Problem with LM2917 Tach Circuit - 14 Pin version

Forum 2005-2010 (read only) Hardware Interfacing
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I'm hoping someone can take a moment to help me and possible restor my sanity! :slight_smile:

I'm stuck with a 14-pin LM2917 - i've got an 8-pin on order now, but damn it, I want to get this to work.

I've built this on a breadboard using a modified version of GitHub - designer2k2/multidisplay: The MultiDisplay Project, An opensource datalogger, boost controller and display for cars . I have then used the datasheet as well as this schematic to convert to 14 pin... :

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So heres what I ended up with - its ugly as a schematic because I've re-worked it to match the orientation/layout of the breadboard to help debug...

And heres my breadboard layout:

Hi-Res Version! <-- that should allow you to see all the components hooked up ok... I hope...

Yellow is frequency in, White is the LM's RC circuit, Blue is Signal Out. To help save hands, I hooked an LED up to the output signal.

Now... I'm not game to hook it to my 'duino nor can I be buggered going out to the bike yet... so I've got a 5V supply from a Minty Boost simulating the 5V from the 'duino. I then have another supply, also grounded to the common ground, thats 14V.

If I pulse 14V onto the "Signal In" port with that powersupply also connected to ground... I get... nothing. The sig-in line goes from 5.08V to 5.6V, and there is no signal on "SigOut".

I've played around with a few variations.. but buggered if I can get this to behave.

Further, if I ground the input signal, I get much better voltage response than if I just ground it to positive... yet this should be designed to read the 12V+ lead off of an injector... The idea is it will take "Signal In" from the 12V+ signal from a Fuel Injector, and then should put out a 0-5V Voltage on SigOut to then read on the Arduino as RPM..

Its driving me nuts... and I refuse to admit defeat with just getting an 8-Pin (eventually) and copying the original schematic... Im guessing now (totally) that my 'test powersupply' is putting 70ma across the base of the 557, which is saturating it. so maybe I need to change the 22k's out ??? I dunno.. I feel well past my depth trying to sort this out...

ARGH!!!! :o :o :-? :-? :-? >:( >:(

Can anyone offer some guidance ? The LM2917's datasheet is at http://jaycar.com.au/products_uploaded/ZL3930.pdf and Its a phillips BC557...

2

How are you pulsing the 14V?

That chip is made to detect RPM, most engines Idle 500rpm-ish, so you're going to need a bunch of pulses or it's going to measure 0.

Ideally, you're going to need 8 pulses per second minimum, 50-80 would be better for test purposes.

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Yeah - I figured that i needed a decent 'frequency' ... I can maintain about ~20Hz according to my 'less than ideal' sound-card scope tapping the bare wire on the terminal block... but even measuring mili-volts out the other side I get zip on SigOut.

Now slightly more awake - Im thinking the grounding of the negative SigIN is 'producing' a voltage because it causes the various caps to discharge - not because it actually runs through the IC... so thats problem I think was just me being dumb ;D

The core problem still remains - Its still otherwise not working properly on the 14V pulse... Once it stops raining here, I'll drag it out and put it onto the bikes injector input at least and see if that makes it behave better... :o

I think the problem is my Internal Zener references.. according to this quote from a thread I found on another forum:

The 8-pins LM2907/LM2917 needs AC (voltage goes above and below ground) for its input. You are feeding it pulsed DC.
The 14-pins versions can have their reference input biased so that the input works with pulsed DC.

So I need to figure out how to do that - and can also ditch the BC557 I think.... since I don't need an AC frequency like the 2907??

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I think you problem lies with the input circuitry of the LM2917. Pin 11 and Pin 1 are the two inputs of a voltage comparator. You have fixed Pin 11 at about 0.7V by using that diode. Therefore to get the chip to do anything you have to take pin 1 above and below this point. However you have no DC biasing on Pin 1 so it will just swing about with the AC coupled input and not do anything.
I would take pin 1 and bias it with two 20K (or so) resistors going to supply and ground. Then put a 10K resistor to the cathode of D3 and the other end of it to ground. Then you stand a chance of getting the input to switch.

5

Thanks Mike..

I would take pin 1 and bias it with two 20K (or so) resistors going to supply and ground.

Ok, I'll give that a go along with the D3 mod. Just to confirm, does that mean build a voltage divider and have the centre net link to pin 1 ?

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does that mean build a voltage divider and have the centre net link to pin 1 ?

yes :slight_smile: