Update of my Nixie tube car gauges project

[Thomas Kummer]

My schematic works, and I am currently designing a circuit board that will fit into a standard 2in/52mm gauge housing. I'm thinking about using the IN-17 as the back of the tube has a rubber spacer that could potentially act as a shock absorber for the vibrating nature of cars. My first gauge will be a voltmeter for the battery. My buddy is writing the code. Right now the code is only able to identify voltage or no voltage, (indicated by a 3 or a 2) but the project is progressing, a bit slower than I would like, but progressing none the less.

https://drive.google.com/open?id=1cEutkdOmCQ9_nVmBiTlx892b_CyOe850

[gregebert]

Are you using any discrete A/D converters, or are you using one already integrated into a microcontroller ?

I found a lot of quirks with the 16-channel A/D on my current project, so ping me if you want to learn from my war stories.

[Thomas Kummer]

I’m using one already integrated into a micro controller, an Arduino Nano with a volt sensor that can detect 0V-25V. 25V is the max voltage any more and the resistors may fry. Im hoping it won’t be a problem as most car batteries don’t go above 16V, but the amps worry me a bit. 

Sent from my iPhone

On Feb 11, 2019, at 13:03, gregebert <greg...@hotmail.com> wrote:

Are you using any discrete A/D converters, or are you using one already integrated into a microcontroller ?

I found a lot of quirks with the 16-channel A/D on my current project, so ping me if you want to learn from my war stories.

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[gregebert]

Be extra careful about ESD/surge events; the automotive electrical environment is quite nasty, especially in older vehicles that have mechanical voltage regulators.

Depending upon where you tap the power & ground, your circuit can be affected by large di/dt events, such as starting, charging, A/C compressor cycling.

A volt-sensor is probably just a resistor-divider to shift the voltage down to a level that can be safely read from a GPIO pin; probably not a full-on A/D converter.

If the resistor values are in the 100K range, or higher, you wont have to worry about ESD or reverse polarity.

[Thomas Kummer]

The voltage sensor is just a resistor divider, I just measured the resistance of it to 37.5K. I may add the diodes or, I may look for a voltage divider with a higher resistance. As for the micro controller and the HV power supply those will be ran off of either the 12V cigarette lighter or a the 12V supply that is connected to the computer or the lights, so that the gauges come on when the car does. I’m not too worried about their power supplies as I tested a nixie clock on the 12V cigarette lighter and it was fine even when starting the car 4-5 times. My theory was if you can change a cell phone on it with no problems I don’t see why you couldn’t run a 5V and HV power supply on it. I believe I was correct as it worked. I may try to run the voltage sensor off the 12V cigarette lighter, and see if I get a reading of 13V or 12V, if it’s 12V then I know that it’s not the true battery voltage. If it’s 13-14V then I know that it is the true battery voltage, or close enough at least. 

Sent from my iPhone

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[Dekatron42]

Don't forget that you don't want to disturb the car electronics so that you crash your car and run someone over or hurt yourself - that's one reason why car electronics is tested against standards.

There are a lot of good materials to be found on the Internet that chip manufacturers like Bosch, NXP, TI and other companies have uploaded for free to read for anyone interested in their products, there you can learn a lot about how to properly protect measurement inputs like on A/D-converter inputs and also how to properly design your circuit so that it doesn't disturb the rest of the car electronics.

Properly protecting an A/D-input is not just to protect it so that you don't damage it but also (or should I say mostly) so that the input doesn't disturb the rest of the die (chip) so that you get correct measurements and also so that the chip works as designed - most A/D-inputs can only tolerate a few hundred milli-volts above and below the supply voltage and ground signal (usually some +/-0.3V) before the chip is affected. Google for instance "A/D-input protection" or "A/D injection current" and you'll find a lot of articles and application notes on the subject.

/Martin