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I Can't See All My Gauges!
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Tue Oct 13, 2009 4:55 pm    Post subject: Reply with quote

RC wrote:
So in other words Scorpio, most of that went way over your head.

Does it make you feel intellectually inferior knowing that a chick could understand?


Ouch!
Ha Ha Ha,
Sharp answer RC

We know ya only playing with us Scorpio

I have no impedance meter but can measure the resistance tomorrow. can't imagine the gauge will take much current to get it moving (call it womens intuition). If it does i will look at using an operational amplifier i guess.

I'm waiting for the MAP sensor to arrive before i can go any further. Got one from a scrap yard with a plug and wires for £6 inc postage.
Also ordered a mixed pack of various value 10 turn trim pots so I'm sure i could easy rig a lower value potential divider or shunt the one i have designed.

I am playing the waiting game at the mo. there is a postal strike here in the UK

As for meter direction. MPG meter reads 0 MPG at +800mV FSD (full scale diff being on the left) and,
Infinite MPG with 0V and no deflection (gauge needle being to the right in its "home" position ).
As for the MAP device, little or no vacuum gives 4.9V (WOT for a N/A engine)
29.6 inches of mercury gives a voltage of 0.3 (i believe it has a linear response)

Will continue to post finding as the project develops


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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Tue Oct 13, 2009 5:07 pm    Post subject: Reply with quote

Cheers RC.

I'm getting more thant slightly annoyed at the postal wildcat strikes, either strike or don't - don't mess with E-bay purchases!!
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1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Tue Oct 13, 2009 6:38 pm    Post subject: Reply with quote

I'll use an LM358 - mostly as I have loads of them... Will this work?
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1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Tue Oct 13, 2009 6:57 pm    Post subject: Reply with quote

hmmm! Low Power Dual Operational Amplifier hay, interesting.
Food for thought

just measured my MPG Gauge. it is 196.2Ω
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Tue Oct 13, 2009 7:09 pm    Post subject: Reply with quote

Was thinking an LM358 will easily take a few mV and crank it up to usable voltages. It is also pretty tough and easy which I like!
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec
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RC  



Joined: 25 Mar 2007
Posts: 2636
Location: Australia

PostPosted: Tue Oct 13, 2009 11:19 pm    Post subject: Reply with quote

9XX Girl! wrote:
Quote:
As for meter direction. MPG meter reads 0 MPG at +800mV FSD (full scale diff being on the left) and,
Infinite MPG with 0V and no deflection (gauge needle being to the right in its "home" position ).
As for the MAP device, little or no vacuum gives 4.9V (WOT for a N/A engine)
29.6 inches of mercury gives a voltage of 0.3 (i believe it has a linear response)

Will continue to post finding as the project develops


Thats cool then, the meter deflection is phased thge same as the MAP output. With the meter at 200 ohms just try a 1 to 1.5K pot in series with the MAP signal. Should only draw around 5 mA and most likely the MAP will cope OK.

It`s an interesting little project that not many here would be prepared to take on.

Rich H wrote:
I'm getting more thant slightly annoyed at the postal wildcat strikes, either strike or don't - don't mess with E-bay purchases!!

Ah crap! Wasn`t aware of that. Have 3 ebay purchases from UK coming ATM that are taking longer than usual. One is surface mail, another I`m hanging for but the other is for mum`s birthday thats less than 2 weeks away.

A couple of LM358s should be fine configured as a balanced input instrument amplifier. Not as accurate as a dedicated IC but ample for this non critical application. Suggest using all 1% or better tolerance resistors though. Should find a basic circuit on some comprehensive data sheets but can dig something up if needed.

Considering the cost of the sensor and the time factor, I think 9XX Girl is on the right track with a cheap auto spec MAP sensor. Think GM also have 2 bar absolute units or at least 2.5 or 3 bar ones that would work for a turbo.

Of course, here, its been posted with the part #.
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Wed Oct 14, 2009 12:09 am    Post subject: Reply with quote

Problem seems to be wildcat strikes - they are all over the place, the official strikes haven't started yet adn will quite possibly kill the royal mail once and for all....

Haven't had a parcel for at least 2 weeks, normal post is getting through though.

Yes it's sad that your job is changing but if you piss off all the punters you have no job at all.
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1994 Lotus Esprit S4 - Work in progress...
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Wed Oct 14, 2009 12:10 am    Post subject: Reply with quote

I'll get a sensor at the weekend and play about see what I can get.
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Wed Oct 14, 2009 5:51 am    Post subject: Reply with quote

RC wrote:
With the meter at 200 ohms just try a 1 to 1.5K pot in series with the MAP signal. Should only draw around 5 mA and most likely the MAP will cope OK.


Ok,
I tried 1.5K pot in series with a regulated 5v supply and the gauge and, got a very rapid FSD.
I then tried 5K pot in series with a regulated 5v supply and the gauge and, got a rapid FSD
So tweaked the pot until i got soft FSD, removed the pot and measured it with an ohm meter. it was 2k3 ohms!

I then removed the 5v regulated supply and injected 1.3v from a tired AA battery hoping to get a little less than half scale deflection. The needle moved very very slowly but eventual made it to FSD.

