Who does not know the classic “river crossing puzzle” of a farmer on his way to market with a wolf, a sheep (or a goat) and a cabbage? Fortunately for the farmer, there is a small boat that he can use. But he can only take one of the three at a time in the boat and must make sure that the left-behind wolf doesn't eat the sheep and the sheep doesn't eat the cabbage if they are left on the riverbank together without the farmer's supervision. The task is to get wolf, sheep and cabbage to the other side of the river with as few crossings by boat as possible.
Those who like a little tinkering with electronics naturally solve this problem in style!
The farmer had probably already read more than 10 years ago, in 2015, in a German technical magazine (ft:pedia 1/2015, 'Wolf, Schaf und Kohlkopf'), how he could solve this problem with his fischertechnik set and some farmer's common sense before he would have gone out to the market.
My own suspicion is, that this was a farmer in the border region of Germany and the Netherlands. And that somewhere in the process of paddling back and forth, this farmer left his wolf unguarded on the other bank. The animal probably started walking westward, because it can hardly be a coincidence that in 2015 the first wolf was spotted (again) in the Netherlands. That wolf indeed originated from Germany, traveling hundreds of kilometers to get to the Netherlands. Isn't it wonderful to realize that there are Dutch fischertechnik fans (like myself) who occasionally travel hundreds of kilometers in the other direction to visit fischertechnik meetings in Germany?
So it has been demonstrated long ago how the problem can be ingeniously solved purely electromechanically. By moving rods in the model described in that magazine, the position of the wolf, the farmer, the sheep and the cabbage should be indicated. A thoughtful method that works purely mechanically. However, some imagination is needed with that method to see through each situation and solve the puzzle.
I have spent a long time thinking about how the movements of the boat, the cabbage and the animals could be made more visual. For example, with a model in which the boat and the animals could actually be moved along the metaphorical river.
At one point I had almost started working on a controller solution with a real boat in which the figures could be recognized by the NFC chip in their base, but I decided to build a technically simpler solution first. After all, it's too easy to lose yourself in overcomplicated solutions. And, hadn't the above referenced article already shown the true charm of mechanical simplicity?
Yet the idea of sliders or elements that could be slid across the imaginary river by the puzzler himself continued to excite. If each slider, as long as present on a bank, activated a push button according to the figure below, all that remained would be to implement logic. And that would be perfectly possible with traditional 'Silberlingen'. Note that the wiring of the push buttons here already follows the negative logic of the 'Silberlingen'. By reversing the farmer's pushbutton, his signal is already inverted logic immediately.
Let's logically map the situations to be avoided when the farmer is not present on the bank. In the figure below, each corresponding signal is “active” when the object or animal in question is on that bank. Of particular interest to the farmer is when he is not on the bank in question. Therefore, for him a logically inverted signal, which is thus active when he is not on the bank, makes more sense. These signals can easily be made with push buttons as shown here.
The two logical AND gates each detect a situation to be avoided if the farmer leaves the bank in question. The upper one detects the danger that occurs if the cabbage and sheep are left unattended. The lower one detects the undesirable situation in which the unattended sheep could be grabbed by the wolf. An OR gate combines both these dangerous situations so that a red warning light will come on in these cases. If there is no danger, then the green “all clear on this bank” lamp will light.
Thus, two AND ports with (at least) three inputs are required per river bank. In addition, a separate inverter (or switching relay) is needed to drive the green signal lamp. Those who want to solve this with discrete components from the traditional 7400 IC family will be able to choose, for example, the 74LS55. This also has already the OR gate we need. Only an inverted output signal then needs to be created to drive the red LED. The green one can be connected directly to the inverted output of this IC.
Those who have enough of the traditional fischertechnik electronics modules in stock (or rebuild them, as I did) obviously prefer to solve the problem with these so-called 'Silberlingen'. The figure below shows the connection diagram of a solution with these 'Silberlingen' in which the signals K (cabbage), W (wolf), S (sheep) and B ('the farmer is gone') are logically processed. The power supply with the fischertechnik rectifier module is not drawn. The top row of three logic gates shows the situation on the left bank of the river. The row below shows the situation on the right river bank. The green lamp lights up when everything is safe, the red one turns on as soon as an undesirable situation occurs on the respective bank.
