Topic: DIY key fob battery box

[Nick_Riviera]

Hey everyone!  First post here.  I've been kinda lurking for a while and finally felt I had something to contribute to the site.  I just got into taping relatively recently, motivated by some tapers you might know by name, and have been working on my setup while going through some trial and error at a few shows.  Right now I'm working with an Roland R-07, a mix of DIY and "commercial" battery boxes, and a pair of Sound Professional omnidirectional binaurals.

I decided to build my own key fob battery box so that I could have some custom specs that match the rest of my set up, and I went a little overboard and made a few so that I could have a variety to choose from (roll-off, no roll-off) if I were to find myself in different recording situations or changed my mics up, etc.  Note that I had very little experience working with electronics before this.  Just motivation, time, and a little extra money

My next project will be a pair of cardioid mics from scratch.  If anyone can point me in a good direction for the best way to mount/use a set of cardioids, that would be awesome - I think I have a good plan for making a pair!  I'm having a hard time finding something on the site.

Here's what I used:
1. A 2020 Kia Soul key fob.  These can be found online for $20-30, and they're fairly simple to strip and make room for the battery box guts.
2. Capacitors - I bought a kit with a wide selection, but I wound up using either a pair of 0.22uF, 1uF, or 2.2uF, each for a different fob.  These variety packs can be found online for ~$10.
3. Resistors - I'm using 9.1Kohm for this project; online for about $6
4. Battery holder/terminals - I'm using 12V batteries (which should give ~40-50hrs recording time), and found a set of 10 holders online for ~$8.
5. 3.5mm connectors - I used these from mouser because of their lengthy port that could get me through the fob wall: https://www.mouser.com/ProductDetail/490-SJ-3541A-TR-67, which ran me about $20 for 6 of them, shipped.
6. A $25 soldering kit found online.
7. Drill bits and a dremel tool
8. 2 part epoxy
9. Glue gun

First thing I did was strip down the interior of the fob using various dremel bits and a pair of snips, and cut off most of the key portion:



Next, I drilled out where I'd put the 3.5mm connectors using drill bits and forming with a dremel bit:


Then, I trimmed one side of the battery holder to give myself another 1mm or so room in the fob - I think this is probably necessary to fit the 3.5mm connector on the long side of the fob, unfortunately, but it's not too difficult


After that, I mounted it in the fob using 2pt epoxy and about an hour of time:


Now the hard part, soldering the connectors, resistors, and capacitors.  I used the schematic from the sony insiders forum here: https://forums.sonyinsider.com/topic/14343-how-to-build-a-stereo-microphone-and-battery-box/.  For this, I essentially started by soldering the capacitors to the 1st 3.5mm connector using the appropriate terminals.  Then, I soldered the resistors to the capacitor leads.  Then, I took some quick eyeball measurements, trimmed some of the negative leads from the caps and soldered them to the corresponding terminals on the 2nd 3.5mm connector.  Finally, I soldered a ground wire between 3.5mm connectors and left an open section in the middle without sheathing to allow me to ground the battery wire to it.  Basically, when I was done I had this (note the missing ground wire at this stage, I added that soon after):


Next step was to mount that in the fob and get everything in the best spot and making sure none of the connectors were touching.  I spent a good bit of time ensuring continuity where appropriate, and 12v+ and ground where appropriate.  Once I had it in, I connected both ground (to the common ground for the 3.5mm connectors) and 12v+ to the paired resistors:


Then, I checked continuity, ground, and 12v+ again before finalizing everything by securing the 3.5mm connectors with epoxy and filling the empty spaces with glue.  Can't stress enough how important it is to get all of that right before permanently mounting:


Here's the end product, a run of the mill Kia key fob...or is it?:


Last thing I did was trial each of them with my car stereo, using my R-07 and binaurals - so far, so good.  The roll-off seems to help a bit, but not sure how often I'll use that one.  Time and practice will tell.

Happy to help if anyone has any questions or comments!

[breakonthru]

nice work! 

[Gutbucket]

Right on.  Welcome to the message board and Go DIY!

A few ideas for you that you may or may not want to consider trying in future builds:

Might use locking mini-plug/jack connectors (they screw together) to assure non-intermittent connections in pocket.
Might use locking mini-XLR connectors for the same reason (I prefer these, partly because I'm routing four channels instead of two).
Alternately, you may wish to gaff-tape the connections to the battery box (and recorder) to help prevent non-locking mini-plugs from partly pulling out.

