Fox and Hound

640px-Two_Vulpes_vulpes_pupsNo, not an article on QRP fox hunts (which I miss, being a bit out of range), but a construction project to make a pair of devices for tracing down wiring. The fox sends a tone down a wire and the hound sniffs around until it finds the right wire by detecting the tone. I found some commercial versions online at a reasonable price, but there isn’t much to these devices, so I figured I should be able to put them together from parts lying around. In what has to be one of the few examples of truth in advertising still to be found on the web, I came across neat circuits for these devices on

The Fox

I built the fox generator circuit as presented on neatcircuits because I had the exact parts required on hand, although the N2907A was a crusty KN2907A (plastic package) that  I had ripped out of something years ago. Likewise, the CD4001 was in the prehistoric section of the junque box, but still tested okay. The circuit gives either a steady or one-second alternating tone (~930Hz / 1160 Hz) depending whether one input in grounded or high.

IMG_20160424_183219Since I had the project breadboarded, I experimented around a bit to see if more readily available parts could be substituted. First, I replaced the 910k resistor with a 1meg resistor in what is a essentially an RC timing circuit, so just about no change. Next, I replaced with 2907A PNP transistor with a 2N3906, and it worked just fine. For symmetry, I then replaced the 2N2222 with a 2N3904, which also worked. So, the choice of transistors does not appear to be all that critical.

Finally, since I think inverter chips are more common than quad dual-NOR gate chips and almost every NOR gate on the 4001 is wired to make it just an inverter, I replaced the CD4001 chip with a CD4069 chip. To retain the steady versus high/low selection feature, the first gate needs to be made into the equivalent of a NOR gate, which can be done by using diodes to OR the input to the inverter. The input needs to be pulled down to assure that the inverter outputs high by default. Otherwise, I think the inverter circuit is simpler and a bit easier to wire since most of the connections are made to one side of the chip. Also, on a chip with six inverters, it gives you two more to play with for other purposes. Here is the schematic using a cmos hex-inverter chip (yes, I built it and it works):


In prototyping these, I did not use the circuit tracing from neatcircuits. I’d note that the circuit trace on that site differs slightly from the schematic on that site – the output is taken from the third rather than fourth NOR gate, but since the third and fourth are symmetric, they are interchangeable so this doesn’t matter. I doing my layout, I did the same, since the physical placement works better. Here is my layout using pencil and paper version 1.0:


The Hound

The hound is a high-gain pickup circuit centered around an FET input and amplified by a LM386 operating at the default 20-fold gain. Again, I substituted more readily available, standard value parts. I did not have 22M resistors, but found that 10M resistors work fine – I doubt that at the extreme high end of resistance the value is all that critical. For the resistors on the drain and source of the FET, I substituted 270 ohm for 260 ohm, and 220 ohm for 240 ohm. For the FET, I did not have either of the ones mentioned in the write up, so I used a J310G. Here’s my circuit and layout using the substituted components:


In packaging the fox, I brought the output to a BNC jack since I have a few BNC-to-alligator clip test leads in the workshop. Out of habit, I used some RG174 to take the signal from the board to the BNC, but of course at audio frequencies any wire would have worked. I used some nice switches for the on/off and tone selected. A slider switch would have made more sense for the tone select, I suppose, but I didn’t have one and it would have made it more difficult to put in the case. I added an LED to provide some indication of whether the unit is on, since replacing the battery means unscrewing the case. In retrospect, a very useful modification would have been to add an auto-powerdown to shut the power off after an hour. It would be annoying for the fox to shut itself down too soon, but an hour should be plenty of time and the feature could save the battery.


The hound was built into a pen case that snaps open nicely, but stays shut otherwise. The speaker was harvested from an old pair of computer headphones and hot-glued in place. An insulated wire was brought through the case and wrapped on a nylon screw and then taped with copper tape to form the probe tip. I also brought the ground out the side in case I want to ground the unit, but this shouldn’t really be necessary. The hound is turned on momentarily by push-button, so I am not too worried about the battery running down (also, it’s easy to open the case, so I think I will store it without the battery).



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