Driving Vacuum Fluorescent display with a MAX7219

Discussion in 'The Projects Forum' started by gotramaval, May 26, 2013.

  1. gotramaval

    Thread Starter New Member

    Jan 1, 2011
    Hello everyone:

    I got some VFD tubes, and have some MAX7219 led controllers which I want to use to drive the tubes. The problem here is that the MAX7219 is designed to drive common cathode 7-segment led displays, so the pins for driving the segments are at 5V+, and the pins for multiplexing between the digits sinks current to the ground.

    To interface the MAX7219 with the VFD segments I'm using a UDN2981 (8-Channel source driver) supplied with 15V. This part is working.

    The problem resides with the driving of the grids, that also needs 15V, so I'm using a PNP transistor as shown in the schematic. If I measure with the oscilloscope in "A" (one of the pins MAX7219 for driving the digits) I get the expected pulsed DC signal (5V-0V). But if i directly measure it on the transistor base, or at the VFD grid, I get a DC constant current of 5V or 15V respectively, so I cannot multiplex.

    What am I doing wrong?? Why am I not seeing the multiplexing pulses on the VFD grid?

    Thank you in advance for your answers!

  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    Can't tell from the drawing, the PNP emitter is connected to +15 v, correct ?
    With the voltage to the base resistor going between 0 and 5 volts, the PNP tr is never turning off.
    You need to shift the 5 v drive voltage level to 15 volts.
    You could do that with some more transistors, opto-isolators, or open collector comparators for example.
    Last edited: May 26, 2013
    gotramaval likes this.
  3. gotramaval

    Thread Starter New Member

    Jan 1, 2011
    thank you tubeguy

    Yes, the emitter of the BC556 is connected to 15V+.

    I see, I have been reading a bit deeper, now I understand that I cannot control the PNP with 0-5 V when I want to source 15V.

    I have been thinking on connecting the microcontroller to a PNP transistor, and the collector of this one to the base of a NPN, that would drive the VFD grid.

    Would that work?

    The load would be at the high level, instead that at the ground level. Would that be a problem??

  4. gotramaval

    Thread Starter New Member

    Jan 1, 2011
    I'm still trying to find out how to invert the output of the MAX7219.

    I have tried a NOT gate (7404) but it is also not working, it seems that is delivering some 3V at the output (Im supplying 5V), so it cannot drive the latter NPN transistor.

    Any ideas, please?