On board air - Dashboard pressure display

[mickeyw]

Something for the collective of electronics genii on here I hope.

I am installing an on-board air system on my 110, and I want a dashboard pressure display.

I do not wish to use a bulky 52mm gauge, as I'm trying to keep the dash reasonably simple and uncluttered. I a not fan of dash consoles from the likes of Raptor et al, which is why I am aiming for a fairly discreet solution.

I had an idea to use a very small 3 way 7-segment display that I could build into the standard instrument binnacle. This is the kind of information that will only need occasional viewing, so doesn't need high visibility.

Now my geek level is very low, so such a plan will need a fair bit of help to design. What I am good at is soldering small components onto PCBs and Veroboard, so at least assembling a circuit shouldn't be a problem.

To lay down a few working figures, I shall be to looking to show values of 0-110 PSI.

The workings of this need to be as compact as possible, and it may be useful to have the display divorced from the main board.

I have found a pressure transducer that has a 1-1.2 MPa (0-168 PSI) working range, and outputs 0.5 to 4.5 VDC, so a 4V total indicating range.

My understanding of the steps required are as follows:

Take output voltage from the transducer, and convert to PSI.

4 volts split over 168 PSI = 0.0238V/PSI, plus the 0.5V initial voltage

Send the output to the display via a 7-segment driver. No idea how to do this

Now I realise this is pretty a basic interpretation of the operation, but it is about as far as my knowledge of electronics goes.

Over to you guys for comments please. What components/devices/methods would be appropriate for this purpose?

TIA

[Dave W]

It's a fair simple job using a PIC Microcontroller. The biggest problem you'll find is choosing which micro controller !

You need one with an analog to digital convertor built in and this will convert the input, typically into 0-255. The display can then be driven from the same micro controller either directly or through a driver. I normally use backlit LCD character displays for this as you can get them as a complete unit that has a built in driver and you can also vary the backlight so when you switch the lights on the display dims.

With a typical PIC and self contained display you could just about manage the basics without any additional components apart from a voltage regulator (which you'll need for that transducer anyway). Most of them will run at 5V so won't need any special circuitry to take the output from the pressure transducer.

You could also use the circuit to switch the compressor in and out via a relay with the addition of a few extra components.

If you're wanting something where most of the hard work is done for you, you could also look at using Arduino components to do the job (and a whole lot more !). I prefer to make my own PIC controllers but the Arduino is a good alternative. If you wanted to go down the PIC route then Microchip sell evaluation kits that give you a good basis with a circuit board that's ready to go, you just need to adapt it to suit your own needs.

[Dave W]

Just looking again at the transducer, the lower pressure version...
http://www.ebay.co.uk/itm/5V-DC-G1-4-Pressure-Transducer-Sensor-0-0-8MPa-for-Oil-Fuel-Diesel-Gas-Air/191684546917?rt=nc&_soffid=5007939209&_soffType=OrderSubTotalOffer&_trksid=p5731.m3795

Will give you better resolution.

HOWEVER, bear in mind that both those have a working temp range of 0-85 degrees C, the -20-85 version might be better for automotive use although those seem to operate at ridiculous pressures so maybe better to bet that the 0-85 won't stop working permanently outside that temp range.

[Team Idris]

With a masters in Mechatronics I hate to say that a few pressure switches at different settings might be better.

I'd put a 52mm gauge in the roof so you see it in the rear view mirror though.

Or go the full Rasberry and make it do other stuff

[Cynic-al]

I just went for a small pressure gauge on the a pillar with a 4mm air line from the tank because it's infinitely easier and more reliable than conversion to electronics.

If you don't want that do you need an accurate pressure or just full and empty? An led bar graph would be easier than a digital display.

Or could you buy something like a digital tyre pressure gauge for a couple of quid and pull it to bits? They're only a couple of quid.

[Boydie]

I have to ask why the need for a gauge of any sort?

