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Old 06-05-2002, 02:15 PM
Daless2 Daless2 is offline
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Collecting Design Requirements Remote For Radio Stethoscope

Hi folks, it appears I have screwed up.

This morning I put a post out on the Technical Archives Forum.

Here is the link


I was not aware that only the original poster and a moderator can reply on that part of the JeepBBS forum.

So, if you don't mind I will start a new thread here.

If you'd like click on the original link so as to read the handful of comments there from Blaine and Dan-H.

Here is the original post:

Collecting Design Requirements Remote Radio Stethoscope

Hi Folks, I want to build something that I think will be useful for many of us; a remote, radio transmitter stethoscope to be used when trying to diagnose where a sound is coming from under a vehicle.

The idea is to strap this little transmitter (2? cube) to an under body component using a velcro strap.

You would then tune your FM radio to the transmitting frequency and drive away listening for the sound on the Jeeps radio.

It may take placing the transmitter in a few different spots, but I do believe it would work to get down to the component that is causing the noise.

You folks want to give me your input on this?

Here are my design requirements so far:

  • Small, target is a 2-inch cube.
    Easily reproduced circuit using minimal parts. (If you can solder you can build.)
    9-volt battery for power.
    Able to receive stethoscope signal on Jeeps FM radio (87.9 or something like that.)
    Able to easily mount on any non-moving under jeep assembly.
    Somewhat watertight, if not waterproof.
    Under $10 in cost.

I have a friend, Chris B who is a new Jeeper from near Cleveland, Ohio. (I have also invited him to join this forum.)

Chris is not only an Electrical Engineer, but he is one of those really smart ones too! He makes his living designing chips that control CT/CAT scan machines.

Once I collect your thoughts on how this thing should function and what features it should have I am going to enlist Chris?s help in designing the remote stethoscope.

Please share your thoughts.


PS: Blaine, this give me an opportunity to fix my "fiend" error too!
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Old 06-05-2002, 02:18 PM
Daless2 Daless2 is offline
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Here are Dan-H's thoughts from the first thread which I had cut and pasted in.


for some reason the BB won't let me post in your thread on the stethoscope:???: If you want you can copy this into the thread.

Here are my rambling thoughts.

how will the microphone work? will it be protected from wind noise? Will it pickup noises transmitted via vibration into it or will it just listen for noises.

I'm interested in trying one but I'm curious how it would filter out other noises like exhaust noise, engine noise and wind noise.

I'd be happy to test one for you since I have an annoying squeak somewhere on my rear axle when going over speed bumps at about 10-15 mph.

Also, will the transmitter clip to the antenna? or will it have enough power to broadcast. Is a license needed for this? If so, what about FRS frequencies. Then a set of head phones and this could make it useable anywhere.

Thanks for sharing the idea.

- Dan"
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Old 06-05-2002, 02:19 PM
Daless2 Daless2 is offline
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Thanks for your thoughts Dan (It almost feels like I am having a conversation with myself here. )

OK, here's what I have so far.

I have a circuit, and a prototype that Chris built and will be sending to me to play with.

I am not sure how this unit will work in regards to housing or how the mic will be set up.

I'd like to fit it all inside a small box. Absolute max size maybe like a cigarette pack, but preferable a lot smaller. (Have to take into account the big things like a battery.)

The mic will be a tiny thing. I will first try it mounting the mic in the box and then you would point the mic part of the box toward or against the assembly you want to check. If need be I can put a small piece of foam over the mic to eliminate wind noise like they do to eliminate an entertainers breath.

Don't know if this will work or not till I play with it.

Another thought I have is to mount the mic inside of a flexible hose, maybe 12-inches long. Then the box could be strapped down where convenient and the mic point at what you want to listen to. (i.e. Drive shaft u-joint)

The hose would certainly help in making the mic far more directional too.

There will be a small, probably 8-inch wire hanging down from the box. This will be the transmitters antenna. Nothing will run from the stethoscope to the Jeeps radio antenna accept for the radio waves.
(no wires)

As for licenses, there will be no need for that. This transmitter will only put out miliwatts of power. Basically the range of it will be something in the order of 50 feet at most.

FCC don't care about things like that. just like a baby room monitor or a garage door opener. But I will double check the FCC regulation on this.

