Continuity Tester/Signal Injector

[Joe]

Months ago I posted details of my Continuity Tester and said that I also used it as a rough and ready Signal Injector.

I wanted to improve it in this latter role but couldn't devise a way that didn't involve complicated, and therefore expensive, switchery.

A couple of weeks ago the blinking obvious hit me: just fit a second board. I'd got all the bits in stock and even if I'd had to buy them they wouldn't have cost as much as a fancy switch.

It works very well and the revised circuit is below.

   

- Joe

[Yorkie]

Months ago I posted details of my Continuity Tester and said that I also used it as a rough and ready Signal Injector.

I wanted to improve it in this latter role but couldn't devise a way that didn't involve complicated, and therefore expensive, switchery.

A couple of weeks ago the blinking obvious hit me: just fit a second board. I'd got all the bits in stock and even if I'd had to buy them they wouldn't have cost as much as a fancy switch.

It works very well and the revised circuit is below.

- Joe

Very neat Joe - a signal injector/tracer is such a handy piece of kit for pinpointing problems in radios, and once the technique is employed, you wonder how ever you managed without it!

What a wonderful little chip the 555 has turned out to be! It was 40 years old in 2011, having been introduced by Signetics in 1971, since which time billions have been produced by several manufacturers. (Incidentally, the 556 is two 555s in a 14 pin DIL package rather than 8-pin).

Many years ago in Practical Wireless there was a series of simple projects known as 'Take 20' in which the requirement was no more than 20 components, costing no more than 20 shillings. I think the series pre-dated the 555 but they could have run the series just on that one IC, (which has 25 transistors and two diodes) and never run out of ideas.

Internet abounds with websites on projects using the 555, and the following links might be of interest:

http://home.cogeco.ca/~rpaisley4/LM555.html

http://www.kpsec.freeuk.com/555timer.htm

89-page e-book:

http://talkingelectronics.com/projects/5...rcuits.pdf

A couple of other ICs that have proved their worth and stood the test of time are the 741 op-amp, and the LM386 amplifier, which needs just a few external components.

[AlanBeckett]

Oh, dear, I fear I'm in the other camp. I've long regarded 555s and 556s as only fit for amateur twiddlers in 'designs' using a few pots to get it right, or where any sort of stability or repeatability was about tenth on the list of desirable features. Many moons ago I tried, but it was hopeless. If anyone is feeling heroic try buying a few from different Manufacturers and plugging them into a test circuit. They won't even all trigger in the same way let alone produce the same timing. As a one-off, with a bit of fiddling, they're OK, but that's about all.

Alan

[Aerodyne]

That surprises me, Alan. I used the 555 timer chip many times for school GCSE projects - hundreds, probably, over a 20-odd year period - timers, tone generators, keyboards, light sensitive switches, that kind of thing - alongside TTL and CMOS chips - plus using one in my own design of signal injector/tracer. Admittedly perfection was never a requirement with any of the school projects nor for that matter was it with the injector, but for simplicity and versatility the chip is hard to beat. What were you attempting to use the 555 for that demanded stability it could not provide?
-Tony

[AlanBeckett]

Tony,

I quote from David's first link above:-
'There will be minor internal circuitry differences between 555 timer IC's from the various manufacturers but they all should be usable for the circuits on this page.'

Now try going along to the Production Manager and telling him that he'll have to install a couple of pots or turrets for SOTs so they can be set up.

Then go along to the Purchasing Manager and tell him that he'll have to buy Texas chips 'cause they're the only ones that are guaranteed to work, but watch the date codes.

Next go along to Test Dept and tell them to keep checking that the pots or SOTs are in reasonable range - not on the end stops - that the correct Manufacturer's chips have been used and that the output waveforms are good. You'll have put all that in the Test Spec.

Finally, call in the Personnel Department on your way out and collect your P45.

OK, that's a bit of an exaggeration, and you could probably get away with using them if it's a short life low volume Domestic product. They're also OK for one-offs 'cause you automatically make the circuit work with the chips you have. But when it comes to long life or high volume designs they're just not on.

Alan

[Aerodyne]

Point taken, Alan. What I believe you you are saying is that for industrial mass-production the chips are unsuitable as they suffer from batch variations and variations between makers. I can imagine that to be the case, certainly, however for the 'amateur twiddlers' - as you so succinctly put it - who need a useful signal source for their homebuilt test gear, the timer 555 remains a useful, low-cost and easy to use item.
-Tony

[AlanBeckett]

Tony,
Indeed so. Much product is made in relatively small batches and then essentially forgotten about. In that case buying 1,00 or so of the same type and building them into the design is no problem.
I've had the same problems in the past with other chips, and even transistors and FETs. At my last Company only a Motorola FET would work in a particular position in one (long lived) product, and the Manufacturing infro said so. As I said, for smallish batches, or short life products, they're fine, but I'd advise against building them intro anything that you might have to repair or service in 10 or more year's time.
Alan