I have a twin piston ARB compressor mounted under my seat. It is connected to an air reservoir (an old truck air brake tank) 170 dia x 350 mm long and mounted on the chassis rail directly below the pump, it's well clear of any rocks etc and hasn't been struck in over 280,000 kilometers. The pump is set at 110 psi, standard pressure and trust me I can easily hear it, if, due to a micro air leak, it ever comes on! It fair causes a man to jump from his best driving position and has been known to be followed by a pungent aroma from my under regions! The on/off switch and air hose plug are mounted on the plastic side cover to the seat.

The reservoir tank enables me to either inflate my Zodiac boat or raise the tyre pressures from 12 psi to 35 psi if I've ever been doing some soft sand driving and the pump only comes on at the fourth tyre.

[mickeyw]

It's a fair simple job using a PIC Microcontroller. The biggest problem you'll find is choosing which micro controller !

You need one with an analog to digital convertor built in and this will convert the input, typically into 0-255. The display can then be driven from the same micro controller either directly or through a driver. I normally use backlit LCD character displays for this as you can get them as a complete unit that has a built in driver and you can also vary the backlight so when you switch the lights on the display dims.

With a typical PIC and self contained display you could just about manage the basics without any additional components apart from a voltage regulator (which you'll need for that transducer anyway). Most of them will run at 5V so won't need any special circuitry to take the output from the pressure transducer.

You could also use the circuit to switch the compressor in and out via a relay with the addition of a few extra components.

If you're wanting something where most of the hard work is done for you, you could also look at using Arduino components to do the job (and a whole lot more !). I prefer to make my own PIC controllers but the Arduino is a good alternative. If you wanted to go down the PIC route then Microchip sell evaluation kits that give you a good basis with a circuit board that's ready to go, you just need to adapt it to suit your own needs.

Dave, this is the kind of thing I had in mind, but I don't know enough yet to actually implement such a plan. I had wondered if someone would suggest Arduino, but I wasn't sure if it was overkill or not. I know there would be quite a few outputs require to run a 3 digit display The Nano version appears to be pretty compact.

I agree a back-lit LCD would probably look tidier, and it would look consistent with my Td5 speedo. I'd better go and read up on them and see what is available. I fear they may work out a little large though...

I just went for a small pressure gauge on the a pillar with a 4mm air line from the tank because it's infinitely easier and more reliable than conversion to electronics.

If you don't want that do you need an accurate pressure or just full and empty? An led bar graph would be easier than a digital display.

Or could you buy something like a digital tyre pressure gauge for a couple of quid and pull it to bits? They're only a couple of quid.

Al, I started off looking for very small gauges, around 25mm dia, but I haven't found anything less than 40mm. The LED bar graph is an interesting idea, but I like the idea of a digital tyre gauge more. They seem cheap enough to hack about too, hmmm....

With a masters in Mechatronics I hate to say that a few pressure switches at different settings might be better.

I'd put a 52mm gauge in the roof so you see it in the rear view mirror though.

Or go the full Rasberry and make it do other stuff

Could you explain in more detail? How would using multiple switches achieve what I'm after?

I have to ask why the need for a gauge of any sort?

I have a twin piston ARB compressor mounted under my seat. It is connected to an air reservoir (an old truck air brake tank) 170 dia x 350 mm long and mounted on the chassis rail directly below the pump, it's well clear of any rocks etc and hasn't been struck in over 280,000 kilometers. The pump is set at 110 psi, standard pressure and trust me I can easily hear it, if, due to a micro air leak, it ever comes on! It fair causes a man to jump from his best driving position and has been known to be followed by a pungent aroma from my under regions! The on/off switch and air hose plug are mounted on the plastic side cover to the seat.

The reservoir tank enables me to either inflate my Zodiac boat or raise the tyre pressures from 12 psi to 35 psi if I've ever been doing some soft sand driving and the pump only comes on at the fourth tyre.