Obviously, there are many ways to physically build the puzzle with fischertechnik, Lego or any other construction material. As I explained above, I wanted to mimic the analogy of a river with two banks as much as possible. I also thought it would be nice if the objects and animals could each be moved from left to right by the puzzler using their own 'boat'. Here the farmer's 'boat' may be moved to the other side of the river each time without any, or each time simultaneously with at most one other cart or 'boat'. After all, the farmer can only take one of the other three in his little boat to the other side each time.
Since I had just been working on my Sensor Adapter Module, I thought it would be nice to do the sensing contactless with reed contacts instead of push buttons as already schematically shown above. While absolutely not necessary, I enjoyed developing and 3D printing various custom parts for the model.
Since I found it convenient to still be able to change the symbols, I designed and printed a number of custom parts for the “symbol carts. For example, the U-shaped building board brick with window under which the icon of the respective animal can be clamped. A building stone with magnet holder for the round ⌀15x3 mm neodynium magnet is also visible in the photo.
The construction plates symbolically representing the river are also my own design. In that color and size they do not appear in the regular fischertechnik program.
For clarity the bottom rail has been removed to reveal the adaptor module with the reed contact, in image to the right. This drops into a hollow (custom) building block onto which the green building boards are slid.
I printed the various building boards and the hollow building blocks myself so that I could harmonize the blue color with the also self-printed blue fischertechnik 'Statika' plates representing the river. They ensure that the trolleys can be moved smoothly back and forth across the 'river'. Of course, the existing fischertechnik construction plates can also be used for this purpose.
The photo above shows the puzzle, as I eventually constructed it, on display. Because I made a interconnecting cable, the model could split across two base plates. On the left is the “river” where the boat may be moved from one bank to the other with at most one other symbol at a time. On the right is the panel with the 'Silberlingen' logic. As can be seen, I used a pair of mini traffic lights as red and green warning lights above each 'bank' of the imaginary river. It is also possible to connect regular LED light building blocks from fischertechnik directly to the corresponding outputs of the 'Silberlingen.
The model can easily be accommodated on a large fischertechnik baseplate 1000 (39 x 27 cm), but for the sake of transportation I accommodated “the river” and logic with the Silberlingen on two separate baseplates 500. As I still had some old 31-pin connectors lying around, I decided to make my own interconnecting cable so that a modular assembly became possible. The photos show the final result of this on the side of the “puzzle module” with the mini traffic light LEDs. The cable connection powers this part of the model and carries the various signals from the farmer, cabbage, wolf and sheep to the Silberlingen module.
This setup allowed me to transport the two “modules” in a stacked fashion. The first time I did this (traveling by train to the fischertechnik Südconvention in Germany) the base plates had shifted causing some slight damage. Therefore I replaced the supporting anchors I used for this at first (constructed from standard building blocks) with self-designed gray slotted bricks. Since then, the model has been transported several times without damage.
So it has been demonstrated long ago how the problem can be ingeniously solved purely electromechanically. By moving rods in the model described in that magazine, the position of the wolf, the farmer, the sheep and the cabbage should be indicated. A thoughtful method that works purely mechanically. However, some imagination is needed with that method to see through each situation and solve the puzzle.
Finally, a remark about the sheep.... I came across many versions of the puzzle where her role was played by a goat. After much deliberation, mainly to reach as large an international audience as possible, I have replaced the goat in this article with a sheep!
However, the U-shaped passe-partout building block (see the image of the “carts” above) makes it easy to create a “regional version” of the puzzle by replacing the image of the sheep with the icon of a goat. This modular design even makes it easy to create a version of the puzzle in which the farmer has to get a fox, a chicken and a bag of corn across the river. And even the version with a fox, goose and a bag of beans will no longer be a problem.
I unfortunately have to stop now because I see that both the goat and the sheep have started on the cabbage while the wolf is already licking his lips.... 😋