Continuity Tester/Signal Injector

[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.