Speed Sensing circuit.

[wing]

Hi all, This is my first posting and reading through some of the postings I notice that a few of you seem to know how to wire a circuit using the LM2917 (with zener) chip.

 Am a trying to construct a circuit that will turn a warning light/led off when the speed reaches a predetermined point and off again when it increases again. The turning point is when the chip recieves about 9 pulses a sec (9 htz)

 I have constructed two circuits so far, niether worked !!!, based on information from th chip manufacturers (see link and diagram below) I test these circuits useing a borrowed frequency generator (first time I plugged it all on I used a -5v to +5v wave about 10 volts - now using a 0-5volts pulse)

 I have some questions, (as you do!)

        A/ does this chip work on a +- fequency if not, have I blown it, and how do I tell?

        B When connected to my main power source (12 volt car battery) the chip gets very warm to touch.

        c/ It does not seem to be switching - my LED stays on.

Link to .doc File

Stuart. FYI .doc files will not display as images and are only useful for windows users. So I made your image tag a link instead. [Editor]

Any help would be welcome,

regards,

Stuart

North Wales

UK.

[Flux]

Your drawing didn't come out so I don't know what you are doing but the chip shouldn't get hot. It needs a resistor of 470 ohms to the supply for 12v if you didn't include this you will have fried it.

The input signal needs to go + & - so you should have been ok to start with. It will not work with a unidirectional pulse unless you ac couple it with a capacitor and put a resistor from input to ground.

It does work on frequency.

Flux

[baggo]

Hi Stuart,

The data sheet I have was downloaded from the RS website and shows a speed switch cct similar to yours but with a few differences. R1 and R2 are both 5K, there is a 470R resistor between VB and pin 6 (as suggested by Flux) and R3 is omitted.

The circuit as shown is designed to operate from an AC input signal and 12 volts supply. The chip probably will get warm (as do they all). There's a lot of components in a very small package which all generate heat. The ac input is ideally fed from a magnetic pickup and is protected against voltages of up to 28 volts.

John

[wing]

Hi Both (Flux and John)

 Thanks for the reply answers as follows.

 Flux,

      As my drawing was not available this is what I done, Power '+' conected through "12 volt LED" to pin 8, 500k resistor conecting Pins 8+10(chip side of "12 volt LED")Pins, Direct power to pin 9, 10k resistor connecting pins 9+10, and another 10k resistor connecting pins 10+11, then I took pin 11 to ground. The other side of the chip was pin 1 pulse in, pin 2 0.3uf capacitor to ground, pins 3+4 together to groung via a 10uf capacitor and a 100-220 variable resistor, and fianlly pin 4 to ground all as (apart from varible resistor)drawing on data sheet AN162,page 8,fig 9. Are you saying I need another 470 ohm resistor before the led on in parallel? thought as much with the unidirctional pusle (only thought on this one after trying it the other way for a long while - Dum or what)AC couple it with a capacitor? wont that remove the pulse?

John,

     I tried the one from data sheet, ds007942 page 8, but again without the 470 ohm resistor, and this did not work so I went looking and came up with the aother datad sheet (an 162 on the national semiconductors site) I understood that a little better.

And again thankyou very much for your response, tomorrow I will go out and buy a 470 resistor and another chip (number 3) just in case the other 2 have 'fallen over'

Stuart,

North Wales

UK

[Flux]

Hi Stuart

You seem to be using the 14 pin chip which I have never used.

The link that someone has made work seems to call up the circuit you need but in the 8 pin form for 2907.

The 2917 has an internal zener on pin 6 for the 8 pin chip or pin 9 for the 14 pin version.

It is this pin 6 or 9 depending on the package that must have the 470 ohm limit resistor ( not needed for 2907 ).  You have connected pin 9 to supply and that will be why it gets hot.

If it really is a 12v LED with its own limit resistor ok if it is an ordinary Led it will also need a limit resistor 0f 470 ohms to 1k.

I have tried to follow your description of the circuit and there are a couple of things I dont agree with.

Pin 3 and 4 joined have a 10uF capacitor to ground and a variable resistor that should be in the 100 to 220k region not ohms.

When you say pin 4 is grounded, I think you mean pin 5.

On the 8 pin circuit it shows 10 k resistors from pin 7 to pins 6 and 8. these are needed to get the reference point of the op amp in range.  You can replace with a 10k pot with slider to pin 7 to alter the frequency in addition to the variable on pin 3/4 but the range is limited, it has to stay near mid volts.

For your 14 pin amimal it looks as though you need 10 k to pin 9 and pin 11 to have the same thing.

The 14 pin data sheet seems also to indicate that pin 12 should be ground but I am not sure.

To start with I would omit the 500k  latch resistor from pin 8 to pin 10. Without it the LED will not switch cleanly from on to off but it removes one possible source of trouble.

I hope this is enough to get you working.

Flux

[wing]

Flux,

Thanks  for the response, I am using the 14 pin version, my supplier over here cannot get the other LM2907, the two internals look simular apart from the fact that pins 3 & 4 are not connected interally and pin 12 is grounded internally as is pin 8 on the 2907, the zener diode is rated at 7.6 volts and cannot be disabled but reading the data sheets, it seems to be more suited (so they say) towards this sort of application.

 I forgot, sorry, to add the 'k' to the 100-220k variable, I calculated that at 9 htz I would need somewhere in the region of 180k.

It is a 12 volt LED.

I was meaning that the 10uf capacitor is inparrallel with the 100-220k and both go to ground and yes pin 5 is also grounded.

I respect to the LED switching cleanly - switch would be great to start with !lol!

I will have a 'play' probably tomorrow now and let you know how I get on.

Thanks again.

Stuart

North Wales

Uk.

[Flux]

Stuart  the zener presents no problems but please remember that it must be fed from a current limited supply. DON'T connect pin 9 directly to the 12v supply.

That chip never runs hot in normal use, it is fairly tough and stands most abuse but neglecting the resistor on pin 9 will kill it.

Good luck.

Flux

[eureka]

Yes, all the tachometer circuits I have show pin 12 connected to ground.

[eureka]

opps, I meant to say pin 12 connect to ground external to the chip...

[Flux]

Thanks eureka, that is the conclusion I came to. The data sheet is highly confusing on that point.

From the the internal circuit diagram pin 12 seems to feed a current source reference and in normal use it looks as though it needs to be grounded.

I have found a circuit where it is connected via a resistor to somewhere else but I think it is best to ground it for normal use. I don't think it will work if left not connected.

Flux

[wing]

Flux, Thanks for the info about the connection to pin nine - OOOOOOOPS! think I have blown it, never mind, I will have to go and get number 3 tomorrow.

Stuart.

North Wales

Uk

[wing]

All, I have been in touch with my suppliers technical department, sent e-mail over Christmas, and they finally came back to me this week, I did ask them about the grounding of pin 12 and their answer was "All the 0V/Earth points will be connected inside the chip, so only one 0V connection needs to be made.", but I gues being safe is better then sorry.

Stuart

North Wales

Uk