Circuit works when I poke a connection, but the connection is sound

So, I've started playing with vfds since this weekend jaycar (if your an Aussie) is selling 11 digit, 10 segment vfds for $1.90. Cheap as :P. It runs off 12v for the segments, 4.5v for the filament. I'm using 3 4094s to control the segments and an atmega168 to control the 4094s, using 3 transistors to up the voltage between the 4094s data, clock and latch pins and the the atmega I/O ports. I'm pretty sure everythings hooked up correctly but, as with most projects, it's not working first time :P. Here's the thing, it works if I poke either the green or brown connection with just one probe of my multimeter/finger it works. My first thought is that i've hooked up my transistors incorrectly, the base is 5v with a 2.2k current limiting resistor, and the collector is 12v with a 5.1k current limiting resistor. To much? I blew out a resistor in initial testing and it got me a little worried :P Thoughts?

Picture of Circuit works when I poke a connection, but the connection is sound
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robot7977 years ago
is this made with standard arduino code or special avr code

becaus if it is normal code
i want to rebuild it
gmoon8 years ago
Picking up this discussion at the top (or the formating on this will be weird)..

Instead of the added 4011's to invert the signal, why not just add an invert() function and use it with ShiftOut()?

uint8_t invert(uint8_t foo){	return(foo ^= 255);}//--- stuff ---shiftOut(dataPin, clockPin, invert(outputbyte));

There might be a little initialization overhead also, but no biggie.
greendude (author)  gmoon8 years ago
that will just invert the output byte, to properly interface with the 4094 the clock pulses have to be inverted as well ie, the 4094 would register the input on the downwards clock edge coming out of the avr instead of the leading edge because of the inverted signal, and I'm not sure how the avr handles the data pin once it's set it and sent the clock pulse i could just rewrite the shiftout function but i reached for my 4011 instead :P i just saw it as an easy way to do it
gmoon greendude8 years ago
Suit yourself, but the ShiftOut() function looks pretty easy to modify (I'm sure it wouldn't sync with every external chip without mods, anyway.) The 4011 is a good quickfix, but if you're planning on releasing a schematic or pcb, then simpler is better... (I'm sure you're dealing with the "strobe" line somehow via software, too.)
greendude (author)  gmoon8 years ago
yeah, i think you are right, it's not that hard at all to simply set up a new function to do that role but until i get this working the 4011 is nice to simplify things, and I wont post a proper instructable until I get this whole thing rounded out properly
westfw8 years ago
So, do you have a schematic of the way you think it is set up now? I'm finding it hard to visualize from the descriptions (although gmoon seems to be doing OK!)
greendude (author)  westfw8 years ago
Here you go :D, mind my terrible schematic skills :P The only difference between this schematic and my breadboard is the liberal use of 0.1uF caps on my breadboard, 100uf and 10uf caps between 12V+ and gnd next to the wallwart plug and all the outputs of the 4094s are tied to the vfd through 680Ω resistors
vfd driver.png
westfw8 years ago
Protoboards are somewhat notorious for getting stretched internal bits that result in unreliable connections. Try moving that set of connections to a different column?
greendude (author)  westfw8 years ago
Thanks for the response, I wasn't expecting one so quickly :P I just tried another column and, again, nothing, so i manually connected the green and brown with alligator clips, nothing. Except if i poke the brown wire to alligator clip or the green wire to alligator clip connections with my finger it works :S The Atmega168 runs a program that fills up the parallel out ports of the 4094s with HIGH and then latches it every microsecond, I dunno if thats important, it shouldn't be but, just for full disclosure :P
gmoon greendude8 years ago
A quick search for VFDs returns this project. Is that your inspiration?

So the transistors aren't for switching the VFD current, but rather to convert the 5V logic of the AVR to the 15V or so used by the CMOS chips. The NPN transistors are wired essentially as a common-emitter voltage amplifier.

There's also an on-board voltage step-up inverter, 5 to 15V (the FET, 150uF choke and caps.)

-- If you're using the same type of step-up inverter circuit, have to tried an external 15V regulated supply? Are the capacitance filter values high enough to remove any ripple?

-- Are the transistor specs OK? Substituting here will likely change the resistor values...

-- Are you sourcing or sinking current with the AVR? Is it PWM, or continuous? Have you measured the switched voltage on the transistors?

Your finger is probably adding some stray capacitance which could effect the inverter, the output ports or any other AC that's present.
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