Using 555 Timers To Add “free Play” Functionality To Classic Arcade Machines


[John Zitterkopf] is in the middle of restoring a vintage Sega Star Trek Captain’s Chair arcade game for the upcoming 2012 Texas Pinball festival, though one prerequisite for the show is that the game supports some sort of free play mode. At this point he doesn’t have the option of tracking down a freeplay ROM for the device, so he had to come up with a solution of his own.

He did not want to alter the machine’s operation in any significant manner, and this meant preserving the functionality of the coin chutes. To do this, he put together a small circuit that uses a pair of cascaded 555 timers to provide the machine with the proper signaling to simulate coin insertion, while still accepting coins. You might initially think that this could be easily accomplished by shorting a pair of contacts in the coin chutes, but as [John] explains, the process is a tad more complex than that.

If you have some old arcade games kicking around and are looking for a non-invasive way to make them free to play, be sure to check out his site for schematics and a complete BoM.

22 thoughts on “Using 555 Timers To Add “free Play” Functionality To Classic Arcade Machines

  1. With my dad’s pinball machine it’s just a switch that gets hit when a coin falls on it, it’s mostly mechanical thoug, not a single IC, so that probably explains why.

  2. That’s way too much work for what could easily be done with a micro-switch routed through the coin return.

    If it did something cool like sense taps on the coin door or maybe actuated with a magnet through the control panel it would be cool :/

    Good job though on understanding that the game needs a pulse not just a momentary tap from a closed circuit.

  3. Most arcade machines have a free play or maintenance mode that can be set by a dip switch. You might want to look around for one.

    Assuming you don’t have a free play mode, this circuit is a lot cleaner way than putting a switch outside the machine.

    1. I liked the option we used when there was a Punch-Out machine in the basement. Behind the coin tank was a button that added credits. Just leaving the coin drawer open, and the box removed, let you reach under and tap for as many credits as you wanted at the time. I assume that was original, cause we never modded it after we got it from the arcade, and I doubt the arcade would add a free-play that could be activated by hitting the coin door hard enough.

  4. seems overkill considering most pinball infact all arcade games use a simple lever switch in the coin unit so you may be able to hotwire a push button on the machine to add credits (ok if you own the machine and dont care if it works on coins again.

  5. @Kevin
    Sega Star Trek does not have a Freeplay mode.

    I did not want to Drill/add an additional switch to my now mint; nearly complete machine.

    A simple push button would not have work with the Sega G08 boardset anyway.

    On these arcade designs; you actually have to move from a NC to NO config on the switch to throw the S/R FlipFlop into the correct state for the CPU to read it.

    1. that sounds like a cheap way to debounce the coin input. Using the NO and NC contact of switches lets you do that, if you have both contacts available. Switches rarely ever bounce so badly that they go all the way back to the connection they just left.

  6. I know for a fact that many games it IS as simple as shorting some contacts in the coin box. I’ve never heard of a need for some sort of special pulse for any of the cabinets I have worked on.

    1. Yes, for most it is. Not all arcade hardware systems are created equally, though. The Sega G80 system went through a lot of trouble to add timing to the triggering of the circuit. If the circuit is not completed for the correct amount of time (too short or too long), the input will just be ignored. Which is frustrating when trying to coin these manually.

  7. Jesus christ, so many posts say things like “most games have a freeplay, why don’t you use that?” Well the answer’s in your question, genius: “most.”

  8. Reminds me of my time in Electronic Technician school while in the Navy. The Hard Drivin’ game in the break room cost .50 to play, and a couple of us were addicted to it. We snuck in there in the middle of the night, and wired an old car alarm in, using a relay on the “chirp”, to the coin acceptor switch. We’d just reach in our pocket and touch the arm button on the remote, and it would click the relay twice. Free play everytime…

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