Wireless Headphones...

Yesterday, I picked up a couple pair of the Radio Shack Wireless Headphones P.N. 33-283 to try on my CZ3D. I tried them out and they work really well for "me". The ear cup is too small for good sound isolation though. They don't fit over the ears, just 'on' the ears, and might become discomforting after several hours. The headphone unit uses two aaa batteries which can be rechargable or not. The small transmitter unit has a non-replaceable rechargeable battery inside. A recharging wall wart comes with the set and will charge one or both units at the same time (if you use rechargeable batteries in the headphones).

The transmitter unit, which is about 1.5" X 1.5" X 1/2" or so, has a 1/8" male plug on it which is usually plugged into a mp3 player or such. Plugging it directly into most detectors will require a 1/8' to 1/4" adaptor which leaves it hanging out of the headphone jack of the detector and could be easily damaged. Most people I've read about get a short cable of about 8 to 12 inch long, 1/8' female to 1/4" male, and mount the transmitter on the top or bottom of the detector housing via velcro straps. I dislike extra things hanging on to my 'stuff' and so am concidering an alternative solution.

I took the unit to work today and was able get the transmitter apart by unsnapping it. Inside the housing is a small pc board with numerous tiiinnnnyyy components and a somewhat unusual looking flat battery pack. I measured the voltage of the battery and it was 4.16 v. with a fresh charge. I figure it will settle down to about 4v. after a few minutes of use.

I plan on mounting this tiny transmitter inside my CZ3D control box (there's plenty of room) and using a variable small voltage regulator to draw 4v. off of the 2 nine volt batteries that run the detector, thus eliminating the proprietary rechargeable battery. Reports are that the original battery will run the transmitter for 10+ hours (package says 5 hrs.) and so I shouldn't see much of a difference in detector time before having to change batteries. I'll probably have to mount the transmitter between the sheilding and control box and may place it where the small speaker is on top. I should be able to wire it so that it comes on when I turn on the detector, so no additional switches. The input signal for the transmitter will probably come from either the output jack terminals or the internal speaker. The speaker is rated at 100 ohms @ 1/4 v. I don't know what the rated transmitter input is supposed to be yet. It would probably be safer to come off of the phone jack. Will take some measurements tomorrow.

If anyone is interested in keeping up with this mod, let me know and I'll post my progress. I stay pretty busy at work and so it may take several days or even weeks to finish up the job, but hope to have it ready in time to take to Myrtle Beach soon. If it works, I may concider mounting the receiver unit into my Koss QZ99 headphones. Time will tell....

Wish me luck...

C.



Edited 1 time(s). Last edit at 09/10/2011 03:08AM by connortn.

One thing to watch out for: the headphone output may not be referenced to any of the obvious internal power supply voltages. For example, the +ve 'phone connection might be a regulated +6V, the -ve 'phone signal driven to 0V via a 200 Ohm resistor (every detector is different - as the output is only ever designed to drive 'phones, they can put whatever voltages they like on it, as is convenient).
The small TX battery sounds like a 'foil-pouch' type Lithium-Polymer type (Li-Po) with a capacity probably less than 150 mAH. These will have a freshly charged voltage of 4.2V, falling to 3.4V when nearing the end, and an absolute useable limit of 3V. It's probable the TX will have a regulator (3V) for stability, so provided you generate something in the 3.5 to 4.5V range you should get results.
One thing to watch out for - the detector might not care for having a radio transmitter operating right next to it, even if it is only a milliwatt. It would probably be best to fit the TX on the elbow-cup, with its battery, but properly wire up the various connections.
Good luck.

Pimento: After inspecting the circuit of the headphone jack, it appears the speaker is picking up its power via the headphone jack. If a pair of headphones are plugged into the jack, the jack opens the speaker circuit and feeds the headphones, so whatever is driving the speaker is driving the headphones. With this in mind, I should be able to just use the speaker leads to wire directly into my Auvio wireless headphone transmitter, as the transmitter works fine plugged into the headphone jack. Being that the speaker is wired directly off of the ph. jack, there shouldn't be any difference. I will still be able to insert some wired headphones into the "phone jack which will disconnect the transmitter and feed the wired 'phones.

I would think you're right about the battery being a 4.2v It has the foil around it as you said. I picked up a variable voltage reg. today and will look into what I need to do to replace the internal transmitter battery. It looks like the neg. of the two 9v. batteries go to the on/off vol. switch. Looks like I can pick up directly off of the 18v and run to the voltage reg. and through the on/off switch to turn the transmitter on when I turn on the detector.

I'm hoping to replace the speaker with the transmitter. I may see some interference and may use a removable lid 9v battery holder to hold the transmitter unit and add some shielding. I will probably make a new speaker cover to hold the 9v. battery holder.

Thanks for the input!

C.



Edited 1 time(s). Last edit at 09/08/2011 08:14PM by connortn.

Pimento...

