All About Seven Segment Display


Together with the Nixie tube, it was in the early days of digital technology, the first type of display, which produced human readable digits: The seven-segment display. It is so called because the decimal point is not counted as a segment. From the remaining seven light bars can easily form all the Arabic numerals. In contrast to the Nixie tube, God bless her, the seven-segment display has established itself as an output medium to this day. Not only her minimalist style, but also the different dosage forms of her lighting elements have ensured her survival. Seven-segment displays come from light bulbs, vacuum fluorescent tubes, liquid crystals, and many other forms. But probably the most commonly used medium is the LED.

And in this form it is also used in my next alarm clock project. There, a group of eight seven-segment displays forms the output medium of the clock and this helped the project also to his name ...
How do you manage to pour a digital display completely into glass? Not at all. The vast majority of electronic components would not survive sinking in liquid glass. So, to figure out how an ad that spills into a vitreous body will look like, there must be another possibility. The obvious thing is not to use a massive glass rod, but a glass tube, in which the display, including electronics, stowed like a bottle ship. Thereafter, the tube is flooded with a substance that is transparent and non-conductive and bonds to the glass so that there are no boundaries at the optical junction. At first, I thought of silicone oil. Silicone oil fulfills the three properties, but has the disadvantage of being liquid. The problem would not show up right away but only after some time. The viscosity of the oil progresses over time and causes puddles. In return, an air bubble forms at the top of the tube, transforming the indicator tube into an oversized spirit level. So, no oil.

But there is an alternative: pourable silicone. This is available commercially for the purpose of isolating outdoor distribution boxes. It is mixed together from two components and hardens to a jelly, a kind of silicone jelly. The substance, which is completely clear, is easy to process and virtually fuses with the glass. The result is a glass rod with integrated Octet-7 display. And the whole thing would even be waterproof.

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The main task is carried out by the already aged but functional driver module MAX7219 of the manufacturer Maxim. As usual with digital displays, he controls the display in multiplex mode. In this case, the ground lines of the numbers in turn are switched through at 800 Hz (scan rate), which therefore requires that segments with common mass (Common Catode) are used. The Octet-7 display uses SC36-11 (Kingbright) seven-segment displays in yellow. They have a height of 14 mm (digit height 9 mm) and fit easily into the glass tube. The decimal points of positions 4 and 5 are not interconnected with the LEDs of the segments, but are led out as separate LEDs on the board (jumper-configurable). Their task is to provide status information of the clock. D1 lights when the alarm is set, D2 indicates error conditions.

So that the chip does not get too hot in its silicone bed, the maximum current of the segments is limited by R1 to approx. 25mA. In addition, the IC was still a narrow heat sink donated, which facilitates the heat dissipation.

The communication between display and controller is realized by a 3-wire SPI. Despite the moderate transmission frequency of just under 2 MHz, you should consider interference and not let the connecting lines too long. The Octet 7 o'clock is about 30 cm. The schematic of the display can be seen on the left, as well as the layout of the board, which is specially shaped to fit into a 19mm inner diameter glass tube.

The data of the display as well as that of the other components are summarized in the technical document. It also contains parts lists and pin assignments.

The Control Unit

The combination of operating unit and controller module forms the Octet-7 control unit. It is housed in a small shell made of aluminum (type AKG 55 24 50 from Fischer). The control unit, here referred to as a console, consists of only three parts: rotary incremental encoder, LED and photoresistor. All sit together on the small "Console" board, which is mounted behind the front panel so that the controls pass through the front panel, making it possible to operate the clock from the front. Behind the console board is the controller module. The layout of the controller board is designed in such a way that on the one hand it enables exactly the contacting of the perpendicular console board, but on the other hand it can also be pushed into the board rails of the aluminum housing. Here is the installation space concept. Also seen in the dimensions and recesses of the front panel. It is noticeable that in addition to the three components of the console board, a fourth part appears in the front panel: It is a passive IR motion sensor of the type AMN3, as it already comes with the alarm clock used. Due to its size, it could not be included in the console board and therefore had to be placed directly on the controller module.

Let's stay with the controller module. The central unit is an Atmega328P type microcontroller. The comes this time in the flat TQFP32 housing therefore, so SMD. This saves space, so that together with some other SMD components, the controller module fits on an area of ​​only 51x33 mm. Functional support is provided by the controller through the DS1307 real-time clock. It is connected to the controller via I²C and serves to keep the running time constant and to save it over shorter power failures. However, the support capacitor C14 required for this purpose is somewhat weak at 220μF, so that even with interruptions of more than one hour the real-time clock chip forgets the time. Furthermore, there is a piezo signal generator on the underside of the controller board, which serves for acoustic signaling. If the signal generator is too quiet during operation, the sound can be made easier for the sound to the outside by means of a small hole on the corresponding side of the housing, thus increasing the sound pressure. Apart from a few decoupling capacitors and debounce modules, these are the essential things that are installed on the controller module.

Maybe another word about the power supply: The whole clock works on the basis of 5V. There are now two options to provide the clock with the necessary voltage: Either you use a stabilized power supply that provides directly the necessary 5V or you leave the integrated switching regulator the work and supplies the clock with a DC voltage somewhere between 9 and 30 volts. If you use the direct 5V supply, jumper J1 must be closed with a solder bridge. After that, of course, the assembly of the switching regulator can be omitted.

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