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MG MGF Technical - Coolant level alarm circuit

I now have my coolant alarm circuit working as it should, using the new expansion tank with the built-in sensor.

If anyone wants details on the delay circuit, go to.....

http://www.apttony.co.uk/Servicing/CoolantAlarm.html

....where you will find the details.

It will get a prolonged 2500 mile road test over the next few weeks, but so far it all looks okay.

Feel free, but remember to do the usual visual checks and, if you copy it, test the circuit every now and then. I will accept no responsiblity.

Tony
Tony Thompson

That looks like you could shave a lot of compolents!

I wouldn't have tied the output of the unused op-amp, as it could easily float and burn out in that configuration.

How much did it cost you?
Will Munns

Did you use a voltage regulator of some kind to protect the opamps from spikes on the 12v? Otherwise I would use a zener with a resistor. Also, I agree that you should have the output of the unused opamp floating, and connect one of the inputs to +12v with a 10k series resistor. This will prevent the opamp from swinging all the time. Keep the other input connected to ground.
A.P. van Wijngaarden

Interesting comments.

Some of the components are ones that I had available, especially the var. resistors.

However the Data Sheet for the Op Amp clearly stated that they should be tied down!

The idea of the 2 caps in the lower lefthand section of the circuit was to prevent such spikes.

Tony

Tony Thompson

Tony,

Well done. Looks like a great DIY achievement!

Just be careful how load you play that new music in the car when you are driving on your prolonged 2500 mile road test. LOL.

Cheers,
Branko.





Branko

Echo Branko

Well done Tony.
Have a wonderful safe trip and give us feedback upon return.

Safety Fast!

Tony
Tony H

Great work Tony! Now looking forward to Will and Alex's suggestion for a Mk2 version! :oD
Rob Bell

Tony

I read this with interest as I am currently building a delay circuit using a 555 timer for the coolant level switch

Here is the link to the circuit I used

http://555-timer.clarkson-uk.com/operation/delay.html

The components cost 60p ( excluding buzzer ) from Maplins though I understand it might be a good idea to put a 0.1 micro F capacitor from positive to earth to protect against spikes


Very interested to see 4 second delay works well

Chris

C Bowes

Nice simple one Chris, and a useful site.

Tony
Tony

Looks useful indeed for all you need
http://555-timer.clarkson-uk.com/operation/
Dieter


Yep, the number of things you can do with a 555 is astounding!

RE Tony's dia:

The two caps on the far right are missplaced in the diagram, it looks like they are somthing to do with the input, wheras they are smoothing for the supply to the Op-amps.

You can lose everything to point 1,
You have two diodes here to prevent the input running over the rails, but this can't happen as the input comes from the same supply and is governed down by the variable resistor, all the circuit to point 1 does is to make sure that you get a clean on/off signal at the RC circuit, This doesn't matter on this circuit, because the only time you shouln't get a clean switch is when the coolant is sloshing, and you are going to ignore this at the next stage anyway.

I guess the 201k variable resistor is beacuse you had them lying around ;-) This is only for testing and you could replace these three resistors with one fixed resistor of a suitable value. The RC'ness of the circuit will do the filtering of the previous stage for you, but you should put a high resistance across the cap to drain it down (or else it will top up gently).

Finally the two seperate output stages could be made one, as they do the same job.

This should leave your diagram with one comparitor, one cap and resistor for the RC circuit, two resistors for the potential divider and two smoothing caps.

I would also be worried about the current handling capability of the OP-amp and feed the output thru a mosfet (reverse the input to the op-amp to get a negate sense signal for driving the FET)
Will Munns

Only having the most rudimentry grasp of electronics, I like the simplicity of the diagram that Chris shows on that website. Very very good :o)

If one were to put a 0.1 uF cap into the circuit to protect against spikes, is it just a case of popping it in between the positive and negative rails of the circuit?

If I understand the behaviour of the circuit correctly, if the circuit reads high for a set time (say 4 sec), the output goes high (illuminate lamp, sound buzzer etc). If the input then goes low, the output would also then immediately go low?

