The Amazing Marble Slot Machine

This machine was first built in 1974, although the verison illustrated here with the wooden and Meccano chasis was a re-construction in 1976.

The slot machine uses marbles rather than coins, since these roll along the internal cardboard shutes much more easily. A marble must be dropped in the chimney at the top before the machine can be played and any winnings are paid out around the side, also in the form of marbles.

How to Play

Rather than having three spinning barrels, like most fruit machines, our slot machine is a game of skill involving three lights. Each light is controlled with a switch.

To play, a marble is inserted in the top shoot. This causes the top light in the column of three on the right hand side to go off and the second one down to come on. From the top, these lights are called READY, PRIMED and RUNNING. When primed, one can pull the handle. There is no actual handle, just a piece of string coming out the front panel. You pull the string instead. The original intention was to fix a handle on the side with an internal capstan to operate the string, but I could never find a suitable handle to attach. When the string is pulled, a timer runs for 12 seconds during which the third light is on. While the light is on, we say the machine is `running'.


While the machine is running, the user(s) must try to get all three barrel lights on. These are independent of each other. With each light there is a switch, called a hold switch. When the switch is down, or when the machine is not running, each barrel light is permanently on or off. It is held in that state. However, when the machine is running and the switch is up, each barrel flashes on briefly at regular instants. However, the thermal inertia of the blub is so long that these flashes can't really be observed. This makes it harder! The aim of the game is to throw the switch down while the light is on. If successful at this, then light will then stay on. However, most users rapidly switch the switches up and down and so even when they catch the light on, they can't stop themselves in time and so they switch up again, thereby releasing the caught barrel. This adds fustration. The whole game is a trade off between switching the switches as fast as possible, so as to have as many trials as possible before the timer stops, versus reaction time and concentration. It's actually quite a good game. As there are three switches to do, and most people have two hands, it is also a challenge to decide how many switches to try to operate at once. Friends can be enlisted. In general, getting one light on per run is a good aim, so on average, three marbles must be paid in by the single user to get them all on.

Once all three lights are on, the WIN light comes on and the player has the choice of gambling or collecting, by pushing the black lever to the left or right. A collect pays out about 8 marbles whereas as gamble pays out a lot more or none. The lower three lights indicate the user's outcome: jackpot, lose or collect.

There are some added features to the game. The first is NO HOLD. When the marble is inserted, the HOLD light occasionally goes off. This means that for the next run, it will be as though all the barrel switches were UP, in the no hold position. Any lights currently caught will be lost and it is random as to which lights might get caught at the end of the run. A win from a NO HOLD is very rare indeed.

Another feature is KNOCK OFF. After a win, the next run is always a NO HOLD. However, it is possible to avoid this by paying tax. To pay tax, the white button is pressed in and a marble inserted while the button is held. Instead of priming the machine it pays the tax. The PAY TAX light goes off and the tax PAID light comes on. I can't quite remember what paying tax does, but I think it avoids a knockoff To avoid cheating, it is not possible to pay tax while the WIN light is on, so late paying or paying for multiple wins in succession until a NO HOLD occurs is not possible. In a multiplayer game, each user in turn inserts one marble and has one run, collecting any winnings payed out in their run. Paying tax then becomes a gamble that the paid tax will be beneficial to the person who paid it and not to a competitor.

A final feature is the STORE. When the store light is on, it seems the machine can be primed with the STORE store switch instead of inserting a marble. I don't recall what makes this light come on.

Internal Views

If we take the lid off, we can see the 30 or so relays that implement the logic. Capacitors are used for the timing functions and there are one or two diode gates as well. Transistors are only used as current amplifiers to enable a longer delay to be generated from a relatively small capacitor. The unit runs on 20 volt DC. Since some of the relays are 50 volt post office type relays, a certain amount of contact bending was needed to get them to run at the lower voltage. Also, a design habit of charging capacitors with no current limiting resistors also tended to reduce reliability.

Back View

As can be seen, a small amount of mecanno was used for the rear frame of the unit.

Side View

The three relays to the bottom right of this view hold the state of each of the three barrels. Also visible is the input from the external DC power supply. In a recent trial, the machine was no longer working very well, and all the lights were dim. Possibly the PSU rectifier has degraded and needs changing to a silicon one!

Payout Mechanism

The payout system uses a mechanno motor to generate a reciprocating action. Each movement dispensed one marble into the payout tray. Actually, there was no tray, so the marbles would roll out onto the table or floor! The exact number of marbles dispensed was somewhat uncontrolled, because the motor was turned on for a fixed duration rather than a fixed number of rotations. An additional page in the schematic illustrates a method of counting out the payout exactly using a uni-selector, but this feature was never implemented.

Top View

Internal Schematic

Most of the schematic is shown on this sheet, although there are that we have not scanned in.

Operation: When the marble is inserted, relay RL-1 latches and turns on the PRIMED light, labeled (2) on the schematic. When the string is pulled, RL-1 is delatched while a bolt of energy is dumped in the capacitor/RL-3 circuit. RL-3 is closed while the capacitor discharges and this constitutes the RUNNING timer. A second relay, RL-X, is slaved to RL-3 to provide additional contact pairs.

RL-4 is the NO HOLD relay and it overrides the three hold switches. Each of RL-6,7 and 8 is a pulser channel that oscillates during RUN mode and latches during stop or hold mode.

RL-15 is the win relay and is open if all of RLs 6,7,8 are open and also RL-12 is open. When RL-15 is closed, the WIN light comes on and the two-pole (on)-off-(on) biased switch is activated to latch either RL-14 (gamble) or RL-17 (collect). When gamble is slected, RL-18 is used to select LOSE or JACKPOT. According to a collect win or a jackpot, one of the two capacitors is discharged into RL-11's coil. The capacitors have different values to provide two different payout times. The payout motor turns while RL-11 is closed. The motor cannot withstand the 20 volt supply, but requires additional current to start it, so a reistor/capacitor shunt arrangement provides the output load. The choice between LOSE or JACKPOT is made by having RL-18 oscillate during each run and hold at the end - thus providing a random bit generator. A similar circuit, using RL-14 as a random generator, implements the NO HOLD random function.

Relay RL-11 implements the tax paid latch and RL-20 is activated briefly at the end of any payout where tax had been paid. RL-20 sounds an old post office buzzer (visible in the photo's) and delatches the tax paid relay.

(C) Century 21, DJ Greaves.