I picked up a NTE1900 IC adj. voltage regulator today. I went to: [diyaudioprojects.com] and found their "Typical Voltage Regulator Schematic" for the LM317, LM338 and LM350 regulators. I think this schematic will work for the NTE1900 also. If you think otherwise, please advise. I may not need C1 (0.1uF) or C2 (1uF) being that I'm not using this in a high end audio application, and also using DC.

Using their Voltage Regulator Calculator, I put in 1K ohms for R1 and 2K ohms for R2 which gave me an output voltage of 3.75V. I can bump it up a little if you think 4V would be better. Evidently the ratio between the resistors is what determines the output voltage. If I put in 500 ohms for R1 and 1000 ohms for R2, I get the same results... 3.75V.

I'm not knowledgable enought to figure out the mAH with these specs. You mentioned above that the internal battery in the transmitter probably has a capacity of less than 150 mAH. The spec sheet for the regulator says "capable of supplying in excess of 100mA over a 1.2 to 37V output range" If you would, please check the spec sheet for the NTE1900 for me and see if you think this is workable using the diyaudioprojects voltage regulator schematic.

Thanks...

C.

It looks like that regulator has the industry standard 1.23V reference, so 2K and 1K resistors in the feedback cct will give 3 x 1.23V =3.7V. This should be OK, a bit higher wouldn't harm.
The current consumption of the TX will be less than 5mA, I'm sure, as it runs for many hours on the small battery. So the 100mA max current of the regulator is fine.
I still think you may have misunderstood my comments about powering the TX from the main detector supply.Unless the audio out of the detector is referenced to one of the main battery voltages, simply powering your TX from the main battery is likely to not work. I'll see if I can find any info on the detector audio cct that may help.

Edited....

On the way home from work, I realized the mistake I had made in this post and deleted it due to possibly leading someone astray. ....that, and didn't want to show my ignorance too much :-)

C.



Edited 1 time(s). Last edit at 09/09/2011 09:58PM by connortn.

I found a cct diagram for a CZ5. That, conveniently, has the speaker negative going to circuit ground (battery negative), the speaker positive thus has the audio signal on it, which is what you need. But your machine may be different...

On the headphone side of things, you could easily remove the electronics, re-house them in a small plastic 'project-box', adding a 3.5mm jack socket (or 1/4 inch) to suit your preferred phones.

Maybe Tom can check his diagrams and see if both the CZ5 and CZ3D have the speaker negatives going to circuit ground. I didn't misunderstand your comments about powering the TX from the main detector supply earlier. My problem was that I didn't understand it at all :-) ...still don't, so I'm glad you at least found out about the CZ5. As you have probably deduced, electronic knowledge is not my forte'. My father and brother were/are 1st class FCC license holders from back in the days when you really had to know your stuff to have said licenses. I leaned toward the mechanical instead.

I talked with my brother tonight via phone and he recommended breadboarding the voltage regulator with variable resistors to nail the exact specs I was looking for, then replace the variable resistors with fixed ones. Sounds like good advice so I'll start on it asap. I'm pretty confident this will work once I get the variables worked out. I'm still waiting for my Koss headphones to show up, and will look into getting the receiver from the Auvio headphones into the Koss'.

I appreciate your discussions on this project and will keep everyone informed as to how it works out.

C.



Edited 1 time(s). Last edit at 09/10/2011 12:16PM by connortn.

No need for variable resistors, fixed is easy enough - you need the two resistors to have a ratio of approx. 2.3 to 1. This will give you about 4.0V. Examples include : 1K & 2K2, 1K & 2K4, 1K2 & 2K7, I'm sure you get the idea.....
If you have a multimeter, you can check for continuity between speaker -ve and the battery -ve.

Stayed over at work last night and finally completed the headphone mod. I removed the small rechargable (3.7V @ 270mAH) battery from the circuit board. There was a blank PC board area under it about the size of a postage stamp. I used that area to drill and mount the very small IC voltage regulater (would remind you of a small transistor) a 510 ohm resistor and one 1K ohm resistor. I spliced into the -18V battery lead that came from the battery compartment just before it plugged into the detector circuit board. I picked up the +18V on the on/off switch so the TX would come on when the detector was turned on. After soldering all the leads togather and checking everything out 3 or 4 times just to make sure I was getting 3.7V, I soldered the -ive and +ive to the original TX battery inputs and Viola!

I picked up the audio from the speaker leads and everything worked great with pleeennnntty of volumn. I have only been able to test it a few minutes outside this morning and I believe it's going to be a winner! About the only interference I can detect is when pinpointing. I can hear a slight warbling of the tone for some reason. I noticed a loss of high freq. in the detectors output thru the 'phones, but being that I negelected to test the headphones very well in the field before the mod, I don't know if it's a side effect or if they were that way to begin with. I'll report my findings as I use them more.

There is a good bit of spare room inside the detector housing, but by the time you get all the wires running around and the TX IC board inside it, you have to be careful while putting everything back inside the box. I used velcro and attached the TX PC to the area the speaker normally goes.

