123 Game - All MCU-free

This electronic game pits a human player against the ‘machine’. The opponents use a common ‘game token’ and take turns moving along a path by one, two or three steps, and the winner is the first one to reach the goal exactly. Incredibly enough, this simple version of the ‘123’ game can be built without a microcontroller, and it’s almost impossible to beat. The electronics for this is built using only diode logic (Figure 1).

The ‘ input inter face’ consists essentially of 30 miniature sockets to which a probe tip can be connected to mark the position of the ‘game token’. To make the game more compact, the sockets are arranged in a grid so the route along the sockets follows a serpentine path (Figure 2). The starting position is at the bottom right, and the goal is in the middle of the playing area. The electronics becomes the ‘active player’ when the button is pressed.

The number of steps it wants to move is shown by three LEDs (one, two or three LEDs light up) at the top of the playing area. Naturally, the human player must move the ‘game token’ for the machine opponent. The winner is the first one to reach the goal exactly. How can such simple circuitry represent such a formidable opponent? As already mentioned, the path from the start to the goal is formed by 30 sockets. Each socket has an associated ideal next move.

There are three possibilities, of course: 1, 2 or 3. As you can see from the schematic diagram, switch S1 closes the circuit (which means the player asks the ‘computer’ how many steps it wishes to move) if the probe is touching one of the sockets. All 30 sockets are classified into three types, represented in the schematic diagram by one socket for each type. All sockets belonging to a particular type are simply connected together electrically, which is not shown on the schematic diagram for the sake of clarity.

Circuit diagram:
123 Game circuit schematic
123 Game Circuit Diagram- Microcontroller-Free

This is how the LED display works:

The player touches the right-hand contact with R4 (only LED D3 lights up), the left-hand contact with R3 (LEDs D1 and D2 light up), or the middle contact with diodes D4 and D5 (all three LEDs light up). The two diodes prevent all three LEDs from lighting up if the player touches the left-hand or right-hand contact. The key to all this lies in the assignment of the 30 sockets to the three types of logic, which means the three types of ideal next move.

Working backward from the goal, no further move is possible when the goal is reached. For this reason, the last socket is not connected to anything. At the socket just before the goal, the ‘computer’ naturally wants to be exactly one step in front. Consequently, this socket is connected to R4. At the second socket before the goal, the electronics wants to move by two steps. This socket is thus connected to R3.

Obviously, three moves before the finish, a three-step is best as it leads to instant victory. Consequently this socket is connected to D4/D5. The correct response of the ‘computer’ is shown in Figure 2 by the number next to each position. As the two opponents take turns playing, the electronics always tries to arrive at a strategically favourable position (marked by the arrows). If the electronics manages to reach one of these positions, it’s impossible for the human player to win. This means that the human player can only win by starting first and always making the right move.
Stefan Hoffmann
Elektor Electronics 2008