by Robert Brand
At the DCA Regional Training School at Waverton, one of the first projects that they let you do was to built a simple tone injector in a pocket torch. This happened before my year and at least for a year after my attendance. It may have continued for more years, but others would have to confirm that. By today’s standards, the tool is bulky and it has discrete transistors, but back in 1968, we were constructing a wonder of the modern age. The unit was a two transistor astable multivibrator with the probe tip coupled by a capacitor. The circuitry took the place of one of the two AA batteries used in the pocket torch. Not a quality injector, but useful when chasing down a wire in a bundle.
The unit was constructed on a small piece of veroboard cut to fit the missing battery. A small metal tab made contact with the positive terminal of the battery below and the switch on the side completed the circuit providing the negative power connection. The switch had three settings: off; momentary on and permanently on. The tool was very useful when learning about audio projects.
The project was finished off with some clear heat-shrink and worked well. It still works! It was stored without a battery, but still corroded a bit. I will clean up the corrosion on the spring inside the barrel and I will clean up the collar where the on/off switch connects. I had told my 10 year old son about this and he is keen to make one. He also has his amateur radio license – VK2FJAB.
I pulled the probe out of my old OTC tool kit (previous post) where it had hidden in plain site for about 30 years since it was last used. It is nearing its 45 birthday soon.