Audible tester for small changes in resistance

Something of a push-for-reference function to set your zero point?

The only difficulty I can see if that my multi-meter (digital) bounces ALL over the place prior to stabilizing. But if we could buffer the input, you'd get a slightly ~slow~ response that might serve you.

Sounds to me like a job for an oscilloscope (dual trace), so one trace can show the contact voltage, and the other the timing.

Well way back when I was a boy setting ignition timing on magneto motorcycles like BSA Triumph etc- we used a cigarette paper between points- a slight pull gave direct knowledge of point opening!.

Thanks,

That cigarette paper is the method that I have been using with success with a helper, but it ties up one hand, which is needed to hold the magneto from rotating on one side of the bike, while the pinion is tapped onto a tapered armature shaft and tightened with a spanner on the other side of the bike. I'm trying to turn a two man job into a one man job, by freeing up eyes and hands. I'll still be one hand short, though, because it is necessary to wedge the timing gears to prevent turning the engine from the prescribed position before top dead center while tightening the magneto pinion retaining nut, at the same time one prevents the points (which turn with the magneto armature) from turning on the other side of the bike, but I have some ideas on how to solve that that in a way that does not tie up my hands, which are not relevant to the question. So, it is necessary that the electronic solution not require that one's eyes be diverted from the magneto pinion nut, and that one's hands not be involved in the detection of the opening of the points, and that it be possible to adjust the rotation of the magneto to the correct position without looking at it, because it is impossible to look at both ends of the magneto, which are on opposite sides of the bike, simultaneously.

Granted, this is an exercise in the unnecessary, as it is easy enough to find a helper, but I'm as interested in the electronic circuit question as I am in the application. I am aware that there are tools on the market which one can purchase for this task. I'm just looking for ideas on circuits I can build myself to experiment with what works best, and then hopefully to understand why it works best, even if that is re-inventing the wheel.

Try using a Micro-Ohmmeter with programmable limits. These limits could be set to within a percentage of your value, and then set off an audible alarm when these limits are exceeded either high or low.

If you were to use one of the better ones, they can measure upto 50 readings per second, so would not have any problem with your application.

Try www.cropico.com I have been using one of these in my work on development, and it was exceptional in perfomance.

Hello Snowman 910.

When I read your description I thought of two devices in the market:

a) The Polar Toneohm Testers (www.polarinstruments.com, there are different models depending on the features (550, 850, 950). The older models come up on ebay sometimes.

b) a manual cable tester used for medical cables:

http://www.bispingmed.de/en/?Products:KT500_Kabeltester

it is not only capable of detecting short continiuity interruptions but also milliohm changes. The detection circuit is published in a german patent application:

Abstract of DE 19728902 (A1)
The device has a resistor (R) in series with the conductor to be checked. The presence of an intermittent circuit resistance change produces a change, e.g. U in the resistor voltage (U2) which is amplified by an acoustic amplifier (V1), to generate an audible signal. Visual indication may be provided by LEDs in a second amplifier (ST) circuit, where a red LED (D1) indicates a faulty circuit and a green LED (D2) a healthy circuit.

Hope this helps further.

snowman910,

A fairly simple circuit could be assembled, using a wheatstone bridge and either a 566 or 74124 VCO (Voltage controlled oscillator) chip...

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Just my $0.02...

556? Or even a 555.

dkwarner,

NO...

555 & 556 chips are TIMERS. 566 is a voltage controlled oscillator.

Granted, you could base a VCO circuit on a 555 timer chip operating in an astable multivibrator mode, but you would need a good deal more discrete components.

Here is a link to an online pdf of the National Semiconductor data sheet for the 566:

http://pdf1.alldatasheet.com/datasheet-pdf/view/8985/NSC/LM566.html

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Just my $0.02...

OOPS! I guess I'm showing my age a bit - I didn't know about the 566. Thanks!

The 566 is probably a better choice than the 4046 but, the 4046 is readily available from Digikey. eg.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=296-2052-5-ND

They want you to ring them to get a price on a 566.

Thanks for your posts and the digikey link, Randall.

Yes: a Wheatstone bridge like this will give you a rail to rail response for a change in resistance from 0 to 1 Ohm (if I've done my sums correctly):-

Any Op amp will do (the LT1001 was the first in the list). Using a straight VCO on the output will give you a continuously varying tone from 0.1 Ohm to about 0.9 Ohm. You can put in a (some?) comparators if you want a discrete change.

If you don't put in any protection mount the Op amp in a socket.

Thank you very much. This is very helpful. What would you recommend as an exciter voltage ?

Anything convenient: I think I would go for a 9V PP3, but you could get the whole thing to work with 3V (a couple of AA or AAAs).

Many thanks to all who took the time to share their thoughts on this. If there are any other approaches that would be worth considering, I enjoy wiring things up so I'll try anything. The suggestions made were great and I'll start ordering components and see how I do.