I very much prefer multiple connections to be located on the same side of the device and parallel to each other whenever possible.  That fits much better in a pocket, creates less lumps, gaff-tapes up more easily if needed, and puts much less stress on the jacks.

^ See a common theme so far? ; ) "Self-worn" rigs take a lot of handling stress over time and will benefit from being setup with smooth operation in mind, and made robust in the right ways to survive without failure.  A lot depends on your particular usage. The most appropriate strategies are likely to become clear with use, but best to set things up from the start to help avoid potential failure where possible.

Another option is using a single battery-box connection (usually via a multi-pin mini-XLR) that handles both in and out.  In that case the wires from the mics run to the multi-pin connector, then back out again from that same connector and on to the mini-jack connection that plugs into the recorder. This eliminates one connector and the separate wire making the connection between battery box and recorder. Looking at it, it would appear that there is only one wire running from mics to recorder, with the multi-pin connector located somewhere in the middle.  You only need to plug/unplug one connector at the battery box. 

[edit- its a bit more challenging soldering multiple connections on the back of the mini-XLRs than the mini-plugs/jacks though]

In my experience, managing wiring and connections is the biggest hassle and key to success.

[rocksuitcase]

Excellent idea and work. Thanks for sharing the DIY steps.
Welcome to TS. Having reliable components is key to long term success.

[ol' dirty taper]

Great build, I might try and make one this winter for fun.

[Nick_Riviera]

Great build, I might try and make one this winter for fun.

Definitely a fun project. Took me most of a morning once I had a strategy. I know there's a box out there that looks like a fob, but I wanted to try to match the specs I thought I might need, and I see these sorts of DIY things as part of the hobby.

Now that we've got some great feedback from @gutbuster, I might have to give it another go... Or at least have parts on hand for the future. I did try to plan for minimizing parts stress (and ease of remembering which plug was in and which was out, to be honest!), but I like the idea of a single plug. I'll have to consider that when building my new mics over the holidays. I'll try to see what I can find on mouser now that I have a better idea of what works and what doesn't.

Thanks everyone for the welcome as well!

[Nick_Riviera]

Right on.  Welcome to the message board and Go DIY!

Might use locking mini-XLR connectors for the same reason (I prefer these, partly because I'm routing four channels instead of two).

Another option is using a single battery-box connection (usually via a multi-pin mini-XLR) that handles both in and out.  In that case the wires from the mics run to the multi-pin connector, then back out again from that same connector and on to the mini-jack connection that plugs into the recorder. This eliminates one connector and the separate wire making the connection between battery box and recorder. Looking at it, it would appear that there is only one wire running from mics to recorder, with the multi-pin connector located somewhere in the middle.  You only need to plug/unplug one connector at the battery box. 

Looking into mini-XLR I was able to find some smaller forms on Mouser - about 12mm diameter is the best I could find.  Trouble is then finding a fob that can accommodate that width.  I think I found a potential aftermarket fob shell though; I've ordered one ($10ish) and will get better measurements when it arrives. 

I like the single connection the best - it would save a bit of space and in all honesty looking at schematics of e.g., this itemhttps://www.mouser.com/ProductDetail/Amphenol-Audio/AG5MCC?qs=t8VhaDIDl4uKhP%252BcF9idjg%3D%3D, the soldering would be way easier than the incredibly tiny, closely spaced tabs on the 3.5mm connectors I've been using.  One downside - it's easier to kinda conceal the 3.5mms, with dustcovers even.  The mini-XLR connector might be a bit more obvious.

Will report back on my progress

[Gutbucket]

I like your use of a key fob case.

A few details:

To accommodate a single in/out quick-disconnect for a stereo pair of 2-wire mic channels, you'll need 5 conductor paths through the connector (R in, L in, R out, L out, shared ground).

Both 5 and 6 pin mini XLR connectors use a pin arrangement with a center pin. For much easier soldering/assembly, use a 6 pin mini XLR rather than a 5 pin and leave the center pin disconnected.  Also, I like a male connector on the device side, which is easier to find in panel-mount style and probably most appropriate for this application, although the cable termination style is narrower.. but longer, which might fit better in a narrow fob but would need to be epoxied in).  A female connector (TA6F) on the cable side will be shorter than a male one (TA6M), making everything a bit more compact and reduce leverage stress on the connection and cable.

[breakonthru]

trrs plugs are a slimmer alternative to minixlrs. they even make trrrs

[Gutbucket]

^Will require 5 conductors to use a single connector for in/out. TRRS only 4.

