signal injectors

[tiv48]

A young lad came to see me for advice on building a simple signal injector with an output upto RF.
He and his dad have some old transistor radios to experiment on,
i suggested a simple mutivibrator circuit  and showed him some of the ones i have built.
he had a few questions.
1.what is the ideal frequency output?
2. why can't i use a 555 chip?
I checked the one that seems to work best and found the output 250kh at 8.7 volt p.p.
I remember trying a 555 but it did not work very well.
so has any one have an answer to the ideal frequency.
Has anyone made a 555 injector that worked well.
Thanks  
Rob T

[ppppenguin]

Can't see why not a 555. Perhaps its edges are rather slow and so not much RF.

I'd probably use a 74HC14 schmitt trigger. One section with an R and C as oscillator, another section (or sections in parallel) as an output buffer. Will run on a pair of AA cells though obviously with only 3Vp-p output. This ought to be enough.

[tiv48]

thanks Jeffrey i will pass on your suggestion and if i find time experiment myself with a 74hc14,
Rob T

[ppppenguin]

There are also loads of circuits like this out on the web: http://www.zen22142.zen.co.uk/Circuits/T..._trace.htm

[Paraguayo]

A single 2 transistor multivibrator at 1khz will work well, sorry for ny bad english

[tiv48]

Well the lad and his dad came round this morning to show me there first attempt .it uses 2 bc108 and the output is 27khz .
We tried it on a roberts r500 starting at the volume control working back though the if stages and finally the ferrite rod.
it worked very well.
I have suggested that they may like to join this site .
but the dad has had a not so good experience on an other site so is dubious
I have asked them to let me know how the get on .
Rob T

[Yorkie]

Well done to them.

Sorry to hear they had a bad experience on another forum - no 'roost rulers' or 'tall poppies' here - no dramas.

A handy companion to the signal injector is a signal tracer - basically just a small amp. Lots of circuits about on the net, some using transistors, others using an IC such as an LM386.

I've mentioned before that Vellman do an excellent tracer/injector kit ('K7000') on a compact plated through hole PCB with a volume control for the tracer amp, and a level control for the injector. You get a nice clean sine wave from the injector - nominally 1KHz, but  as often as not it's a bit higher than that. Not important, it has harmonics going way up in frequency anyway. I've built several over the years, often as gifts to friends. I've attached some pics of one that I recently built, and a scope pic and frequency counter readout of one that I built a little while ago (1.25KHz). 

Doesn't need to be in a fancy box  - a plastic project box is fine.  They soon earn their keep.

The kit is widely available from several sources for about a tenner. (Speaker, sockets, knobs, switch etc not included).

https://www.esr.co.uk/products/frame_test-equipment.htm

 The construction manual is here:

https://www.esr.co.uk/manuals/k7000.pdf
 
Hope that's of interest.

[ppppenguin]

You get a nice clean sine wave from the injector - nominally 1KHz, but  as often as not it's a bit higher than that. Not important, it has harmonics going way up in frequency anyway.

Puzzled. If it's a clean sinewave the harmonics will be minimal. The schematic shows a classic phase shift oscillator which generates a sine wave that's realtively clean and free of harmonics. https://en.wikipedia.org/wiki/Phase-shift_oscillator

[pwdrive]

Standard sig injector back on the bench in my day was a DIY 1Khz multivib stuffed into a Radiospares BY100 packaging tube with a probe stuffed in one end and battery supply leads coming out of the other end, harmonics up to way on when as I recall.

Lawrence.