It could be tuned to transmit inside the FRS bands, but then it would require someone to have one of those radios. my current thinking is that most vehicles would have an FM radio in them. And if they didn't then any portable (walkman) would do the trick.

This right now is a half baked idea and certainly needs some work. In posting it I wanted to collect any ideas and have discussion like this one about it. I'd like this to be the best we can make it up front. But we can always go back and redo things as needed to.

So far with the circuit I have now the cost are sitting at about $1.56.

This does not include a box, battery holder, battery connector, circuit board and a few other items, but does include all the electronics (little parts and pieces) including the mic.

I feel confident it can be done for under $10.

Dan, after we play with this a little bit and fine tune it some I will make a few of them and would be happy to send you one to test and play with for me. Feedback would help a lot!

Might be a week or two before we are there but you can count on getting one.

Thanks for your thoughts.


PS: I think I cleaned up my mess a bit. Sorry folks.
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Old 06-05-2002, 03:36 PM
Dan-H Dan-H is offline
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I had a few more thoughts and sure don't want you to go stir crazy talking to yourself

I'm sure Stu or any ham can set us straight on what frequencies can and cannot be transmitted on.

Many years ago, the last time I went to the drive-in movies they had a setup where you clipped something to your antenna and tuned to a specific freq to get the sound. Something like that might work. I don't think a section of wire will be a problem for guys like me that buy cable ties by the carton.

[edit after re-reading your last post.]

The mike in a section of vacuum tube sounds like a good idea for keeping it directional.

Overall, it sounds like a great idea. Much better than me hanging over the side with one end of a 4 foot piece of vacuum tube taped in my ear and the other end zip tied to a broom stick while someone else drives over bumps.

- Dan
01 TJ sport
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Old 06-05-2002, 06:55 PM
dwells dwells is offline
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Off the top of my head... (not much else left up there)

FM "eavesdropping" transmitters are common DIY electronic kits. It's been a few years now but I recall them going for $5-10, crummy mic included. The parts are worth maybe 1/3 that.

How about a simple microphone to speaker deal? I don't mind dangling 12ft of spkr wire for a drive down the road.

Sounds are sneaky. They travel down material and seem to originate where they didn't. Mic placement could become tiresome. Strategies will change for different frequencies. I've gone so far as to use a tiny NTSC surveillance camera & a 13" TV in the passenger's lap to find clunks.

Keep us posted!

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Old 06-07-2002, 05:13 AM
Daless2 Daless2 is offline
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Hi Folks,

I have a very workable solution for a Remote Stethoscope to be used to help find under Jeep noises.

First a word of thanks to Dan and Dave for your thought provoking comments. It never ceases to amazing me how much better an idea can turn out when you put it out to folks to consider and contribute their thoughts on.

I went to Wal-mart yesterday and found the solution hanging from a metal shelf peg in the electronics department. At a cost of $8.64 each I purchased two ?Bellsouth Two Way 22 Channel FRS/GMRS Communicator? (Part number 2231BK).

Here is a front and side view of the FRS radios.

To say these two units do the job is a true understatement. I experimented all day and the better part of the night.

Here are some attributes:
  • These two units cost $8.64 each. (Wal-mart did have Motorola units that are weather resistant for $14.96 each.)

    Each unit puts out 500mWatts of power (1/2 watt), which is more then enough to transmit from one unit under the jeep to another unit inside the Jeep.

    These units have a VOX which is a voice operated transmit mode having 5 sensitivity setting.

    They also have a Lock Button, which when pushed disables all the front panel controls until manually released.

    There is also a phone jack for a remote microphone.

    They weight 3.1 ounces with batteries. (They are the same size as a Motorola Star-Tec cell phone.)

First thing I did was test them and learn how they operate.

I was quite surprised by the sound quality.

Then I set the channels and wrapped one unit in a covering of ? inch foam to pad it and insulate it from vibrations.

I cut a hole over the microphone area and then wire tied it to my front axle by the u-joint, turn on the VOX, locked the keypad and went for a ride, listening via the second unit!


I was amazed at what I could hear. I found a ticking noise I didn?t know I even had. Probably time to replace that u-joint.

For the next 5 or six hours I strapped that little radio on everything I could find under my Jeep; Control Arms, Differentials. T/case, Trans, everything I could think of.

The sounds you can hear will amaze you! I attached it to a rear shock and could hear the valve open and close.

