Associated circuits. By Lee Davison.

These are the associated circuits used to make the demonstration VFD light up with something interesting. These circuits are in place of whatever you would use to drive the display be it discrete logic, a dedicated chip or a processor of some kind.

The oscillator is just a 555 timer IC configured to run as a multivibrator. None of the circuit values are critical though it needs to be fast enough to avoid flicker.

The diode is there to make the output ratio closer to 50:50, without it the ratio would be nearer 66:33, though it is probably not needed in this case.

The output of the oscilator goes on to drive the binary counter.

The counter is a three bit binary up counter and is set to free run. The 74LS161, which is actually a four bit counter, was chosen just because I had one on the bench, any three bit binary counter would do in its place.

The A, B and C outputs from the counter go on to drive both the grid and anode decoders.

The outputs from the counter are decoded into eight active low grid drives and wired so that a count of 000 drives the rightmost digit while a count of 111 drives the leftmost digit.

The outputs go to eight of the anode/grid drive circuits and then to the display grids.

I would have used a 74LS47 BCD to seven segment decoder for this but I didn't have one available so I programmed a GAL16V8A as a hex to seven segment decoder instead. The pld file for this decoder can be seen here and both this and the resultant jed file can be downloaded here .

The outputs from the counter are decoded into seven active low anode drives and wired so that a count of 000 displays a 0 while a count of 111 displays a 7 so the whole display will read 7 6 5 4 3 2 1 0.

If the D input on pin 4 is taken high a count of 000 now displays an 8 while a count of 111 now displays an F so the whole display now reads F E d C b A 9 8.

The outputs go to seven of the anode/grid drive circuits and then to the display anodes.

Last page update: 28th July, 2009.

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