The growing popularity of VFD displays, their relatively low price and interesting appearance prompted me to build a clock based on such a display.
The heart of the clock is one of the oldest specialized Soviet microcircuits for electronic clocks made in the p-MOS technology КР145ИК1911 (KR145IK1911) in a 40-DIP housing. The system has the ability to control external devices or simply can be used as an alarm clock and timer. In my solution, I did not use these functions. The whole was assembled on a universal printed circuit board.
The power section remained on the original board from the damaged clock. The КР145ИК1911 microcircuit requires a DC power supply of -27V, it comes from a stabilizer based on a rectifier bridge, zener diodes and a transistor. A low-power transformer with two secondary windings is used, one to obtain a voltage of -27V, and the other to obtain an alternating filament voltage of 5V. Since the heating circuit of the display is also the cathode, the transformer has a tap in the middle of the winding. In order to extend the life of the display, the filament voltage is reduced to about 4.8V. The clock is also equipped with a main oscillator backup system based on two A23 batteries connected in series.
At the outputs of the microcircuit 12-18, a seven-segment digital code is generated. To indicate the numbers, the VFD display ИВЛ1-7/5 (IVL1-7/5) with multiplex control is used. The signals from the КР145ИК1911 system control the potential of the anodes of individual digits and service marks, ensuring their alternate activation. Only one digit is displayed at a time, but the transition from one digit to another occurs at a frequency of 500Hz, which makes the digits on the display appear to be continuous to our eye. The flashing of the service marks (dots) of the display is done by giving a 1 Hz signal from the output 22. The clock also has the function of blanking the insignificant zero when displaying one-digit hours, based on a transistor and two resistors.
The main oscillator is inside the chip connected to a standard 32.768kHz watch quartz oscillator. The oscillator was initially connected to a resonator system and two 20pF capacitors. However, its accuracy was not satisfactory, which I wrote about here: https://www.elektroda.pl/rtvforum/topic3852502.html.
So I decided to add additional capacitance to the system in the form of a 4.2-20pF trimmer capacitor. Despite numerous attempts to adjust the frequency and replacing the capacitors with a slightly larger capacity (22pF), I decided to look for another solution.
The original version of the clock:
Finally, as an oscillator I used a specialized DS32kHz system from Dallas, which is a temperature-compensated quartz oscillator (TCXO) with an output frequency of 32.768kHz. It is powered by the 78L03 stabilizer with 3V. It also has a power backup, which is based on the backup power supply of the main oscillator of the КР145ИК1911 chip. The used DS32kHz system, according to the manufacturer, has a deviation of ±2ppm, which gives ±1 minute / year (0.17 seconds / day). The system works perfectly, the movement of the clock is very accurate. Now, after three months of uninterrupted operation of the clock, the deviation from real time was about +0.03 seconds / day (+0.9 seconds / month), which gives an accuracy of ±0.3 ppm!
According to the manufacturer's data sheet, it is worth supplying the DS32kHz system with a voltage close to 3V to achieve greater stability, as you can see it gives very good results.
Final connection diagram:
Plate:
Clock appearance:
The heart of the clock is one of the oldest specialized Soviet microcircuits for electronic clocks made in the p-MOS technology КР145ИК1911 (KR145IK1911) in a 40-DIP housing. The system has the ability to control external devices or simply can be used as an alarm clock and timer. In my solution, I did not use these functions. The whole was assembled on a universal printed circuit board.
The power section remained on the original board from the damaged clock. The КР145ИК1911 microcircuit requires a DC power supply of -27V, it comes from a stabilizer based on a rectifier bridge, zener diodes and a transistor. A low-power transformer with two secondary windings is used, one to obtain a voltage of -27V, and the other to obtain an alternating filament voltage of 5V. Since the heating circuit of the display is also the cathode, the transformer has a tap in the middle of the winding. In order to extend the life of the display, the filament voltage is reduced to about 4.8V. The clock is also equipped with a main oscillator backup system based on two A23 batteries connected in series.
At the outputs of the microcircuit 12-18, a seven-segment digital code is generated. To indicate the numbers, the VFD display ИВЛ1-7/5 (IVL1-7/5) with multiplex control is used. The signals from the КР145ИК1911 system control the potential of the anodes of individual digits and service marks, ensuring their alternate activation. Only one digit is displayed at a time, but the transition from one digit to another occurs at a frequency of 500Hz, which makes the digits on the display appear to be continuous to our eye. The flashing of the service marks (dots) of the display is done by giving a 1 Hz signal from the output 22. The clock also has the function of blanking the insignificant zero when displaying one-digit hours, based on a transistor and two resistors.
The main oscillator is inside the chip connected to a standard 32.768kHz watch quartz oscillator. The oscillator was initially connected to a resonator system and two 20pF capacitors. However, its accuracy was not satisfactory, which I wrote about here: https://www.elektroda.pl/rtvforum/topic3852502.html.
So I decided to add additional capacitance to the system in the form of a 4.2-20pF trimmer capacitor. Despite numerous attempts to adjust the frequency and replacing the capacitors with a slightly larger capacity (22pF), I decided to look for another solution.
The original version of the clock:
Finally, as an oscillator I used a specialized DS32kHz system from Dallas, which is a temperature-compensated quartz oscillator (TCXO) with an output frequency of 32.768kHz. It is powered by the 78L03 stabilizer with 3V. It also has a power backup, which is based on the backup power supply of the main oscillator of the КР145ИК1911 chip. The used DS32kHz system, according to the manufacturer, has a deviation of ±2ppm, which gives ±1 minute / year (0.17 seconds / day). The system works perfectly, the movement of the clock is very accurate. Now, after three months of uninterrupted operation of the clock, the deviation from real time was about +0.03 seconds / day (+0.9 seconds / month), which gives an accuracy of ±0.3 ppm!
According to the manufacturer's data sheet, it is worth supplying the DS32kHz system with a voltage close to 3V to achieve greater stability, as you can see it gives very good results.
Final connection diagram:
Plate:
Clock appearance:
Cool? Ranking DIY
Where can I get some copies? I haven't found anything in Poland, only eBay and it's a shipment from Russia, so there is currently no option for it to arrive. Maybe there are other, more accessible VFD clocks, also implementing all the functionalities in one chip?