You can hear clear as day shifting the t/case.

Yes you can hear a humming noise as well, like constant background.

On my front axle, I could hear the u-joint and the axle spinning but could clearly hear a ticking that was speed sensitive and became somewhat modified when turning corners as well. (Yup! U-joint)

I took it over to a friend?s house. He has a bad UCA bushing we are going to replace this weekend. I strapped the unit on the upper control arm and like magic you could hear the thumb clearly.

We then put it on the rear Detroit, and could hear it clicking.

Today I am going to experiment with some mounting systems for this. There is a belt clip on the backside that is removable. It might come in handy as a mounting point., else I will remove it.

I also want to make a foam sleeve, like a pocket that I can slip the unit into and then wire tie or Velcro it on where I want to listen.

I will also try one of my PC boom microphones. Maybe mount a magnet on the bottom of the boom to attach the microphone to the Jeep and use it to point at the drive shaft u-joints.

Let me tell you folks, this works, very very well. Far exceeding the sound quality I was expecting out of my own design. And for less then $20 for both units it isn?t worth it to build my own.

Please let me know of any ideas you have on how to best mount this unit on a Jeep for what you would like to listen to.


PS: I can see the future now: ?Blaine, here is a MPEG of my Jeep. Can you tell me what that sound is??
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Old 06-07-2002, 09:15 PM
Stu Olson Stu Olson is offline
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That was great.....the answer being right under one's nose the entire time!

I have a pair of them (different model made by Motorola) and will have to give them a try too.

I have a couple of raido pouches, much like you described, made from skin diving suit material.....they have a velcro closure on them and have a belt loop sewn into the back side through which a cable tie or similar could be used. They can be easily had from the local ham radio store or bought on-line.
Stop on by....
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Old 06-08-2002, 09:42 AM
Dan-H Dan-H is offline
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I sure am glad one of *us* thinks when provoked

I totally overlooked using an FRS out of the box. LOL, sometimes the simple solutions are so obvious I can't even see them.

I think I'll go pickup a cheap pair this weekend. A friend lost one of my Motorola's last year on the trail. So, having a low cost set can work also as my loaner FRS's and I won't worry as much about it getting lost or broken.

Great idea !

- Dan
01 TJ sport
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Old 06-08-2002, 02:46 PM
Stu Olson Stu Olson is offline
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I just checked my FRS radios....no VOX function available

William, could I ask a favor of you?
Stop on by....
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Old 06-09-2002, 07:36 PM
Daless2 Daless2 is offline
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Hi Folks,

I have figured out a means of mounting and protecting the FRS Stethoscope to my Jeep?s under-parts.

I sandwiched the FRS (stethoscope) radio between two blocks of black closed cell neoprene foam.

I believe this provides these benefits:
[list=1][*]Radio protection[*]Insulate from vibrations[*]Funnels, or channels sound to microphone[*]De-forms to shape of what it is mounted to[*]Provides a mounting block to strap to an assembly[*]Deadens other sounds you are not interested in[/list=1]

Here is a drawing of what I made.

Foam Blocks

I used closed cell neoprene foam that I had in my shop. Each block is 1.5-inches thick, 3.5-inches wide and about 5-inches long. The size you will need will depend on the physical size of your FRS radio.

Block ?A?

This block has an oval shaped hole cut all the way through it. This hole should be positioned above (in front of) the microphone of the FRS radio. This hole will act as a channel, or funnel for the sound to be directed to the microphone.

Block ?B?

You need to carve out a place for your FRS radio to sit in this block B. Put your radio on the face of the foam block and trace the outline with a marking pen. (A Sharpie works well). Then take a utility knife and cut the outline you just traced. DO NOT cut all the way through the foam. Cut only deep enough so that the FRS radio can be craddled within the foam.

With your fingers begin to tear out the foam where the radio will sit. Keep doing this until you can cradle the radio nicely in the indentation you are making.

If you ever plan to use the remote microphone function of your radio you may have to cut a small channel in the foam in order to plug it in. This will depend on where the jack is located on your radio.

As you can see in the drawing above, I put four Velcro tabs on the face of block B. There are four corresponding Velcro tabs on the backside of block A. These are used to hold the bocks together, with the radio inside, prior to mounting the FRS stethoscope under your Jeep.

