In the previous section, we concluded that it would be a further improvement if the four control cycle steps were guaranteed to alternate seamlessly without undefined and undesirable intermediate moments or overlap. The basic idea is that the four different control cycles are run electronically allowing a split between the control phases and the final “decoded” lamp combinations. This is made possible by a counter that counts from 1 to 4 (or from 0 to 3) and a separate 'decoder' with logic gates.
I wanted to build the electronic solution with the classic electronics modules from fischertechnik. A small problem is, however, that the original 'Silberlingen' assortment does not provide a digital counter with two binary outputs.
For the electronics we will limit ourselves in this article to the traditional Silberlingen. The electronic control cycle can thus be made with two so-called Grundbausteinen (36391) as described in fischertechnik's Hobby4, Volume 3. An overview of Electronic counters is given in Volume 5 of the same edition. The outputs of both Grundbausteinen hereby go through four unique output combinations. Note, however, that this “count” is not purely binary. Although anomalous, this counting offers the advantage that only one signal changes each time so that the underlying decoder will never receive undefined short transition signals. This count order of the two 'bits' can be thought of as a so-called Gray encoding.
This period duration of the four individual steps of this four-step pulse generator constructed from two Grundbausteinen should be about the same length. As the oscilloscope image on the right shows, this is more or less correct, but unfortunately this is only true for the higher (with GB1 controllable) frequencies. For longer cycle times, which are more realistic for traffic lights, the mutual period times do not allow themselves to be easily mutually controlled. The proposed feedback with a resistor of 22 kΩ to extend a period time did not give me the desired solution with different combinations of (original) Grundbausteinen.
The logic functions for the individual lamps are given at the bottom of the table. A small additional advantage is that the steps in which the yellow lamps should light (steps 2 and 4), conveniently called 'Gelb I' and 'Gelb II', can be determined with the following so-called XOR function:
A disadvantage, however, is that the logical XOR function will have to be composed from the available logical functions available in the so-called “Silberlingen” (fischertechnik AND and/or OR gates) and that when built with two Grundbausteinen, the pulse length ratio of the two succeeding pulses cannot be easily set.
There are several ways to create an XOR gate with a composition of OR/NOR and/or AND/NAND Silberlingen. This can be done with five OR/NOR Silberlingen or four AND/NAND Silberlingen. Since all Silberlingen provide both the normal digital output, and the inverted variant of it, it is even possible to build the XOR gate with only three Silberlingen as shown in the figure above. In practice, however, the construction of the decoder with four AND-Silberlingen is preferable because with this construction the control signals for the two green lamps are already available on the outputs of respectively the 2nd and 3rd AND port. The control signals for the red lamps can be taken directly from the outputs of GB2. This control signal is taken via a 1MΩ resistor to minimize the feedback to GB1.
The circuit diagram of this traffic light control is shown below (enlargeable). As drivers for the five differently distinguishable lamp functions, I used my self-built 'Quadruple Driver' module. Instead of this, also four RB I Relaybausteinen 36392 can be used for switching the various lamps. As voltage supply, of course, the regular fischertechnik rectifier module 36393 can serve. I used one of my own power supply modules.
Note that my 'Quadruple Driver' modules work with postive logic, while the Silberlingen traditionally work with negative logic. However, this is not a problem since all the necessary control signals are also available inverted. When using the original fischertechnik Relaisbausteinen, however, the complementary control signal for switching the lights should therefore be used. When using a Relaisbaustein, of course, simply the other contact of the changeover contact can also be used for switching the respective lamp.
As with the electromechanical solution, the traffic control is split into a four-step counter and separate lamp decoder. However, an additional advantage of this electronic variant is the tight definition of the four different stages. However, there is still little control over the mutual timing of the different steps.
An improvement on the previous design would be if we could control the individual time duration of each step in the sequence. This would allow us to configure the red/green times of the two directions of travel and the time that the yellow lights are on completely independently.
To control the individual time duration of each step in the sequence, I experimented with four monoflops as the timer of the control cycle. The combined clock signal from these modules can then serve as the clock signal for a four-step binary counter. With a normal binary counter, control proceeds as outlined in the table below. Below again are given the functions for the individual lamps.
A possible construction with homebrew variants of the so-called fischertechnik 'Silberlingen' is given below. As with the previous circuit, the power supply can be done with the traditional fischertechnik Gleichrichter Baustein and the driver modules I used can be replaced by (five) fischertechnik Relaisbausteinen.
With the four monoflops in the top row, the duration of each of the four cycle steps is fully independently adjustable. After connecting the supply voltage, this feedback “seqencer”/timer must first be “started” by pressing the pushbutton once. The Dyn-AND Baustein DA (36483) on the far left combines this pulse signal with the feedback output signal from the fourth monoflop.
After pressing the push button, the four monoflops go through the four steps of the control cycle. The time per step is adjustable on the respective monoflop. This makes it possible to set the “yellow” phases (2nd and 3rd step) more realistically and shorter than the red/green times of traffic lights.
To merge the four output signals of the monoflops into one combined clock pulse for the flip-flop counter, again some Dyn-AND Bausteinen come in handy. The steps of the counter/sequencer thus created, cause the two flipflops to count neatly in binary from 00 through 01 and 10 to 11. The sequence of the two flipflops was chosen so that the regular binary count can be easily read from the LEDs of the (homebrew) flipflops. Only an AND/NAND Silberling is needed for decoding this time for each green signal of each traffic light.
The mini LED traffic lights provided enough inspiration to investigate the different ways in which traffic arrangements were realized throughout history. To stay in the theme of roads and traffic, it therefore seems appropriate to conclude with a well-known saying: many roads lead to Rome! But what is certain here is: whoever actually undertakes this journey will encounter slightly more traffic lights on his path today than in Roman times.... 😜
