measuring oil pressure with a picaxe

craigcurtin

Senior Member
Guys,

I run my car on WVO and am looking for a sensor i can directly interface to the picaxe for measuring the fuel pressure (waste vegetable oil) - this is to allow me to see when the fuel filter is starting to clog and putting too much back pressure on the fuel pump.

The pressure is in the range of 0-15psi

I would like some form of oil safe pressure sensor that can (hopefully) interface directly to the picaxe

regards

Craig
 

Jaguarjoe

Senior Member
If it's oil pressure you want to measure then why not use a plain old oil pressure sender from just about any automobile? If you have access to an auto breaker, you'll get one for almost nothing.
Normally, they are variable resistance units so you'll need a fixed resistor to feed them some current to work with.

An automotive low oil pressure switch might work too. They trip at a few psi.

You could skip all the pressure stuff and just monitor fuel pump motor current.

Why not have two filters (one in use, the other clean and ready to use) and a 3-way solenoid valve that would alternate between filters when one gets plugged? That way you won't run out of filter at an inopportune moment or place.
 
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craigcurtin

Senior Member
Joe, I will check out your suggestions thanks

In terms of a standby filter - the idea was in my head already but i doubt i will have enough room under the car to make it happen

regards

Craig


If it's oil pressure you want to measure then why not use a plain old oil pressure sender from just about any automobile? If you have access to an auto breaker, you'll get one for almost nothing.
Normally, they are variable resistance units so you'll need a fixed resistor to feed them some current to work with.

An automotive low oil pressure switch might work too. They trip at a few psi.

You could skip all the pressure stuff and just monitor fuel pump motor current.

Why not have two filters (one in use, the other clean and ready to use) and a 3-way solenoid valve that would alternate between filters when one gets plugged? That way you won't run out of filter at an inopportune moment or place.
 

defi

New Member
Guys,

I run my car on WVO and am looking for a sensor i can directly interface to the picaxe for measuring the fuel pressure (waste vegetable oil) - this is to allow me to see when the fuel filter is starting to clog and putting too much back pressure on the fuel pump.

The pressure is in the range of 0-15psi

I would like some form of oil safe pressure sensor that can (hopefully) interface directly to the picaxe

regards

Craig
You could use two oil pressure senders (gauge type) One on the inlet one on the oultlet, if you start with a new filter you would have the clean filter pressure drop, when the pressure drop increases its time to change the filter.
I think some of the picaxes have adc comparators
 

westaust55

Moderator
Not knowing your location makes giving a recommendation difficult.

OceanContols in Victoria (Aust) have a range of pressure sensors
 

craigcurtin

Senior Member
Sorry Westaus

I have updated my profile -location is Sydney

I have just now checked out the ocean controls stuff - as i will need 4 of these they are a little out of my price range !! $200 each approx !!

regards

Craig
 
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craigcurtin

Senior Member
Thanks - moniotring current will not work

The fuel/oil pumps that i intend to use have a fixed psi limit and cycle on demand - so there is no reasonable way to measure their current and extrapolate from the

I will check out Oil pressure sensors from cars and see what i can find

Craig

If it's oil pressure you want to measure then why not use a plain old oil pressure sender from just about any automobile? If you have access to an auto breaker, you'll get one for almost nothing.
Normally, they are variable resistance units so you'll need a fixed resistor to feed them some current to work with.

An automotive low oil pressure switch might work too. They trip at a few psi.

You could skip all the pressure stuff and just monitor fuel pump motor current.

Why not have two filters (one in use, the other clean and ready to use) and a 3-way solenoid valve that would alternate between filters when one gets plugged? That way you won't run out of filter at an inopportune moment or place.
 

craigcurtin

Senior Member
Would something like these standard car ones work

Guys,

Would something like this interface with the Picaxe OK ? I assume i would drive the sender at 12v but it would just return a varialbe resistance on its measurement leg - or would i need to put a voltage divider in to reduce this down to the picaxe safe 5v range ?

http://cgi.ebay.com/ebaymotors/New-Oil-pressure-sender-sending-unit-100-psi-_W0QQcmdZViewItemQQhashZitem3a619d7384QQitemZ250745811844QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories#ht_500wt_956

As i will have two fuel lines and two fuel filters i intend to have 4 of these one before and after each filter. and switch between them based on the selected fuel input

regards

Craig
 

MartinM57

Moderator
Direct feed to the gauge is incorrect. You will need:
12v---->resistor-->G pin (sensor) -pin--->Gnd
...and take the ADC feed from the junction of your resistor and the G pin. Also put a 22K resistor (a la download circuit) between the take off point and the ADC pin to provide protection to the ADC input.

Note the resistance range of the sensor (240R - 33R) to work out a suitable value for your resistor so that you do not exceed 5v at the ADC input and you still have a useful ADC range (exercise for you!)...and work out the max current flowing through it and hence the power being dissipated so that you can get the right power rating. Have a go - it's only Ohms Law stuff - and report back what you think the answers should be if you're not sure/want an independent check.

