Fun project

[Anonymous]

Hi All,
One of the things I would like to do is make my GQ clone as light and easy to use as possible. Presently, I have everything shaft mounted and am using Li Ion batteries to reduce the weight. This setup works well, but there is one thing that still is a problem and it is the cord on the headphones.
In the field or when moving from one place to another, if one isn't careful, the headphones will almost always tangle with something. It doesn't matter which headphones I use, as long as they have wires, the problem exists. This has always been a problem.
A good friend and fellow hunter had a saying that if he ever fell out of an airplane, the one thing he wanted to have with him was his headphones. He figured he would never hit the ground since the headphones would tangle with something before he got there.
Since I seldom fly, I figure that if I could eliminate the cabling problem, that would be one more step in building the ideal detector. So, I decided to try to find a satisfactory wireless headphone setup. This way, there would be nothing to tangle. The problem was, just what design would work with a PI.
After a few attempts trying different FM types and ending up with a lot of noise, I tried a couple of IR models. The first attempt didn't work, but one model did and worked really well. The model is the Koss HB 70. The unit is compact,easy to mount with Velcro, and works extremely well, considering the noisy environment.
What really surprised me was the how well they seemed to work when moving the control unit all over the place. I did end up with a lousy signal when I placed the control unit behind me, which normally will not happen, but outside of that, I couldn't get the unit to fail to provide a clear signal even though they are supposed to be line of sight. In fact, I deliberately tried to get a lousy signal, since I just wanted to see how well it would work and how forgiving it was form lateral movement.
Now, the next decision was to decide just how to power this new setup. Rather than tie into the powersupply of the PI which might introduce noise, I decided on using an external battery and ended up using a surplus LI Ion setup.
Although the transmitter power is normally 12V I found it would work reasonably well at about 7V so two light weight LI Ion cells were tried and seem to work just fine. The sensitivty is great and the only real down side is very strong target signals are a little raspy because of the lower voltage. Weaker signals are extremely clear. I might increase the voltage later but right now, I am just trying to find the best and lightest setup. There are some li Ion polymer batteries that are lighter and I just might try them in the future.
Reg

 

[Anonymous]

Reg,
If you want more fun, try matching the low output impedance of your detector to the 10K ohm input impedance of the Koss HB70 transmitter through a transformer. You could obtain gain with no noise, given the 10K ohm input impedance of the transmitter module. For optimum performance, look for a miniature output or input transformer 50 ohms to 10K ohms. That should give you a voltage gain of about 14. Other step up audio transformers in your junk drawer will also work just to quickly see if better matching provides any benefit.
To find the turns ratio (equivalent to voltage gain) from published impedance specifications just divide the square root of one winding impedance into the square root of the other winding impedance.
Along the same thought path of using IR headphones, look at this. The Korg PX4 is a cigarette pack size digital guitar sound processor that also has an IR output for wireless IR headphone use. It is powered by 4 AA batteries. An added benefit is that you can play with the compression and expansion and other guitar effects related to gain, save your settings, and do some serious post processing of the metal detector signal to squeese the maximum gain out of the smallest whimper. It even has an adjustable noise threshold to go silent just above the noise floor. The unit has a belt clip which makes attaching to the metal detector very easy. A 6" to 12" 1/4" plugs on each end of shielded cable is all you need to connect it to the headphone jack.
The smallest and lighest RF transmitter I have ever seen is a small AKG UHF guitar/bass wireless transmitter bug that is powered by one 1.5V battery. The 1/4 inch plug is just inserted into the metal detector headphone jack. It has a companion UHF receiver but that is much larger and not necessarly meant to be portable but could be with batteries. The fidelity is very high but so is the price. Now if that same type of transmitter could be used with a small, low power, low cost transmitter and receiver, that would be the real deal for metal detecting.
The closest commercial RF alternative that might work on your metal detector is a wireless microphone transmitter module and receiver module used on portable camcorders. The transmitter module is about the size of a pack of smokes. I believe the receiver works on a single 9V battery in the VHF range and is about the same size. I had a unit made by Azden many years ago when I was doing TV productions. Some newer models have an automatic squelch to minimize noise.
Have Fun Hunting
bbsailor
Shades of Spring are around the corner. Next weekend is the Philadelphia Sailboat Show.

 

[Anonymous]

Hi bbsailor,
I looked at the Korg PX4 and it looks like it can do a lot, but I will stay with my setup for the time being.
As for your suggestion to impedance match, I am sure it would increase the gain, but the Koss already introduces a fixed gain/ and has adjustble volume. So, I already do see a gain increase.
Since I have added a gain control to my GQ clone, I have more than enough gain. In fact, with all at max, I have too much now.
The key objective of this project was to keep it as simple as possible and still be able and have a wireless headphone setup,all for a very reasonable price. So far, the heaphones, cost $35 shipping and all (Ebay used), the battery, $6.50 (surplus), the special Li Ion charger, $4.50 (surplus), and a little wire and connector for the battery. I did add a connector in the charger so I can just plug in the battery when I want to charge it.
The battery and transmitter are attached with Velcro so removal or installation is quick and easy. Both are light weight so there is very little additional weight added.
As you can see, the transmitter is quite small and the battery is reasonably small also. It is mounted under the GQ clone control unit. I have the transmitter mounted at an angle for best transmission. I might raise the transmitter a little before it is all said and done, to assure a good link . Right now, I have not encountered any problems, but that is on even ground also. So, raising the transmitter should help in case problems arise when hunting on a hill or in a hole.
Reg

 

[Anonymous]

Hi Reg and BBS,
Interesting experiments. I think IR is safer than a radio link. R.f. transmitters could cause noise and feedback problems, although the frequencies used are well outside the bandwidth of the detector. This is because input protection diodes demodulate any rf that gets in and you are then left with the modulation, which is the audio that the detector is outputing. Could result is a very wobbly threshold.
Eric.

 

[Anonymous]

Hi Eric,
I never really did try the RF transmitters much. There was simply too much noise from the start.
Now, as an update to using IR types, Radio Shack has their IR headphones on sale for $19.99, regular $49.99. Because they were so cheap, I gambled and tried a set of them to see how they compared to the Koss. They were almost Identical. They look different but they sure act the same. One advantage the RS models have is the batteries for the headphones are rechargeable and can be charged using the standard power supply that comes with the headphones. So, overall, they appear to be a better setup.
Anyway, at $20, this drops the price even lower for those interested in trying to build an wireless headphone system.
Now, as a mod, I am considering mounting the transmitter vertically to give it added height. This might take care of the few "blind spots".
Reg