Not good

I then rigged the 5k trim pot as a potential divider with the MPG gauge (and DVM in Parrnell to it) connected to the wiper and 0V. I then connected +5V across the pot. Got FSD at about 134mV on the DVM but when i removed the MPG gauge, the DVM went up to 344mV.
Therefore the MPG gauge is loading the circuit, causing volt drop, and throwing things a skew.

So just been thinking while eating chicken. Would i not be better shunting the MPG gauge rather than putting a resistor in series which arguably is just limiting current? If i parrnell a trim pot to the MPG gauge and drop 5V across (and then 1.3V) i should have much better control.

Will eat some more, an go have another play.


RC wrote:
It`s an interesting little project that not many here would be prepared to take on.


hmmm.., wonder why?

oh yer, i see you guys twiged there are 1, 2 & 3 bar units
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Wed Oct 14, 2009 7:59 am    Post subject: Reply with quote

anyone know anything about potentiometric or null-balance instrument theory?

I have come to the assumption the MPG Gauge in question (for calculation purposes) has a 1mA meter movement and a coil resistance of 200 ohms.
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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RC  



Joined: 25 Mar 2007
Posts: 2636
Location: Australia

PostPosted: Wed Oct 14, 2009 10:04 am    Post subject: Reply with quote

Rich H wrote:
Haven't had a parcel for at least 2 weeks, normal post is getting through though.

Wat ya mean?
Ebay parcels ARE NORMAL POST, its the bills etc that are junk mail!

9XX Girl! wrote:
Quote:
Therefore the MPG gauge is loading the circuit, causing volt drop, and throwing things a skew.

Yes, exactly.
The MAP unit is intended to feed into the high impedance "load" of an ECU, with extremely little current drawn. No idea if its output is purely analog or more likely PWM which will exaggerate the loading factor.

Quote:
So just been thinking while eating chicken. Would i not be better shunting the MPG gauge rather than putting a resistor in series which arguably is just limiting current? If i parrnell a trim pot to the MPG gauge and drop 5V across (and then 1.3V) i should have much better control.

No, shunting the meter will increase the load current and make things worse.
Clearly now this set up does require a buffer stage between the high imp MAP and low imp meter. The easiest and cheapest option is a basic single op amp voltage follower.

From Wikipedia:

Quote:
A unity gain buffer amplifier may be constructed very simply by connecting the output of an operational amplifier to its inverting input (negative feedback), and connecting a signal source to the non-inverting input. For this circuit, Vout is simply equal to Vin.

The importance of this circuit does not come from any change in voltage, but from the input and output impedances of the op-amp. The input impedance of the op-amp is very high (1 MΩ to 10 TΩ), meaning that the input of the op-amp does not load down the source or draw any current from it. Because the output impedance of the op-amp is very low, it drives the load as if it were a perfect voltage source. Both the connections to and from the buffer are therefore bridging connections, which reduce power consumption in the source, distortion from overloading, crosstalk and other electromagnetic interference.

Simple hey? There`s nothing critical about it. Practically any common op amp will work as long as it is able to run on a single 5V supply and sink an output current of say 20mA. The LM358 mentioned earlier is a suitable cheap choice.

http://www.national.com/ds/LM/LM158.pdf

There`s a circuit of a voltage follower about half way down. Only 5 connections required. Try feeding the output of the 15K/5K pot voltage divider of your original circuit without the cap. That will load the MAP by under 2mA but it may require much less. As input current on this IC is down to the nA range a voltage divider of a mega ohm is reasonable.
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Wed Oct 14, 2009 9:05 pm    Post subject: Reply with quote

I am just messing with the gauge and a PSU at the mo. I think i will wait till the MAP arrives and see how the two behave together.
May order an op amp just in case,.. if they are so cheap,

Thanx for you input RC
_________________
924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Thu Oct 15, 2009 12:22 am    Post subject: Reply with quote

LM358 will cost under 50p I would think.
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Thu Oct 15, 2009 12:24 am    Post subject: Reply with quote

http://cpc.farnell.com/jsp/search/browse.jsp?N=411+500001+1001902&Ntk=gensearch_003&Ntt=358&Ntx=

19p + vat
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Thu Oct 15, 2009 5:34 am    Post subject: Reply with quote

Rich H wrote:
http://cpc.farnell.com/jsp/search/browse.jsp?N=411+500001+1001902&Ntk=gensearch_003&Ntt=358&Ntx=

19p + vat


i went and had a look. Haven't used Farnell or RS for a while

hmm... but I need to place a bigger order to make it worth my while

Goods Subtotal:_________£0.19
Timed Delivery Charge:____£0.00
Handling Charge:________£5.95
VAT: __________________£0.92
Order Total:____________ £7.06

Plus, making the order bigger could slow things down. Letters seem to be getting through ok as they are sorted electronically, it's parcels that are being delayed.

So i splashed out on http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=400074951816&ssPageName=STRK:MEWNX:IT

£ 2.48 for two,

Plus if i miss a parcel when i'm out, i got to go 5miles to the sorting office to collect it. This way i should get it in an envelop in a couple of days.

I got some Vero board, so should be ok to build when it comes. I will read the data sheet and make a diagram in the meantime.
_________________
924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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