Using a VFD IV-3A tube to build a simple counter – Part three


In part two an ULN2023 is used so we can connect the IV-3A segments to 20V, while driving the segment with 5V. In this part the IV-3A is connected to the ULN2803A, and a 74LS48 which is BCD to 7-Segment Decoder.  By using the 74LS48 we can use 4 bits to drive the 7 segments.

Connecting the IV-3A

To connect the IV-3A to the IV-3A follow the following table:

IV-3A connections

IV-3A ULN2308
5 8
6 7
10 6
1 5
2 4
3 3
4 2

The last connections to make are the grid and heater. The schematic looks like:

IV-3A connections
IV-3A connections

Connect the 74LS48

The datasheet for the 74LS48 can be found here. Before connecting up the 74LS48, we need to invert the signals. Since the ULN2308 is a NPN Darlington array, we  need to invert the signals, so that a segment is activate when the input signal is going HIGH, instead of LOW.

An easy solution is to use a HEX invert. Since a HEX invert, as the name implies has 6 inverters, so we need two of them. So 2x a 74LS04 is going to be used.

The schematic looks like:

Display Driver schematic
Display Driver schematic

When connecting the all the BCD (248) inputs( pins: 76,2,1) to GND, the IV-3A should show a “zero”.   Like wise, if we connect these pins to 5V, a 8 is shown. The table below show the BCD coding for the pins:

BCD (248) conversion table

A3 A2 A1 A0 Digit
0 0 0 0 0
0 0 0 1 1
0 0 1 0 2
0 0 1 1 3
0 1 0 0 4
0 1 0 1 5
0 1 1 0 6
0 1 1 1 7
1 0 0 0 8
1 0 0 1 9

In the next article the HP8175A is going to be used as a binary 4 bits counter.

Using a VFD IV-3A tube to build a simple counter


Some time ago I figured out the pins of of IV-3A VFD tube. In the upcoming articles series I’m going to build a simple counter, and planning to use some of my LAB equipment to test and build this counter. Just for fun.

Roughly I’m thinking of planning the following articles :

      • Implement the PSU for the different voltages (1V,5V,20V)
      • Design and implement the display driver
      • Use the HP 8175A to simulate a 4 bit binary counter
      • Using a Hp8110A as a serial data generator to simulate a 4 bit counter
      • Finally build the counter

Implement the PSU for the different voltages (1V,5V,20V)

One of the first challenges when working with a VFD tube is getting all the different voltage rails required. The IV-3A tube needs the a couple of voltages: 1V,5V,20V

The get the 5V rail is not a real problem, 1V and 20V can be more difficult.

To start with the 1V rail, one might think of a simple voltage divider, but this is not as simple as it sounds. When trying to drive the heaters, this will add a load to the voltage diver. Which lowers the voltage, resulting in a voltage which is to low. This could be addressed by adding a opamp, as a buffer. However the circuit is going to get more and more complex.

So in this design I’m going to use small DC-to-DC converters. Or step-up converts to be more precise. And I use two modules:

      • One module to step up the voltage to 20v
      • One module to down convert the voltage to 1V

The modules ‘m going to use are:

      • XL6009E1
      • HX-mini-360

These modules are cheap, and easy to use.

The downside to this is that the heater could be driven “to hard”, on the other hand, these tubes can take somewhat of a “punishment”. I’ll keep the rest of  the voltages low by driving the tube with 20V instead of the max 30V.

Both of the DC-to-DC converter modules are driven from a 5V supply. This makes it easy to implement the modules.

In the next article the display driver is going to be implemented.