In this article, I would like to add a clock source to the previous circuit to make it a VFD clock.
The previous article is here.
50 Hz clock
The clock source for the MM5316N uses an AC signal from an electrical outlet. Therefore, a clock with a frequency of 50Hz or 60Hz is required.
Also, the clock input voltage has to be 25V, the same as VSS.
I will use a 1.638400MHz clock and divide its frequency by 15 to get 50Hz. Since the circuit runs on 3.3V and the output of the 74HC4040 is also 3.3V, I use a transistor to convert the level to 25V. The transistor can be any kind of NPN transistor. I used a chip transistor, S8050.
Here is the 1.638400MHz oscillator.
Here is the 74HC4040 divider IC. Since it's a surface mount component with a narrow pitch, I bent the leads to the top so that only the necessary pins touch the board.
Wire the board according to the schematic. I was able to confirm that the oscilloscope successfully produced 50Hz.
Adding to the breadboard
Added the clock module to the breadboard, the clock is now supplied and the clock is counting up!
Make the heater AC driven
Problems when driving the heater with DC
The IV-6 heaters have a voltage gradient of 1V per heater; when four heaters are connected in series, there is a 4V voltage difference between the lowest VFD, where the heater is connected to GND, and the highest VFD, where the heater is connected to the power supply. As the voltage of the heater increases, the voltage difference between the heater and the grid decreases. When the voltage difference between the heater and the grid decreases, the display becomes dimmer.
This causes a brightness difference between the lowest IV-6 and the highest IV-6. To mitigate this, the heater is driven by AC, and the display alternates between bright and dark, so that on average, the brightness appears equal.
Reversing the voltage with an H-bridge
An H-bridge is a circuit that can switch the flow of electricity in the forward and reverse directions, and is used to drive motors forward and reverse. By adapting this circuit to a heater, we would like to switch the current of the heater in the forward and reverse directions to achieve AC drive.
The table above shows the truth table of an H-bridge IC, where the current can be switched between forward and reverse directions by setting the IN1 and IN2 pins to H,L or L,H respectively.
Therefore, we can generate alternating current by inputting the 50Hz signal and its inverted signal to IN1 and IN2 of the H-bridge. Also, the 50Hz signal is 3.3V, but since the H-bridge IC is driven by 5V, we need to convert the input signal from 3.3V to 5V.
This is an inverting circuit using a transistor, which also performs level conversion at the same time. The FM116B is used as the H-bridge IC, but the L9110 has the same function, so the L9110 may be easier to find.
I soldered the AC drive circuit to the left side of the 50Hz clock module I made earlier.
Adding to the breadboard
Now that the heater is AC driven, all digits are displayed with the same brightness.
Modified the DCDC converter to 25V only
The DCDC converter module is a variable voltage type, so to fix the voltage to 25V, remove the multi-turn volume and replace it with a resistor.
This DCDC converter module uses an IC called MT3608. The fordback resistor, R2, originally had a 200Ω resistor. So I calculated R1 to be 8.2kΩ so that the output voltage would be 25V.
So, I attached an 8.2kΩ resistor to the removed volume, and made it into a 25V-only DCDC converter module.
My clock using the IV-6 and MM5316N is now complete!
There is something mesmerizing about the light from the VFD, as well as the Nixie tubes. It is very beautiful.
The light from the VFD is also something mesmerizing, just like a Nixie tube clock.