[jefflester]

To accommodate a single in/out quick-disconnect for a stereo pair of 2-wire mic channels, you'll need 5 conductor paths through the connector (R in, L in, R out, L out, shared ground).

Core Sound uses only a 4-pin mini-XLR on their single connector battery box/mic plug.  R/L in/out and I guess ground is only carried in the shield/shell. Don't see a handy photo of this on their (his) site other than hooked up.

[Nick_Riviera]

I like your use of a key fob case.

A few details:

To accommodate a single in/out quick-disconnect for a stereo pair of 2-wire mic channels, you'll need 5 conductor paths through the connector (R in, L in, R out, L out, shared ground).

Both 5 and 6 pin mini XLR connectors use a pin arrangement with a center pin. For much easier soldering/assembly, use a 6 pin mini XLR rather than a 5 pin and leave the center pin disconnected.  Also, I like a male connector on the device side, which is easier to find in panel-mount style and probably most appropriate for this application, although the cable termination style is narrower.. but longer, which might fit better in a narrow fob but would need to be epoxied in).  A female connector (TA6F) on the cable side will be shorter than a male one (TA6M), making everything a bit more compact and reduce leverage stress on the connection and cable.

Here's the one I'm waiting on: https://www.ebay.com/itm/204334617353, it looks like it might have the width, though yes, it'd have to be epoxied I'm betting.

Appreciate the insight on 6p vs 5p - that makes sense. The one I linked earlier has a 6p variant as well.

[Gutbucket]

Core Sound uses only a 4-pin mini-XLR on their single connector battery box/mic plug.  R/L in/out and I guess ground is only carried in the shield/shell. Don't see a handy photo of this on their (his) site other than hooked up.

That's a good example of the arrangement.  Hmm, maybe does use the shell connection for signal ground.  Nice to have it on a pin connection though too I think.

[Sebastian]

This is a work of beauty! Congrats on the great DIY work and welcome to ts.com!

[Nick_Riviera]

Fresh prototype using a different fob and a 5 pin mini XLR.

Parts:

• Capacitors and resistors as noted above
  
• Key fob like this one, which conveniently comes in a 2 pack for <$15: https://www.ebay.com/itm/195806113383
     The fob mentioned in my initial post would likely work as well, with some caveats as mentioned below
• A 5 pin mini XLR: https://www.mouser.com/ProductDetail/523-AG5MCC, I'll mention why below
  

I started out by clearing out the guts just like in the initial post - removing all the excess plastic inside to make room for the battery and XLR/resistors/etc.  From there, I drilled using increasing bit sizes until I got to a 7/16" bit.  It would have been helpful to put the fob in a vise to keep the "box" from separating and making the hole easier to size.  The bottom portion of the fob can be removed, which is helpful in the process. 

The easiest part this time - unlike the original, was soldering the caps/resistors to the XLR connector.  Super easy with the pins laid out they way they were and of good length.
 
I plan on making a few of these just to get it right, and it took me all of 5 minutes to solder everything for three connectors. 

After drilling out the bottom and soldering the connector, I set it in the tunnel with a bunch of epoxy and clamped the bottom together to get it permanently epoxied.


Then, I superglued the buttons to the shell to keep them in place and eventually put it all back together.





What I like about this version: It took way less time to assemble.  The next two I do I will try to shift the connector a bit more towards the middle of the bottom fob face, so it sits more flush.  There's a screw hole (and that darn flip key) that is centered that I'd have to avoid, though.  I like that the bottom of the fob can be removed and replaced separately - I think it made securing the connector with epoxy easier, and permanently epoxying it I think will last quite a while. 

What I don't like about this version: The mini XLR stands out a bit more.  I bought some caps here: https://www.digikey.com/en/products/detail/menda-easybraid/34229/5892065, though I don't know if they fit.  They should make it a little less obvious, I hope.  Second, it requires you have a mini XLR setup on your mics.  I'm in the process of building a set of cardioids and plan to incorporate a line that I got from Amazon (https://www.amazon.com/dp/B09SWYF613), but it makes it harder to retrofit anything you might already have.  Last, the battery fit is a little tight.  The next two I'm going to trim some of the bottom of the battery holder to get it to sit just a mm or so lower. 

 I do think overall the XLR is a superior design, as @gutbucket suggested. I might have used a 6pin, but the 5 pin soldering was easy as it was and the line from Amazon was 5 pin - I could not find a premade 6p line on Amazon.