Iain, I run a Denso A/C pump as my compressor on a serpentine V8, same as I did on my last truck. You don't hear it at all, unless the engine is at tickover, then you notice the revs drop a little. I have used a T-Max compressor before, which is why I changed over to the A/C pump method. No soiled seats in my truck ya know

I use the air for a set of horns that get the attention of BMW drivers after they've cut me up (drivers of other car marques do this too, but it's mainly BMWs), inflating tyres, air freespool operation, and general blow line duties. The main reason for having a dash pressure display is to show me that air is present to operate these systems.

I too have a receiver mounted underneath, and I'm using a workshop compressor style switch to operate the compressor clutch. This provides adjustable shut off pressure, as well as adjustable hysteresis. They are rated for quite a few amps at 240V, so have no problem switching a mere 12V for the clutch.

All the above kit is tucked away out of sight, hence my wish to have a pressure indicator in the cab - but like I said, I'd like it compact as discreet.

[simonr]

Something like this?

Edit: The resistor values for BAR & PSI are the wrong way round! Doh!

It uses the sensor above:

http://www.ebay.co.uk/itm/5V-DC-G1-4-Pressure-Transducer-Sensor-0-0-8MPa-for-Oil-Fuel-Diesel-Gas-Air/191684546917?rt=nc&_soffid=5007939209&_soffType=OrderSubTotalOffer&_trksid=p5731.m3795

And a meter from eBay:

http://www.ebay.co.uk/itm/DC-0-200uA-3-1-2-Digital-Red-LED-Panel-Ammeter-Ampere-Meter-/121755159398?hash=item1c592c4b66

Si

[Dave W]

Dave, this is the kind of thing I had in mind, but I don't know enough yet to actually implement such a plan. I had wondered if someone would suggest Arduino, but I wasn't sure if it was overkill or not. I know there would be quite a few outputs require to run a 3 digit display The Nano version appears to be pretty compact.

I agree a back-lit LCD would probably look tidier, and it would look consistent with my Td5 speedo. I'd better go and read up on them and see what is available. I fear they may work out a little large though...

You can get a simple character matrix LCD display with an on board driver, they're really simple to drive as you just set the "cursor" to the start and then send each character code you want to display one after another. Timing is the main challenge.

You can also get 7 segment drivers that use the same communications so once you have it running you can swap displays fairly simply if you want to. The problem with the 7 segment LCD and LED displays is that you need to make your own PCB for the driver and display(s). The alphanumeric displays though generally have the driver IC on board.

A good starting point for going the PIC route would be to get a PIC 16F886 evaluation board...

http://uk.farnell.com/microchip/dm164130-3/pickit-lin-28-pin-demo-board/dp/1798097?ost=DM164130&categoryId=700000005161

It's a bit overkill for what you need but you can use it for dev or use one of the blank PCBs for the final project if you don't want to make your own PCB. The header pins on the PCB have all the connections you need fro the display so you can just use some ribbon cable between the two. With the flexibility of this package you can potentially have it monitor other things too and rotate the display between them. I have one of these running a fuel and battery monitor, showing battery voltage on two batteries, showing fuel level on two different tanks and controlling a solenoid to link the two batteries together when the main battery voltage comes up over 13 .5 volts. It also drives the analog fuel gauge and low fuel warning light based on the currently selected fuel tank.

You just hook that up to a simple LCD display like this one...

http://uk.farnell.com/midas/mc10811a6w-fptlw/lcd-8x1-fstn-black-on-white/dp/2218949

The Microchip PICKit 3 programmer will allow you to debug and programme the PIC in situ, that and the free Microchip development environment (MPLab) work well on Mac and PC and give you a free C compiler with in circuit step by step debugging.

I have some working code that you can have a copy of to drive the LCD if you want to go down this route.

The biggest issue I normally have is making a surround for the display, a 3D printer would be great for that if you have access to one. You can buy bezels though which finish off a panel mounted unit.

[HoSS]

Why not just buy a gauge. I use some from Keyence AP30 series. They are smaller than 52mm gauges and have settable alarms and outputs (you could switch the compressor off at pressure etc) You can find them on ebay for as little as 20 quid.

