Homebrew, open source, repurposed, hacked, software defined, open hardware

Friday, 27 May 2011

Finishing the signal tracer

Like a lot of amateur radio operators, I have quite a few partly finished projects lying around, usually at the point of being a working circuit board, but not boxed up and labeled.

Waiting for the amoxycillin and clavulanic acid to kick in for the head cold, and waiting for the batteries and hardware for the roller shutter mods to arrive,  I avoided the urge to start a new project, and thought I'd actually finish one for a change.

I bought a signal injector and tracer kit from oatley electronics a while back.


It is very similar/essentially the same as the published silicon chip design:


A 555 putting out an audio signal at a few KHz is made to sweep up and down in frequency by another 555 which is doing the frequency modulation, and this becomes the signal for injection into your cable with many harmonics.

The tracer has some front end amplification with the 2N5484 followed by a transistor stage which achieves some detection as well.

I thought why not add a simple audio tracer function and complicate things by adding a 3 pole 3 position switch to take care of power, AF and RF input routing, and LM386 input routing, and add a potentiometer for AF gain.

The LM386 input is hi-jacked for the AF signal tracing, and the RF input stage is only powered up when the input BNC is routing the signal through the RF front end.

So, the kit gets to do double duty as an RF signal tracer and an AF signal tracer.

The photo shows the innards of the tracer, with the signal routing and power routing on the rotary switch.

The dividers making up the 9V battery compartment were made with tin snips and some spare ABS panel from a lexmark printer found in a dumpster. I prefer to use an external power supply for gear that is not used frequently, and added the switched DC barrel connector socket, as well as an external 8 ohm audio jack.

All done. I won't lose the screws for the lid now that the lid can be left on permanently.

I've now made it a habit to label the power connector so I don't have to dig up the specs a year later when I've forgotten what it needs for power.

1 comment:

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