You might be able to use the Kia key fob above, but it might be harder to secure using epoxy and the fob shells are about 2x as much.

I'll post some pictures over Thanksgiving when I have the other two built.

[Gutbucket]

Very nice.  I'm considering building a four channel version myself.  Thanks for sharing the build here at TS.

Agreed that a small plastic cap on the miniXLR would make for an excellent finishing touch.

I think you'll also find the locking mini XLRs preferential in use, in part simply by eliminating the need to secure  connection with tape, even though converting to that connector required re-terminating the microphones.

[goodcooker]

The challenge with reterminating your mics is making them incompatible with other gear - like off the shelf battery boxes or recorder inputs - but if you plan to use this key fob exclusively and you have a backup in case it develops an issue you should be all set.

Nice work. This is really cool.

[Gutbucket]

Yes, good point.

But best to configure things in whichever way best optimizes for the primary usage of the rig, I feel.. as long as I've clearly determined what that is.  Of course I need to accommodate whatever inputs the recorder has.. unless modding that too, which is more extreme but has been done by some TS members.  I don't go that far.  My rigs used into handheld recorders use miniXLR up to the end of the signal chain where small right-angle stereo mini TRS plugs are needed to plug into the recorder.

To retain the ability to to use mini-jack equipped battery boxes or preamps, say as a backup or whatever, you can build short adapter cables to convert back to mini-plug/jack, and will only need to use them when using that alternate gear.  That way the primary configuration remains optimized, but you have fallback options.

For instance, I've set things up both ways in the past using the DPA miniature mics that feature a built-in cable and terminate to a coaxial microdot connector.  If I want to retain the microdot connectors on the mics I make up a female microdot to mini-XLR adapter cable (or to stereo miniplug or whatever).  Whereas if I'm setting up the rig for specific regular use and optimizing for that, I'll cut off the dots, trim the mic cables to the most appropriate length and re-terminate directly to my preferred connector.  In my current custom fitted four channel rig all four DPA mics terminate to a single multipin miniXLR and the four mic cables are of different lengths.  That eliminates adapters and connections as well as the need to loop and secure excess mic cable length.  But I waited until I knew I wasn't changing the rig further to do that.

If I build one of these with four channels it will be for use as a backup/higher-security-alternate to the mini-XLR input 4channel preamp I currently use.

[kindms]

just a suggestion or idea etc

Ive been looking at phone case with hidden compartments. they seem to manufacture several with the idea being a hidden wallet or etc for credit cards.

but id imagine an empty phone case that you are holding as you go through security filled with whatever would be a great option with more space

love the key fob idea btw

[Nick_Riviera]

Put together the final design for this, a version I'm most happy with.  Only thing I haven't been able to source is a mini-XLR cap.

Parts summary:

• ~2020 Kia Soul Key Fob shell: $18 (https://www.ebay.com/itm/285295208930)
• Capacitors: $10 for a set (https://www.amazon.com/gp/product/B08V5815K7)
• Resistors - I've tried both 6.2K and 9.1K: $6 (https://www.amazon.com/gp/product/B07QG1VWT7)
• 12V battery holder/terminals: $8 (https://www.amazon.com/gp/product/B09L87S74L)
• 5-pin mini-XLR male terminal: ~$15 delivered (https://www.mouser.com/ProductDetail/523-AG5MCC)
• Two-part epoxy: $8 (https://www.amazon.com/J-B-Weld-50139-Plastic-Adhesive/dp/B01IBOK7FE)
• And of course soldering equipment and supplies

I also built a couple of pairs of binaural mics, one set of omnis and a second set of cardioids

Parts summary:

• Omni-directional mics: $2ea (I bought 8 so I could have at least one set that matched closely - https://www.mouser.com/ProductDetail/665-POM-3535L-3-R)
• Cardioid mics: $4ea (bought 8 again - https://www.mouser.com/ProductDetail/665-PUM-2745L-HD-R)
• Mini-XLR 5-pin cable: $12 (https://www.amazon.com/gp/product/B09SWYF613)
• Monster 3.5mm cable: $10 (https://www.amazon.com/gp/product/B015RA5P0C)
• 20' of Mogomi 2697 cable for the mic stretch: $15 shipped (https://www.markertek.com/product/2697)
• A set of Ukes sunglass holders: $13 (https://www.amazon.com/gp/product/B07N75CYTL)
• And again, various soldering supplies and shrinkwrap

For these I basically followed the instructions at the sonyinsider site, which - when you don't have a lot of experience soldering, takes some practice!  Suffice to say there were a few screw-ups, a lot of learning, and once assembled I tested them using online frequency waves to make sure they matched well enough (within ~1db).  Then assembled the mics/cables to the Monster 3.5mm and then a split with the mini-xlr cable, making sure to feed the mics through the sunglass holders before welding everything together! 