Look in my build threas here, halfway down. I have one for OBA and one for Vac.

http://www.terrangbil.net/forum/index.php?/topic/11062-my-restoration-and-conversion-to-overland-camper/

ebay search

http://www.ebay.co.uk/sch/i.html?_odkw=keyence+ap31&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xkeyence+ap30.TRS0&_nkw=keyence+ap30&_sacat=0

[Simon119]

something like this small enough??

http://uk.rs-online.com/web/p/analogue-positive-pressure-gauges/4359615/

would just require the right adapter to whatever size push fit connector you need

[mickeyw]

LR Pressure.jpg

Something like this?

Edit: The resistor values for BAR & PSI are the wrong way round! Doh!

It uses the sensor above:

http://www.ebay.co.uk/itm/5V-DC-G1-4-Pressure-Transducer-Sensor-0-0-8MPa-for-Oil-Fuel-Diesel-Gas-Air/191684546917?rt=nc&_soffid=5007939209&_soffType=OrderSubTotalOffer&_trksid=p5731.m3795

And a meter from eBay:

http://www.ebay.co.uk/itm/DC-0-200uA-3-1-2-Digital-Red-LED-Panel-Ammeter-Ampere-Meter-/121755159398?hash=item1c592c4b66

Si

I knew I was over thinking this.

We are so lucky to have Si among us to keep it simple and economic. Big thanks Si.

something like this small enough??

http://uk.rs-online.com/web/p/analogue-positive-pressure-gauges/4359615/

would just require the right adapter to whatever size push fit connector you need

Simon, that is very much small enough, thank you. I see there is a 27mm version too.

Note to self - must give my Googling skills a tune up.

You can get a simple character matrix LCD display with an on board driver, they're really simple to drive as you just set the "cursor" to the start and then send each character code you want to display one after another. Timing is the main challenge.

You can also get 7 segment drivers that use the same communications so once you have it running you can swap displays fairly simply if you want to. The problem with the 7 segment LCD and LED displays is that you need to make your own PCB for the driver and display(s). The alphanumeric displays though generally have the driver IC on board.

A good starting point for going the PIC route would be to get a PIC 16F886 evaluation board...

http://uk.farnell.com/microchip/dm164130-3/pickit-lin-28-pin-demo-board/dp/1798097?ost=DM164130&categoryId=700000005161

It's a bit overkill for what you need but you can use it for dev or use one of the blank PCBs for the final project if you don't want to make your own PCB. The header pins on the PCB have all the connections you need fro the display so you can just use some ribbon cable between the two. With the flexibility of this package you can potentially have it monitor other things too and rotate the display between them. I have one of these running a fuel and battery monitor, showing battery voltage on two batteries, showing fuel level on two different tanks and controlling a solenoid to link the two batteries together when the main battery voltage comes up over 13 .5 volts. It also drives the analog fuel gauge and low fuel warning light based on the currently selected fuel tank.

You just hook that up to a simple LCD display like this one...

http://uk.farnell.com/midas/mc10811a6w-fptlw/lcd-8x1-fstn-black-on-white/dp/2218949

The Microchip PICKit 3 programmer will allow you to debug and programme the PIC in situ, that and the free Microchip development environment (MPLab) work well on Mac and PC and give you a free C compiler with in circuit step by step debugging.

I have some working code that you can have a copy of to drive the LCD if you want to go down this route.

The biggest issue I normally have is making a surround for the display, a 3D printer would be great for that if you have access to one. You can buy bezels though which finish off a panel mounted unit.

Dave, thanks ever so much for all this information. This is the kind of project that I would find immensely satisfying to make.

However time and simplicity are key ruling elements in my life right now, so I think I shall keep this idea of the shelf for future enjoyment.

Thank you everyone for your excellent contributions. I shall endeavour to update this thread with the final outcome, you may need to be patient though.

