Gauge control

[NITROPIXIE]

Is it possible to control a gauge e.g. a smiths temperature gauge using PWM?????

[brentp]

gut the gauge and put a hobby servo motor behind the needle.

[Funky Diver]

Is it possible to control a gauge e.g. a smiths temperature gauge using PWM?????

Yes, but you'd need some form of microcontroller and a stepper motor as brent has identified

[brentp]

However, if the gauge can handle a basic 12V square wave (i.e. not require the HV kickback pulse) you can just feed it a signal with varying frequency. Should work fine

(silly me for over-engineering the solution)

[Funky Diver]

Came across this on my t'interweb meanderings...

Smiths gauges made after 1964 have two terminals. The gauge case must be earthed only if you want to illuminate it.

The basic temp gauge connections are:
green/blue wire from the sending unit to EITHER of the gauge's spade lugs. (it doesn't matter which one)
light green wire from the second gauge spade lug to a 10V supply (not 12V) (more on that below)
red (or red/white) wire from the gauge lamp to the lamp wiring in the dash.
gauge case = earth (for the lamp). Usually a ring terminal is crimped to a black wire and the ring terminal goes over the threaded stud used to mount the gauge. The other end of the black earth wire is connected to any good earthing spot behind the dash.

The gauges are powered by 10V, not 12V. They are powered by the voltage stabilizer. This is a small rectangular metal canister typically with a mounting ear on it. On center binnacle cars it is attached to the top, back side of the speedometer. On later cars, I believe it is mounted on the back of the gauge cluster. Others will have to tell you how to tap into that 10V source if you want to use what's already there.

There are modern replacements available for the voltage stabilizer and you could also buy one of those, wire it into the dash, and use it to provide the 10V to your gauge.

Source : http://www.theminiforum.co.uk/forums/in ... &p=1719271

I would imagine you have the said rectifer on your car, but obviously if needed you could do it equally well with a voltage stabilizer and some resistance in the circuit to get it down to there.
The other option would be to use 2 x 5v outputs on your Arduino wired in series to get your 10 volts?

Hope that helps

[NITROPIXIE]

Thanks for the help funky.

The temperature gauge is actually a current sensing device (basically an ammeter), which is hooked up to a thermistor (CTS). As the thermistor increases in temperature, it lowers in resistance and therefore more current flows through it. The ammeter senses this and increases its reading.

For an accurate reading the the system uses a voltage regulator (10.5v) which are semiconductors in modern cars and an electro mechanical device which pulses to get an average of 10.5v (such as an old fashioned smiths regulators) in older cars. If you buy a new Smiths voltage regulator, these days you can see the traces in the pcb indicating it is of a semiconductor (more accurate) type. With mini's the voltage regulator supplies 2 gauges, temperature and fuel.

Now bearing in mind the old fashioned voltage regulator pulses and the hysteresis in the gauge means you can't see the pulsing, you could uses PWM and change the duty cycle to control the gauge position in theory. Most gauges work on this principal, except things like volt gauges. A fuel gauge works on a similar principal except it has a variable resistor in the tank which can physically move.

[NITROPIXIE]

I managed to find a spare spare temp gauge i had lying around the workshop, well someone else no longer wanted. So when i get some more components tommorrow i should be able to give it a bit of a test in the next week. I may even be able to come up with a circuit which could fit inside the megajolt V4 case using a simple 8 pin microcontroller to power an external gauge.

More to come

[DannyP]

RC servos require 5 v dc and a pulsewidth of 1 to 2 milliseconds for full range of motion. Most have 90 degrees rotation, but there are 180 degree versions for retractable landing gear. Also, there are sail winch servos that have 360 degree plus rotation.

[brentp]

I think servos are an awesome bit of customizable technology!

How big/powerful do they get? I'm wondering if they could be used for the quasi-silly ideal controlling active aerodynamics.. I've been thinking about that for use on our 24 Hours Of Lemons race car & controlled by a PWM output of Race Capture

[NITROPIXIE]

When i used to work in the local model/hobby shop the standard servos there could push out a few kg's of torque, nothing massive.

I googled these http://www.active-robots.com/products/m ... tors.shtml

What you thinking adjustable wing or air brakes (maybe for cornering)??

[brentp]

:-O now that's a servo!

I had a couple of whimsical applications in mind.

Air brake- variable deployment based on a function of speed and commanded brake pedal pressure
adjustable down force- speed based, of course

for 24HoursOfLemons it has to be pretty Junkyard-Tech. Perhaps a standard servo could be gutted and adapted to a windshield-wiper motor - those have a ton of torque

[NITROPIXIE]

Windscreen wiper motors do have a ton of torque that's very true, that would an awesome junkyard part. You could probably make you own servo with a pot and the circuit board out servo or equivalent IC. Can't be too difficult.

There is some stuff here, so its been done before

http://www.fieroaddiction.com/servo.html

http://www.rcuniverse.com/forum/m_9236698/printable.htm

[Funky Diver]

Ryan

You seen this?

http://www.janspace.com/b2evolution/ard ... ooterputer

Thought I'd post it in case you missed it

[NITROPIXIE]

That's blooming brilliant. Its the sort of thing i would like to finish off with. I like how it is so clean and detailed. Alot of work has gone into there i think.

[NITROPIXIE]

Right done my first real test on a Smiths temperature gauge using PWM from the arduino and so far the results are promising. I used a very basic circuit for the test which could be improved up on (see circuit diagram). I monitered the voltage regualator temperature by touching it with my fingers to make sure it wasn't over heating.

I have included phots for every step

The Smiths gauge has an internal resistance of 60 Ohms (just measured across it with a Ohm meter. I tried just putting a 5v power supply across the terminals of the gauge and it wouldn't even reach maximum, "she just canna do it captain"!!!

So out with a 9v 1A voltage regulator on a 12v source and the max is easily reached.