One thing that puzzeles me is that the detectors headphone jack is a switching jack which disconnects the audio to the speaker (or the TX in my case)when a headphone jack in inserted into it, and I see no way to feed a stereo signal to the headphones. I'm going to assume that the audio output of the detector is mono, and the phone jack is wired to give the same audio signal to both the L and R any time earphones are normally inserted for listening. Being that there is only one speaker in the CZ3D, and I used the leads from it to send audio to the TX, I had to connect the L and R togather on the TX board. When I tried any other option, I only received sound from one ear piece. Maybe Tom can enlighten us as to how the headphone jack works if it truely is giving a stereo signal.

I like not being tied to a wire to my headphones, but I don't think I would do this mod again. You're confined to very small spaces and you have to watch very carefully with wires floating around. During the process of 5+ hours, (I worked sloooww and carefully) I always kept the batteries out of the detector unless I was testing something, then I would immediately take them back out to prevent an accidental short, thus ruining my $$$ investment in the detector. It's probably better to hook everything outside as others have done, and if the headphones or TX go dead, you can just unplug it take it off. I would have to disassemble mine and unsolder everything, and put it all back togather.

It is neat though! No strings (wires) attached :-)

C.

Your detector has a mono output, like just about every other. There are two possible ways that stereo 'phones can be driven with a mono signal, however. (A): connect the two earpieces in parallel, ie. join L and R together, and feed the audio signal to both, relative to the 'ground' of the 'phones. or ( feed the audio to the L channel and ground the R channel, leaving the 'phones ground unconnected. This uses the two earpieces in series connected mode.

Glad you got it working. Some of your remaining minor problems may be due to you feeding too large a volume into the TX inputs. You could try reducing the level with a 'two-resistor' divider. Trial and error come into play here, but for starters: Connect the TX audio input to ground with a 510R resistor. Connect the TX inputs to the speaker drive signal through a 2K2 resistor. This reduces (to 20% approx) the audio drive level. Then fiddle around with resistor values.

Have fun.

Pimento says: ..."feed the audio to the L channel and ground the R channel, leaving the 'phones ground unconnected."

That's exactly what I observed while inspecting the 'phone jack connections. I had never run across it before and didn't trust what I was seeing. I don't know if there's any reason for one over the other, but I may change it to work this way instead of connecting the L/R togather, if only because that's the way it's wired at the headphone jack.

..."Connect the TX audio input to ground with a 510R resistor. Connect the TX inputs to the speaker drive signal through a 2K2 resistor. This reduces (to 20% approx) the audio drive level. Then fiddle around with resistor values."...

Let me make sure I have this right ...desolder the positive speaker lead from the TX and insert a 2K2 resistor between it and the headphone L/R "in" on the TX (being that I have the L/R connected togather at this time). Then to add a 510R resistor from the +ive speaker wire to ground also, so that there will be two resistors on the +ive speaker lead, ...one to ground and one to the L/R 'phone connection on the TX?

Hope I said what I meant :-)

I think you are probably right about this also, as I'm familar with overdriving the audio from the 'younger' generation on guitar amps at work. We used to pay good money to make audio 'clean', and the younger generation pays good money to make audio 'distorted'!

I'm not sure 20% will be enought as I'm having to leave the headphone audio level control almost 'off' to keep the volumn down.

Thanks...

C.

The L and R inputs to the TX module are joined together, and are joined to two resistors. The first is the 510R to ground. The second is the 2K2 to the speaker +ve wire. It's a 'potential divider', Vout = Vin x 510/(510+2200) = Vin x 0.2
Seems you may need more reduction, try reducing the 510R downwards, maybe even to 10R, whatever seems to work.

The AUVIO 33-283 Portable Digital Wireless Stereo Headphones works just fine on every detector I've tried,..........Except my Gold Bug Pro, when fitted on those the audio sound is very faint to the point where it's a strain to hear.

If anyone have the answer as to why it would be much appreciated.

ivanll

It could be related to my earlier comment, some detectors drive both earphones in series. Your wireless TX will be wired to accept signals relative to the ground of the jack plug. It should be possible to make an adapter cable up, going from 1/4 inch to 3.5mm with the wires 're-routed'.

An adapter with a resistor installed and the Auvio wireless H-Phones is working just perfect on my Gold Bug Pro.


ivanll

I am SOOOO skeptical about inducing 'silent' EMI; subsequently, (unknowingly) reducing performance. Different frequency or not........ connecting a transmitter directly on the control head/electronics hardly seems prudent.

Moving the transmitter to the elbow cup makes a significant difference to the amount of RF picked up by the electronics, and would be my personal choice.

Having said that, top end XP machines have built-in RF transmitters, the circuitry is on the main PCB in the control box. Presumably as this is designed-in, it was engineered in a way that eliminates pickup problems.

It also helps a lot if the transmitter is low-power. If the range is described as '10 metres' or '20 feet' then it's probably suitable. If it boasts '100 foot' or '100 metres', it's more likely to cause problems. Radio range is also determined by the reciever sensitivity, but I assume none of these units have exotic recievers to give the extra range.