Apologies for the numpty questions!!!
Rob Bell

>> Looks useful indeed for all you need
http://555-timer.clarkson-uk.com/operation/ <<

I'm liking the 'one shot' function of the 555 Dieter - and am thinking about that idea of being able to drop the window a fraction when the door is opened (and perhaps raise it again when closed?)
Rob Bell

The 555 is just doing the job of the (remaining) comparitor, its just the wires are internal :-)

Yes - and you want to put it as close to the +ve feed to the 555 as possible.

The circuit is powered _thru_ the sensor, so yes, as soon as it goes low it would cut out, _but_ I'm not sure about the internal drain current emptying the cap, and you might get the topping up effect I talked about earlier unless you added a drain resistor.
Will Munns

At the risk of turning this thread into "Electronics course, 101", would you put that drain resistor in parallel to the capacitor? What value would you use?

I also note from elsewhere on that page that the 555 runs off 9v - would 12v fry it?

Thanks Will :o)
Rob Bell

Rob

I understand a 555 is OK up to 18 volts

Should be OK for the car

Chris
C Bowes

>the 555 runs off 9v - would 12v fry it?
First thing I checked - it's good for 5->16 volts

> would you put that drain resistor in parallel to the capacitor?

Yes, high value resistor
Time constant for caps:

T (seconds) = R*C
where T is about 33% and voltage falls away 1/expontially

You want T to be about 2 seconds, but the value of the resistor you use will affect the value you need for the feed resistor (use a lower value because some of the current will go thru the drain)
Will Munns

Okay, thanks. So would I have to recalculate the resistance of the "Ra" resistor by assuming that the resistors are in parallel - ie 1/Rtotal = 1/R1 + 1/Ra where R1 is our resistor in parallel with the 0.1 uF cap protecting the circuit against voltage spikes?
Rob Bell

Hey, Rob,

I did Electronics 101 about 27 years ago....!! :-O

Sometime after that the 555 timer hit the market.

Wow, what a device!!!!

LOL.

(Maybe it wasn't actually called that)


Cheers,
Branko.



Branko

Doh, no smoothing capacitor possible on that circuit because it would charge and "hold on" the power for longer than the blip. That might cause problems.

For drainage the timing capacitor (the one in the dia) needs a resistor in parrallel to it.
Will Munns

OK….
Its Friday afternoon, its hot and I couldn’t be bothered to think, so…
I built…

The Delay On cct from the 555 web site works OK as is (with 0.1uF anti-spike pin 8 to pin 1).

With 10 uF C and Ra 680 kOhms delay is as expected.

When power is removed the voltage across the capacitor goes instantly (to the timescale were talking about), so no need toe a discharge resistor.

The output from the 555 is more than ma enough for a blinding array of LEDs and a buzzer.

The only thing that concerns me is that 16V is a bit close to the charging voltage on the battery. Back to Will (maybe a couple of diodes?)…
JohnB

Charge voltage is 14.5v, you could use a 5v regulator (input 7.5->30 volts) or if you were a sick puppy you could use two diodes in series to drop a further 1.4 volts.
Will Munns

For those of us in the remedial class, what's the problem with the charge voltage???
Rob Bell

There will be ignition spikes on the input voltage rail, probebly not enough to cause problems with a 555 as they are fairly robust, but there in any case.
Will Munns

So basically I can use the 555 circuit with John's cap modification over pins 1 & 8 with no problem?

Hmm - I foresee a trip to Maplins for some bits and bobs! :o)
Rob Bell

I wouldn't use it to run a high noise buzzer without some sort of drive circuit (mosfet?)
Will Munns

Okay ... ignorance again... Mosfet?
Rob Bell

metal oxide field effect transistor
Will Munns

basicly transistors have a 'gain' that is how much current they flow (collector to emmitter) is dependant on how much current you put on the base.
Of course you have to limit the amount of current thru the base or the device will blow up, so it gets tricky.

Also they have a 'rated' current, which is usually low.

MOSFETS operate on 'field effect' ie the amount of current they flow is dependant on the voltage across the base, so they don't consume current to switch. they turn HARD ON (~2 ohm resistance) at 10 volts (ish), lower than 10 volts they act as a resistor and get hot.
Mosfets are generally high currnet capable (the ones in your water pump circuit can push 30Amps if they are heat sunk)

So great, but this means that for the field to be 10 volts the FET has to be switched on with a GND on the input if it is switching the +ve, or a +ve on the input if it is switching the ground.