How To Use

Set both FRS Radio to the (same) channel you want to use.

Activate the VOX (Voice Operated Transmitter) function on one radio. Also set the VOX to the lowest sensitivity level. (You will have t experiment on the sensitivity level for your own radio.)

If you have not already done so, remove the belt clip from the back of the radio.

Place the transmit radio inside the pocket you made in foam block B.

Place foam block A on top of the radio and block B. Press the two together till the Velcro grips.

Mount the FRS stethoscope on the underside component you want to listen to. Please makes sue you have the clearance needed.

I used Velcro straps, bungee cords, nylon straps, and even bailing wire to attach the unit.

Try to press the front of foam block A against the component you want to listen to, in such a way that the foam de-forms around the object. This will seal out other noises.

Start your vehicle, drive it and listen.

If you are not satisfied that you have found the location of your noise simply relocate the FRS Stethoscope and try another location.


The closed cell neoprene foam I used for the block-mounting system held up very well to the temperatures under my Jeep. I mounted the block to axles, differentials, control arms, floorboards, skid plates, t/case, side of transmission, and the bell housing.

I would however be careful not to put it too close to the exhaust system. While I had no problems, I am sure this stuff will melt.

I intentionally dropped one FRS Stethoscope, (wrapped in foam) out of my Jeep on a country highway at 45 MPH. I wanted to see what would happen.

What happened? Nothing!

Radio survived with zero damage.

Even the foam looked good, but boy did it bounce down the road. Looked like one of the super balls jumping all over the place.

I still do not have an adequate remote microphone system. I took a Computer Microphone with one of those small booms and mounted it to an old CB radio antenna magnetic mount. This worked, but the boom was not flexible enough to bend it into positions I would want to listen to (point at) driveshaft u-joints. Any ideas?????

Folks it takes a lot more power to transmit then it does to receive. If you are using this system and your batteries on the transmit radio are getting weak, simply swap the radios, or the batteries with the receiving radio.

Heat is generated while transmitting. While I did not see any negative effects of this, with the radio encased in the foam, you should be aware of this.

Bottom line

I am very please with how this work, yet I still need to work on the remote microphone solution.

For less then $20 total cost I believe this is a great diagnostic tool.

Plus, when not used for as a remote stethoscope I now have two FRS radios to use for their original purpose when I need them.

Any comments or suggestions are welcome and actively being solicited.

I will be doing a small write-up on this in a few days.

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Old 06-10-2002, 04:50 AM
Daless2 Daless2 is offline
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Good Morning Folks,

Stu, Dan, Dave, Blaine, anyone else.

I have quite a bit of this closed cell neoprene foam.

If you have an interest in using this and playing around with it for your radios I would be happy to send you some.

The foam I have is 1.5x4x24-inches. (I'd probably cut it in half to make it easier to ship.)

Let me know. I'd be happy to send some out to you. (It's not so easy to get.)


PS.: Here's a question? How do you package and ship "Foam" to keep it protected?
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Old 06-10-2002, 06:04 AM
Tumbleweed Tumbleweed is offline
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Frank, don't know if this will fit your radio or not. I got a remote, speaker/mic. from radio shack that fits my Motorola FRS for about $9. Works well and has a clip mount. Used to use on ATV with radio in pocket and mic. on collar. Could hear it with helmet on. Might work for you??
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Old 06-10-2002, 07:31 AM
Daless2 Daless2 is offline
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Hi Don, thanks for the tip. I will have to check that out.

What I am trying to do (Cheaply) is find some means of mounting a mic on the end of a flexible shaft. I saw these flexible microphone tubes, but they are expensive ($12).

When I get some time I am going to try 3/8 inch fuel line, about 12-inches long and mount the mic inside, about 1 inch from the end. I am going to try putting some coat hanger wire inside the hose to act as a spine.

I am hoping I can mount this to the magnetic mount I have and be able to form and bend the hose to point and stay where I want it to stay.

In reality I think I could hear if I had a bad U-joint just buy mounting this on the axle tube, t/case or differential as it is. But I would like the added flexibility to really hone in on it.

I have a friend coming over this morning. He has had a noise under his jeep for the last few weeks that he has not been able to find. We're going to try it and see what we come up with using the FRS Stethoscope.

Do you need any foam to make a system for yourself? Let me know. I'll ship you some.

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