Raspberry pi interfacing with older gear

[Andy Tefft]

Anyone been using a Raspberry pi in any nixie projects?

Many years ago I worked on a clock that basically used a frequency counter as a display. Basically it output 1 pps and the counter just counted them to mark off the seconds (with some smarts to handle the rollovers from 50 to 0 seconds and so on). This used an AVR microcontroller, with a bunch of external components - a switch and rotary encoder for setting the time, some power supply related stuff and so on. There would have been additional external components needed to support more stuff I wanted to add, like a battery backed RTC.

While I had it working nicely on the breadboard, I never quite got it right on my protoboard version and soon ran out of free time to work on it.  This year my son wanted a raspberry pi for Christmas and I ended up getting myself one too - figuring I could revisit the counter clock.

All in all the hardware itself is about what I paid in total for the parts I'd used before, and it seems like nearly everything I need is in the pi itself (yes, it is way overkill for a clock of this nature!). I'm thinking I will even connect the pi to my wifi and use ntp as the time source which does away with all the hardware and software needed to set the time. Theoretically I just need a power supply (my counter has +5V available since it's all TTL, though I'm not sure I would use it directly), a connection to the counter's input, and a connection to the counter's reset signal.

Initial quick tests are good with one exception - I had just connected an AVR output to one of the pins on the reset button on the counter and the AVR was able to toggle that output and reset the counter (pretty sure I was powering the AVR with 5V). This same trick does not work with the pi, presumably due to its puny little 3.3v-level outputs. Anyone have a standard, simple go-to for interfacing between old TTL circuitry and a pi or something like it?

[Dylan Distasio]

Some sort of logic level converter like this should work:

https://www.sparkfun.com/products/12009

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[David Forbes]

Andy,

You can use an NPN transistor (2N3904 or 2N222A or whatever) and a
couple resistors to boost the Pi signal to 5V. Pi output through a 10K
resistor to Base, Emitter to Gnd, counter fed from Collector, Collector
also tied to 5V through a 1K resistor. Should work up to about 1 MHz.

On 12/30/15 2:54 PM, Andy Tefft wrote:
>
> Initial quick tests are good with one exception - I had just connected an
> AVR output to one of the pins on the reset button on the counter and the
> AVR was able to toggle that output and reset the counter (pretty sure I was
> powering the AVR with 5V). This same trick does not work with the pi,
> presumably due to its puny little 3.3v-level outputs. Anyone have a
> standard, simple go-to for interfacing between old TTL circuitry and a pi
> or something like it?
>

--
David Forbes, Tucson AZ

[Andy Tefft]

Thanks for the quick feedback, guys.

I'll probably breadboard up a transistor for now, and a little converter like that can permanently go inline in my connecting cable even though I only need a single channel. It would be cool if someone made a breakout board for the pi that had an optional level converter on every (or even several) gpio output, that would be really useful for prototyping.

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[Dylan Distasio]

I was surfing on Tindie tonight, and came across this which I think is what you were asking for:

https://www.tindie.com/products/land_boards/rpio-tiny-2/

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[Quixotic Nixotic]

On 30 Dec 2015, at 21:54, Andy Tefft wrote:

Anyone been using a Raspberry pi in any nixie projects?

Did anyone manage to get one of the Pi Zeros that were free on the cover of Magpi magazine? I tried, but they'd all sold out instantly where I live.

Crazy cheap to buy anyway at £4 or $5 http://swag.raspberrypi.org/collections/pi-zero/products/pi-zero

John S

[Dylan Distasio]

I didn't get one free, but I picked a few up.  I haven't had a chance to play with it yet, though.

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[Andy Tefft]

Yes! that manufacturer has a few variants. Maybe I'll be a guinea pig to see how well they work. Thanks for the pointer.

Got my 2n222 version wired up and working fine. I decided I would like to have the pi be able to read the +5V from the counter (so it can tell whether it's on or not) so I'll probably try out one of those boards for prototyping next but use one of the little multi-channel converters for the permanent hookup.

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