Sorry this is all getting too complex, I need the loo and I am right in the middle of things so cannot really explain well - plus I need diagrams.
Will Munns

How many way's can you skin a cat? Ask an electronics engineer! :-)

Well, if the truth be known, I'm a software engineer with a good understanding of digital electonics, but not so hot with analogue stuff.

I tried various options, tested them on a PC analogue simulator, made them, wasn't happy, then went and asked for help!

As I said earlier, the bits I used are ones I had available, so no doubt there are other and perhaps better or less complicated ways of doing it. The main thing is it seems to work okay. Its been on the car all week and used every day.

The drop-off once the float sensor 'opens' is extremely quick - should have measured on the 'scope, but didn't. :-( I tested it by shorting across the sensor connections, tapping the connections on & off as quickly as I could, but still could not get it to turn the alarm on. Only with a constant supply would it do so, after the 4 seconds.

As Will says, you don't have to have the 2 output stages, but this does allow for a certain amount of flexibilty in choosing the LED and Buzzer. Since they are quite seperate, you can vary the resistor on the buzzer to suit it's output volume.

I also thought of putting an dash mounted switch across the sensor, purely to allow easy testing of the circuit, but in the end opted for the usual visible checking and the occassional manual pushing down the float. Certainly didn't want to include an over-ride switch, since this might get left in the 'off' state and the whole exercise would turn into a white elephant! LOL Also, the buzzer is VERY loud and will encourage immediate pulling-over and switching the engine off !

Tony

Tony Thompson

Tony – I can probably think up more solutions if you like! Using what you have available is usually the most satisfying. Saving an opamp when there are four in the box is not a big deal… Unless you built two circuits and had a windscreen wash level monitor as well :-)

Rob – If Tony’s buzzer is loud enough with a 100k resistor then the 555 timer should be OK to send you deaf. The 555 output is ok for 200mA, an LED usually wants 10mA, find a buzzer that is loud enough that required less than 190mA. That shouldn’t be too hard if you look at piezo buzzers.
Regular testing would seem to be a good idea. And having to actually do the testing at the header tank also allows you to check that the water is really there.
JohnB

Thanks John - but bleeding eardrums weren't on my "want list" ROFLMAO ;oD

I foresee a trip to Maplins in the not too far distant future...
Rob Bell

Quote:
<b>Will Munns</b>

<i>I wouldn't have tied the output of the unused op-amp, as it could easily float and burn out in that configuration.</i>

The LM339 is open collector output, so it is fine to tie it to ground.

A warning though, to anyone designing electronics, the "+12V" supply can go very high when starting the car, or wipers running etc. There are specs for these kind of things, which I won't go into, unless anyone is interested, but you can expect to see 200V spikes when the wipers go, for example, without trying.
Kev

How would you protect an electronic circuit against that kind of voltage surge Kev?
Rob Bell

the higher voltage spikes are fairly low energy, so it doesn't take much effort to protect from them.

In a simplest case, a 200V, 10us long pulse, can easily fall to just under 20V with a 1k ohm resistor feeding a 100n cap to ground, and the circuit input taken from the join between the two.

Tony has used a couple of diodes to clamp any spikes to the supply rail, effectively forming a low impedance to his two 100uF capacitors - this will also make any spikes dissapear nicely. The unopened switch is effectively acting as an antenna in the engine compartment, thus picking up noise from the HT too, so whilst the nominal voltage may be no greater than Vbatt, the "noise" can be much higher.

Of course, no one is going to sell these gadgets as products (as is) anyway, so there is no desparate need to meet any vehicle standards, but is doesn't hurt to be safe, as in Tony's diagram.

Should Tony disconnect his battery, whilst the engine (alternator) is running, or for that matter, a cable breaks, I wouldn't mind betting the LM339 says cheerio, but let's assume he's not going to do that.

