 at 15:41
  • \$\begingroup\$ Hey, thanks for responding! You were right, made a few mistakes in my trace out there, this update should be closer to reality. \$\endgroup\$ Commented Aug 22, 2021 at 16:22

3 Answers 3


C5, at 470 uF, is the main timing capacitor. The cathode almost certainly is supposed to be tied to GND, so there probably is a schematic error there.

To reset the 555 timer, apply a brief (current-limited) short circuit across the cap, discharging it; the output will not change state. The problem is that to do this with tokens, the signal from the coin-op switch has to do two very different things -

  1. When the timer is not running, increment the token counter;

  2. When the timer is running, reset the timing capacitor

Of course this can be done. The question is how badly do you want it? At first pass, it would take a some logic to redirect the token signal from the counter to the reset circuit, based on the output state of the timer. Maybe another CD4093 configured as a demultiplexer. The cap reset circuit is simple, one small transistor and one resistor.

Bottom line, more than a couple of cuts and jumpers.

DIP switch #9 might be a timer range selector.

Clear photo of the bottom side of the board - ???

  • \$\begingroup\$ Yes I think I oriented a few of the capacitors backwards, my mistake. Thank you for the detailed response however! Yes I don't think I want to get too invasive on this old board, was hoping I'd get lucky and it would be a simple "oh just add a wire here, dummy!"; aside from being a bit of a collector's piece, as Dave Tweed replied above it'd just be easier to replace the whole thing with an Arduino and build all the functionality in via code. I'm going to go this route instead, but thanks again! \$\endgroup\$ Commented Aug 22, 2021 at 17:36

The 555 is fundamentally not a retriggerable device, so any mod to make that happen would be nontrivial.

The canonical answer is that anyone implementing this functionality today would replace the whole thing with a small (e.g., 8-pin) microcontroller, which gives you the flexibility to implement any behavior you like.1

BTW, switch 9 simply shorts out the timer adjustment pot, giving you a fixed small amount of play time.

1 For example, an ATtiny, which is supported by the Arduino programming environment. Ever considered getting into Arduino?

  • \$\begingroup\$ Edited your response with my reply due to comment character limits \$\endgroup\$ Commented Aug 22, 2021 at 17:37
  • \$\begingroup\$ Use the comment system to reply don't edit someone else's answer \$\endgroup\$
    – Voltage Spike
    Commented Aug 22, 2021 at 18:11
  • \$\begingroup\$ Questions and answers should not contain discussion \$\endgroup\$
    – Voltage Spike
    Commented Aug 22, 2021 at 18:11
  • \$\begingroup\$ Since voltage deleted my response, I'll just break it up into multiple comments then... Dave, thank you so much for the clear and concise answer! Funny you should mention it, I actually just got my first Arduino Uno specifically for this project, but for a different reason. I have done some simple projects with PIC controllers in the past, but this is my first time doing anything with an Arduino. \$\endgroup\$ Commented Aug 22, 2021 at 18:20
  • \$\begingroup\$ The other shortcoming of this kiosk is when time runs out, the console fully powers off, so the screen just goes black. I'm using the Arduino to generate a composite video signal displaying a "time over/insert token" screen to display when the console power goes out. I designed a little circuit to intercept the video from the console, and the power output from the coin-op timer, to be able to detect when the console powers off and switch between video signals. \$\endgroup\$ Commented Aug 22, 2021 at 18:20

To more easily see how hard a mod would be, consider re-drawing the schematic.

A schematic tells a story; it has a beginning, an end, and chapters in the middle. When possible, signals move left to right, and conventional power moves top (positive rails) to bottom (negative rails). Grounds are shown individually, and not bused to improve clarity; same can be true for power sources.

For example, left to right: Token signal input Debouncing Counter DIP switch 555 timer Driver transistor and relay Switched power in and out connectors

Use four NAND gate symbols instead of the part outline, and place them where then convey circuit function with the most clarity. I try to keep all gates pointed the same direction (left-to-right, just like a schematic) because that is how they are taught; a surprising number of people have trouble reading "backwards" gates.

There are multiple symbol types used for DC power sources and GND, and some people get downright cranky about sing the "right" ones. I care less about that and more about their orientation, which conveys useful information; positive sources point up, negative sources point down, and GND always points down.


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