[Boydie]

One alternative and very simple solution would be, if your electric clutch on the A/C - Air Pump is switched by a pressure switch - automatically - wouldn't a simple indicator light on the dash - supplied from the electric clutch circuit - be enough to tell you that the pump is running, ergo your system is below pressure and when it's not illuminated your air tank is full. Me, I go for simple things - Oh, with the exception of the woman I live with after all, it is she who does the laundry after my pump comes on just when I'm not expecting it!

[Cynic-al]

I have a 52mm pressure gauge on the a-pillar and the switch on the dash illuminates when the pump is on. The reason I like both is if someone is using your kit to put a tyre on the bead you can watch the rate the pressure gauge is moving and if they're draining your tank quicker than your pump is putting the air in you can rev the engine a bit to help them out or drop off a bit so you don't hit max pressure where the pump would trip out and not cut back in until the pressure has dropped off. It's also a useful indicator of leaks in the system, ie if you turn something on and notice it going down a bit quicker it can help highlight what you've turned on to get the leak. If you just notice the pump cutting in more often and your using several things it's harder to pin point which is leaking. The light on the switch is useful for 'it should be pumping' as you can't hear the a/c compressor over the noise of my engine and fuel pump. It's such an easy thing to run a 4mm airline to the dash and fit a cheap gauge I can't see an argument not to do it.

[Team Idris]

For the switch idea you have a pressure switch set at full pressure and another 10psi below that, (both green led), and yellow set below that and a red set at 60psi. I can't see the pressure bands on the eBay ones, but they are less that six quid each.

[Team Idris]

I guess the main argument for a computer one is rate of change. For low use you might have a 'dead band' on the controller of 10-15psi. If it has a sudden drop due to high air use it would be better to cut the compressor relay in sooner. If you monitor the pressure accurately you can write a PID routine into the computer based on pressure-drop vs time.

[jai_landrover]

Maybe a cheap and very cheerful solution?

Why not rip apart a cheap tyre digi guage. I suspect they will fail ale ventilate not being full duty cycle items but worth a play. I suspect the hardest part would be fitting a suitable line to the actual fitting. Or I guess you could just extend the wires so that the pressure side of it stays engine side and you only have the display or electronics dash side. I guess you could extend the sender and pressure to bulkhead. But for £5 and some time may be worth a shout. Failing that a small guage like you get in small tyre pumps mounted in your own housing. I quite like the digital look myself

[mickeyw]

Well I've had a few more thoughts on this.

Our microwave died recently, so I felt compelled to see what useful components it might yield.

The harvest included a small-ish 7-segment display.

I had a read up on the Arduino and how it could drive such a display with enough characters

Next idea involved a very cheap digital tyre pressure gauge from the bay.

Having dissected this device I see beautiful simplicity. No doubt there is much cleverness hidden in there too.

One very small barometric sensor, a very small LCD, a power source, the PCB with a simple tactile switch, and a clever bit to convert the pressure signal to an LCD output.

This all looks very usable. That tiny LCD could be packaged to be very discreet on the instrument binnacle. The wires can be extended to the engine bay, where the sensor can be hooked up to the air hoses.

I have discovered a couple of hurdles though.

The unit auto powers off after 30 seconds or so, which is fine for battery power - not so much for an always on display. Easy I thought, just bridge the switch contacts. Not so simple it turns out, as that is the same as the press and hold required to switch the unit off!

So it seems that the chip under the black blob on the PCB has a clever program on board. I wonder if that can be read and modified.........

Now waiting for that Cwazy Wabbit to tell me that's an easy job

[elbekko]

A simple timer (like a 555 or so?) that closes the contact every once in a while?

[Cynic-al]

Why not run a 4mm nylon airline to the dash then you don't have to extend the wires which might (depending how the thing works) alter the reading. It also keeps the sensor out of harms way of water and heat.

Do you need it on all the time? Could you not have the button you press when you want to read the pressure? Afterall modern cars have information like this on a common screen that you have to scroll through, it isn't always visible.

[CwazyWabbit]

There is a reasonable chance that it isn't reprogrammable, I've been away for training with work so I'll have a think when I'm back.