CWD351 – Digital LED indicator from the communist era

Since its inception, all digital machines were meant to serve man and help him perform his daily tasks in the office, on the factory floor or at home. However, in order for a machine to be useful, something was needed so that humans could understand it. Nowadays, it is quite normal to have all kinds of screens on which digital systems display information that we can understand, but this was not always the case.

In the days when the only common screen was a cathode ray tube, computers used all sorts of indicators to communicate with humans. These could have been light bulbs, nixie tubes and later also LEDs and 7-segment displays. In this material I will tell you about a certain design, which is one of the more interesting variations of the 7-segment display.

CWD351 display

Today’s hero is a display, actually a “digital diode indicator” with the designation CWD351 produced by the “POLAMP” Unification of Lighting and Electromechanical Equipment during the People’s Republic of Poland.

Rather, everyone will agree that this is a 7-segment display, but with a slightly different design. There are no ordinary led segments here, as we are accustomed to in classic elements of this type. Instead, the digit is built on eighteen LEDs, arranged in the standard Arabic numeral typeface. An additional nineteenth LED acting as a decimal point was placed rather unconventionally on the left side of the sign.

The CWD351 is actually built from two separate PCBs connected by a dozen wires. LEDs are soldered onto one of the boards, additionally protected by black plastic, which also acts as a background for the displayed digit. The other part of the structure contains all the control electronics, which we will look at later, as well as the connector for controlling the display. It takes the form of etched copper fields directly on the laminate.

It’s hard to say something about the application of this type of display, I can only suspect that it was used rather in professional control and measurement equipment or in factories, where it might have been part of some larger machine.

Where does the POLAM display come from?

Rosa Luxemburg Electric Lamp Manufacturing Plant Warsaw (

As I mentioned earlier, the manufacturer of the CWD351 display is the Polish company POLAMP. However, if we would like to say specifically from which factory the mentioned design comes, a problem arises. Under the name POLAMP during the communist period, there were actually 30 different kinds of factories, associated around the broadly defined tube industry. Glassworks, manufacturers of metal parts, research and development offices and plants assembling already finished lighting equipment – all these factories were subordinate to the United Lighting and Electromechanical Equipment “POLAMP”.

ZWLE receiving electron tube assembly hall (

POLAMP as a unification was established on December 31, 1971, although the story begins a little earlier. In January 1970, the foundation for POLAMP, the “Unitra-POLAM” Light Source Combination, was laid. From the very beginning, the leading role in the unification was played by the Rosa Luxemburg Warsaw Electric Lamp Manufacturing Plant. With the collapse of communism in Poland in 1989, POLAMP is dissolved, and all the plants operating under the logo of the unification until then have to work on their own account.

70s, operation to apply luminophore to mercury lamp balloons (

The origin of CWD351 cannot be 100% determined, but there are many indications that they were made at ZWLE, the aforementioned Rosa Luxemburg Electric Lamp Manufacturing Plant in Warsaw. The history of the factory is quite interesting, as it was established shortly after World War I in 1922 as the Polish-Dutch Electric Lamp Factory founded by the N.V. concern. Philips.

During the interwar period, the company develops quite rapidly. At first, only incandescent lamps were produced, but over time the product range expanded and electron lamps and radios were added. As the business expanded, more factory halls were also built, employing several thousand people.

The development of the factory located in Warsaw’s Wola is brutally halted by the outbreak of World War II. After the fall of the capital, it is occupied by the Germans and until the Warsaw Uprising, it mainly produces transmitter and receiver apparatuses for submarines, tanks and aircraft.

ZWLE products (

After World War II, the plant is renamed ZWLE and officially becomes a state-owned enterprise. Under communism, the company is developed more and more, more production processes are created, and new products appear in the factory’s offer – LEDs, displays, digital reading panels, or sodium lamps. However, after the fall of the People’s Republic of Poland, ZWLE’s monopoly on lighting products ends. The plant is privatized and divided into 4 companies Polamp-Warszawa, NATRIUM, Halzet and Polam-Elta, which did not survive the transition period. In 1993, the buildings of the bankrupt plants were sold, through the British-Polish company Fintrade, to the Polish company Eurotrade-Pol. A year later, the site was acquired by Universal SA, and after the initial modernization of the building into a banking and service facility, all work was stopped due to too much mercury. From then on, the ZWLE buildings were abandoned and were eventually demolished in 2011.

One interesting fact I found is that ZWLE was one of the first companies to introduce gymnastics during working hours.

CWD351 display control

CWD351 pinout description

The CWD351 is not just a simple board with LEDs soldered on. The engineers have added some electronics to it to make it easier to control the display. Looking at the pinout description, you may already know what circuitry sits inside.

The display has ten pins, on the edges there is a power supply, which should be around 5V. Right next to the positive pole of the power supply was the lead that controls the dot, next to the displayed digit. If you connect it to the plus of the power supply, the diode symbolizing the dot will light up. The remaining pins are used to control the circuit responsible for the displayed character. On B, C, D, A we specify the binary code of the displayed number, while the RBI, BI/RBO and LT signals make it possible, among other things, to force all LEDs to go off or activate.

The inside of the LED indicator

The inside of the display

In order to get inside the CWD351, we have to unsolder from the metal wires the laminate on which the connectors and all the control logic are placed. After disconnecting the two boards, we can see that the design of the display is quite simple. Here we have a single integrated circuit and a few resistors. The brain of the diode indicator is the UCY7447 chip, this is a chip manufactured at Warsaw’s CEMI plant, which is a copy of the popular BCD – 7-segment decoder with an almost identical designation – 7447. This design allows you to drive a 7-segment display based on the BCD code fed to the input. The task of all the resistors placed on the PCB is to limit the current flowing through the LEDs, so as not to damage them.

CQXP 01 or CQXP 02 LEDs

After unfolding the display, the plastic cover of the LEDs can also be pulled off. It reveals the CQXP 01 or CQXP 02 LEDs, which were probably manufactured in ZWLE. They differ slightly from today’s LEDs especially in dimensions, which can be seen well in the photo. Besides, it can be concluded that these diodes are rather from later years of production, as the silicon core is placed in a slight recess. In the early 1970’s a flat substrate was used.

The digit 9 along with the decimal point

Despite the years, the display still works. The typeface of the digits 6 and 9 is quite interesting; it is devoid of the upper and lower dashes, respectively. This is a typical solution in those years, which could be circumvented by adding two NOT gates to the circuit.

Digit 3

You have to admit that the CWD351 is a rather interesting design that is, in a way, the essence of the People’s Republic of Poland – a display manufactured by the POLAMP union at the ZWLE plant, from which the LEDs probably also come. The integrated circuit is made by Warsaw’s CEMI, resistors from Cracow’s TELPOD and everything placed on a laminate from Toruń’s TORAL plant. But whatever one may say in its time the design certainly served its purpose.



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