Also remember that ultimate accuracy will depend on the accuracy of the 12v - unlikely to be accurate to be honest - which is why cars have a, say 10v, regulator feeding instrumentation like this (LM317T would just about do the job, but there are better lower drop out regulators)
 
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RexLan

Senior Member
I would use a 0-30 Psi sender and feed it with a regualted 5 volts through a 100 ohm resistor. Use the ADC input to monitor the pressure and at pressure XX you can make the decision to do something. Rather simple circuit. The senders are reasonably linear and easy to purchase. VDO senders are very common.

http://egauges.com/vdo_send.asp?Sender=30PSI

You can also use a pressure tank and a very good digital guage to build a lookup table. Attach the sender to the tank and your circuit. Have the Picaxe send the data to the PC and slowly change the pressure in the tank 1 PSI at a time ... get the reading and continue to build your table. Quite accurate result and a relatively small table.
 

craigcurtin

Senior Member
Repeatability rather than a definitive number

The overall accuracy is not that important in terms of PSI etc - what i am looking for is a number that starts at (say 10) when a brand new filter is put in on the push pump side and is (say) 5 on the vacuum side of the pump. When the filter is getting towards needing to be changed the figures may be 20 and 2 (there is an integral pump in the injection pump in diesels so the vacuum side of the filter will always go downwards)


I will look at the VDO gauges.

The output from my alternator in the car is running at just over 14v now. If however that is not enough to drive the circuit then i was going to use a stepup transformer to provide another level of protection to the system based on how much people are saying to protect yourself from spikes in the car etc.

Craig
 

JoeFromOzarks

Senior Member
I’m a daily lurker, but I felt some kind of urge to jump in here.

Mouser Part Number: 580-78SRH-5/2-C US$13.95ea
Manufacturer Part #: 78SRH-5/2-C
Manufacturer: Murata Power Solutions
Description: DC/DC Converters & Regulators 8-32Vin 5Vout 2A HSIP Switching Reg.

http://www.mouser.com/ProductDetail/Murata-Power-Solutions/78SRH-5-2-C/?qs=sGAEpiMZZMukgiigmf73gBQy9zXQtmCp

What's nice about these little critters, they fit in the TO-220 780x series pinout and they DO NOT require REGULATED DC power supplies!

Under certain circumstances, the datasheet recommends input caps and sometimes output caps, but since my 7805 circuit would’ve used ‘em anyway… so do I. Use 50V caps on the input, a few pennies more.

$14 might seem a wee bit on the expensive side, but with a 7.5-36V input range giving 5VDC WITHOUT a heat sink and with the exclusion of not requiring a regulated input, how can one go wrong? I’ve used, maybe fifteen of ‘em in various projects, not one has let me down. One runs an mp3 player/radio on a lawnmower, another runs the LED dash lights and passageway illumination on my boat, soon one will power the nav lights.

$14 is pretty cheap insurance to make sure you have a reliable and consistent power for an oil pressure circuit requiring a “reliably predictable” power supply and ADC reference voltage. Some things you can be cheap with, like a set of cheap silverware for a wedding gift (sic) or cheap pizza for a boozed-up football party, – but a power supply is NOT one of ‘em. Do it correctly, do it once.

:) joe
 
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MartinM57

Moderator
The output from my alternator in the car is running at just over 14v now. If however that is not enough to drive the circuit then i was going to use a stepup transformer to provide another level of protection to the system based on how much people are saying to protect yourself from spikes in the car etc.
Well over the top IMHO...YOU choose the voltage feeding the R and sensor so even 5V would do fine (subject to the calcs being OK - I'm sure they are). Then you can use something like an LM2575 switching regulator (look in the datasheet for the circuit and stick to it exactly) to power all 4 sensors - a bog standard 7805 linear regulator might get a bit hot in this scenario. A few inductors and capacitors appropriately placed will see to any spikes you might encounter.

Joe's module are OK if you're happy to pay the price and enjoy the simplicity - they look no more that a SMD switching regulator on a PCB, so nothing you can't do yourself.
 

Dippy

Moderator
Unless I have missed something I agree with Martin (and I'm not so humble).

The pressure sensor linked to is just a variable resistance isn't it? So why all this switched mode PSU ??
This looks like the usual molehil into mountain PICAXE Forum scenario :rolleyes:

Yes, driving that with a standard regulator might warm the cockles up, so why not supply power for half a second and sample? A 10% duty switch would keep things cool-ish and reduce power consumption.

I would always RC filter (low pass) from an unknown quality sensor.
How you physically arrange it is up to you.

And don't get too excited about all this auto power supply stuff.
Generally: Fuse , transient suppression/absorber , filter , reverse-polarity , decouple ... do it properly and with the correct components and job done. A design is based around knowledge, testing and construction - not some kind of Magic Wand ;)
 

hippy

Senior Member
The pressure sensor linked to is just a variable resistance isn't it? So why all this switched mode PSU ??
This looks like the usual molehil into mountain PICAXE Forum scenario :rolleyes:
I have to agree with Dippy, this looks like a standard 'measure a varying resistance' problem which probably has no specific voltage or supply requirements.

If it is a passive variable resistance device rather than an active circuit, wire it up as a potential divider with the sensor between READADC pin and 0V and job done. You can use a 'floating earth' sensor and PICAXE input pin and PICAXE 0V or a 'chassis earth' sensor and connect PICAXE 0V to chassis.

A 'floating earth' sensor could go between PICAXE pin and PICAXE +V, vehicle battery, even a regulated supply, but why do that when it isn't needed, why add the complexity of using a voltage beyond what the PICAXE accepts only to have to bring it back into range ?

As this is really a 'filter clogged' detector you're not particularly interested in absolute pressure values which makes it even easier, gives you greater flexibility to use mis-matched sensors. I'd get down to a car breakers and see what they've got, bung it on breadboard and see how it goes.
 

craigcurtin

Senior Member
Sounds good thanks Guys

I will get some of the sensors and see if i can get any sense out of them and report back

Thanks for all the input

Craig
 
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