If you fancy it, a couple of pages in on http://www.engin.umd.umich.edu/ceep/tech_day/2003/slideshows/ppt/2003PresentationShen.ppt gives some figures. Sorry I couldn't find anything more autothorative quickly on the net - ISO7637 is the spec, but you're meant to pay $$ for that.
Kev

Kev, using the 555 circuit mentioned above, John mentioned using the following modification: "(with 0.1uF anti-spike pin 8 to pin 1)"

Would this not work as intended - i.e. block the voltage spikes that you discussed?
Rob Bell

Hmm. Not especialy, the 0.1uF is the same as 100n, as in my example, and shows these kind of spikes can still reach a level of ~19V with a 1k resistor feeding it, now you're presumably feeding it with a piece of wire.... but we're just likely to power up your circuit for 10us, then off again in this case, which will reset it anyway.

A Voltage regulator goes a long way to help protect circuits for you.

The 200V very short spikes are not a major problem, in terms of damage, they're more likely to cause spurious counts etc on various inputs to circuits. The load dump is where the energy comes from, and for example, a regulator like http://www.onsemi.com/site/products/summary/0,4450,LM2931,00.html is quoted as being ok with this - you can get these from farnell.

This kind of circuit will probably give no indication if it breaks, which is the same as the 'everything ok' indication - for the sake of another 50p you could potentially save yourself a lot more.
Kev

Rob – I know nothing about auto electronics, other than it is much more complicated than designing for a 12V battery. I’d picked up the 0.1uF (100nF) from higher up the thread. Kev’s 1k resistor makes sense you could easily add a 10uF (>40V) cap across the power as well without the 555 timer running on for very long after the switch has opened. The 1k won't make any difference to the timing but it may affect your buzzer and LED.
What we need are some volunteers… Then we can see the results of running these circuits over several months and in wet conditions (wipers on).
JohnB

John, I know far less than you, so any opportunity to increase my knowledge is gratefully received! :o)

Kev, there are a significant number of LM2931 devices listed on that page, and I have absolutely no way of judging which would be suitable for our application.

If you were to build a 555 delay-on circuit with a load-dump protection circuit, what parts would you order? And how would it all be put together?

Apologies to be asking for such basic information, but I suspect that I am not alone in not being able to put together all the necessary components to build a reliable circuit - but I am happy to weld a soldering iron! :o)
Rob Bell

Rob
<<but I am happy to weld a soldering iron! :o) >>
I suspect that welding with a soldering iron will be both difficult and likely damage a few electronic components. :lol: But agree with the intention - electronics from my degree hrmmpph years ago is rusty and so happy to put something together but not really up to designing it.
David M

Try 334-3601 from farnell, 55p each, link here

http://uk.farnell.com/jsp/endecaSearch/searchPage2.jsp?Ntt=3343601&Nty=1&N=401&Ntk=gensearch

datasheet here

http://www.farnell.com/datasheets/15524.pdf

That one is a 5V regulator, there are others, but it was the first I came across that illustrated the protection for automotive use. I guess the buzzer wouldn't hurt running from 5V if there had to be a 100k in series anyway!

I probably wouldn't use a 555 for delayed on.

I'll sketch something and post it in due course - could you let me know which buzzer is to be used? Or shall I just pick one?

John - the 1k ought not affect the buzzer to much, as that was fed though a 100k anyway!
Kev

David - you've spotted my problem! ROFL Meant to type "wield" - d'oh!

Kev, that would be great. Regarding buzzer - well, since I don't have a stock of these lying around, you might as well choose what ever you think is suitable. :o)

Cheers
Rob Bell

I havn't tried building this yet, but I definately wouldn't use a 555 for delayed on - I went for a very similar solution to Tony, and seeing as it was suggested that a 4 second delay was appropriate, put that value in, non adjustable.

http://www.jbet.co.uk/levelSensor.jpg

seeing as there was a spare op-amp, I decided to do a radio mute feed - this would not necessarily work with all radios.

Usual caveats apply.
Kev

Does the float switch have Normally Open and Normally Closed connections?

This does look more 'professional' than the 555 circuit.

The radio mute is a good idea.
JohnB

I've not seen any official one fitted to any coolant tank, ala R25 etc as mentioned, but the circuit could be reconfigured if the other sense was desired.
Kev

nice one Kev, thanks. Will have a go at building this when I get time (i.e. not the foreseeable future unfortuantely, have a garage to rebuild, amongst other things!!!) - but it looks as though it will do the trick nicely!

BTW Love the radio mute idea - unfortunately, my head unit doesn't accept this :o(
Rob Bell

This thread was discussed between 25/05/2005 and 08/06/2005

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