scahr-lab / SC_IV18

SC_IV18_CLOCK

Hello, world. In this article I will tell you about creating the clock project using old-school soviet VFD indicator IV-18 (ИВ-18).

Intro

What where my goals in this project? First of all, of course, I want to know a current time, date and the day of week. Also it would be interesting to know the temperature in the room and anything else. For example, I want to be able to configure the device through USB-connection using the AT-commands in the same way as with dial-up modems ~~in a far, far galaxy...~~ a long time ago.

Let's develop it.

The Theory

Every clock like this can be functionally divided into several blocks.

DC-DC Step-Up converter

To power an indicator itself

VFD Driver

To control VFD's anodes and/or cathodes and the grids

Real time clock (RTC) module integrated into MCU or standalone IC

To keep and provide time and date information

Microcontroller (MCU)

To manage all this peripherals

I also added a buzzer for an alarm clock and a USB-TTL converter for communicating with a PC via a standard USB port as described above.

And now let's go through each block in more detail.

The MCU

Actually, we can use any MCU for our goals. This can be MCUs from ATMEGA, for example, the same that uses in ARDUINO-boards, from MSP (for example, most popular MSP430 series), from ST (lot of STM32 ARM-series). I choose the STM32 MCU, model STM32F030F4Px. Why this one? I'm working with this MCU series, I have several MFUs of this model available, and they are cheap.

The RTC

Here we again have many options for counting and saving the current date and time. Almost every ST MCU has a built-in RTC module, and, of course, we can use this module. But I want to practice more with a dedicated RTC module for coding and testing some circuit/schematic solutions. Thats why I choosed DS3231 IC from Maxim Integrated.

The VFD Driver

As you know, the MCU's operating voltage in case STM32 is 3.3V, but VFD requires up to 70V. So, to operate with this voltage we need a VFD drivers. They allow control this VFD's high voltage with our MCU. I choosed MAX6921AWI from Maxim Integrated. This IC perfectly fits for IV-18 VFD.

The DC-DC Step-Up convertor

And now we must obtain this VFD's high voltage. From where? We can use a DC-DC step-up convertors. Our device will powers from a standart USB, we can step-up this 5 Volts up to 50Volts for our VFD.

The Practice

And now, let's go to the schematics.

To configure the MCU and build a source code template I use the STM32CubeMX. This is a perfect and easy to use tool. Now, after selecting the MCU model, clocks, pins and peripherals configuration looks like this.

The pinout

The clock configuration looks very simple. It is just maximum possible values for this MCU.

The SPI, I²C and USART also has configured with standart params.

The hardware.

Let's begin from power supply. First, we need global power supply for our device. Here is minimum of necessary components as you can see.

I want to supply my device from any USB-host (e.g. laptops, PCs, USB chargers etc…) therefore, the device is designed for an input voltage of 5V. But MCU works with 3.3 Volts and I used pretty simple solution with fixed voltage regulator AMS1117 from Advanced Monolithic Systems. Simple and practical.

Besides all this we need stabilize our input (from USB) and output (from regulator) voltages to reduce interference and power surges. We can do this adding the electrolythic and ceramic capasitors. Datasheet for MCU recommends, how to properly we need add capasitors, but I want to minimize number of components and reduce the cost of whole device. For this device, this will be enough.

J1 — Mini-USB connector

C6, C7 — Capasitors for input voltage from USB

C3, C4, C5 — Capasitors for output voltage from voltage regulator

Now we can connect the buzzer for alarms.

Another one simple solution to operate with buzzer with direct (from USB) 5V voltage.

R5 — Current limiting resistor for MCU pin

R6 — Current limiting resistor for buzzer

Q3 — Almost any NPN transistor

TIM3 PWM configuration for BUZZ pin

The prescaler, counter and pulse values may be vary.

Next. We need count and store time and date to show them and implement an alarm. I like IC DS3231 from Maxim Integrated with an integrated temperature-compensated crystal oscillator (TCXO) and crystal.

Of course, I can use built-in MCU's RTC. But in this project I wants littile bit more practice with «external» interfaces, such as I²C. Among other things, we can read the «room» temperature from this IC.

I will use only the I²C for interfacing with our MCU. And also we need provide a backup power supply for DS3231 for continuous timekeeping. For this I use standart battery CR2032.

And now the most interesting part, in my opinion, in this device. It is a step-up converter for our VFD indicator.

Each VFDs requires two voltages. It is a heating (from 1.5 to 5.5 Volts) and operating voltage (from 25 to 75 Volts) depending of VFD model.

Heating (cathode) voltage for this VFD can be powered directly from USB which is very convenient.

Now we must commutate all anodes and grids of VFD using our MCU. How we can do it? There is two ways at least. At first, we can add many transistor keys. But in this case we needs many of free MCU pins or use shift registers IC for multiplexing our MCU pins for these keys. At second, and it is my solution, we can use some IC, which designed special for this targets. An VFD drivers. And I found this one.

MAX6921AWI from Maxim Integrated again. Perfect IC for this solution. Using this IC I can connect and commutate all VFD pins using SPI interface of MCU.

Now also I added the IC for communicate with USB host to be able to changing settings of device or get some info. It is popular IC CH340G (like FT323).