All said and done, these were fun projects and the sound is pretty darn good.  Looking forward to catching a couple of shows here in the coming months.

[detroit lightning]

Great stuff - this rules!

[shadowfax1007]

I'm very interested in this project. Keen to do something similar myself. I exclusively use 3.5mm gear so I'll be keen to have a crack.

Has anyone else successfully built a key fob BB? Would love to see people share examples.

[mrfender]

I'm very interested in this project. Keen to do something similar myself. I exclusively use 3.5mm gear so I'll be keen to have a crack.

Has anyone else successfully built a key fob BB? Would love to see people share examples.

With some guidance from Nick, I've been in the process of copying his design.

Two things causing me to pause and rethink:
1) The pins on the back of the 3.5 plugs are tiny.  I bought some others that have a better separation of leads (meant for a break out box) but the collar of the plug is much shorter, about 2mm vs 5mm so I'm trying to see what I can do with it.
2) Discussion in the A10 thread that 12v battery power may be the cause of some issues so I was maybe looking at doing a 9v and 12v.  I typically run CA14s which, to my horrible ears, seem to like the higher voltage when using a SP 12v box.

  My progress:

[breakonthru]

Why don’t you sacrifice a male mini xlr and put the key ring thru it?

It would look like those common “breakaway” keysets

[shadowfax1007]

I'm very interested in this project. Keen to do something similar myself. I exclusively use 3.5mm gear so I'll be keen to have a crack.

Has anyone else successfully built a key fob BB? Would love to see people share examples.

With some guidance from Nick, I've been in the process of copying his design.

Two things causing me to pause and rethink:
1) The pins on the back of the 3.5 plugs are tiny.  I bought some others that have a better separation of leads (meant for a break out box) but the collar of the plug is much shorter, about 2mm vs 5mm so I'm trying to see what I can do with it.
2) Discussion in the A10 thread that 12v battery power may be the cause of some issues so I was maybe looking at doing a 9v and 12v.  I typically run CA14s which, to my horrible ears, seem to like the higher voltage when using a SP 12v box.

  My progress:

I saw a similar remark elsewhere but I've personally been unable to find evidence of the battery boxes causing damage. I found one comment where the poster thought that maybe it was the cause but provided no evidence.

Are you able to link to some of the comments in the thread because I've had no luck finding them sorry.

[mrfender]

I'm very interested in this project. Keen to do something similar myself. I exclusively use 3.5mm gear so I'll be keen to have a crack.

Has anyone else successfully built a key fob BB? Would love to see people share examples.

With some guidance from Nick, I've been in the process of copying his design.

Two things causing me to pause and rethink:
1) The pins on the back of the 3.5 plugs are tiny.  I bought some others that have a better separation of leads (meant for a break out box) but the collar of the plug is much shorter, about 2mm vs 5mm so I'm trying to see what I can do with it.
2) Discussion in the A10 thread that 12v battery power may be the cause of some issues so I was maybe looking at doing a 9v and 12v.  I typically run CA14s which, to my horrible ears, seem to like the higher voltage when using a SP 12v box.

  My progress:

I saw a similar remark elsewhere but I've personally been unable to find evidence of the battery boxes causing damage. I found one comment where the poster thought that maybe it was the cause but provided no evidence.

Are you able to link to some of the comments in the thread because I've had no luck finding them sorry.

I haven't been able to find anything else on the topic either.  Googling everything I can think of related to it but nothing specific comes up.  Like you say, no firm evidence of it yet so maybe it's nothing.

[shadowfax1007]

In relation to your own attempt, I've been thinking about the project and if 3D printing could come in handy. I'm thinking if there ends up being a standardized parts lift for people, including the fob, then I could model a harness for the internals to make it easier for others.
Something I'll have a think on further in the near future.

[breakonthru]

Core Sound uses only a 4-pin mini-XLR on their single connector battery box/mic plug.  R/L in/out and I guess ground is only carried in the shield/shell. Don't see a handy photo of this on their (his) site other than hooked up.

That's a good example of the arrangement.  Hmm, maybe does use the shell connection for signal ground.  Nice to have it on a pin connection though too I think.

i have one of these and yes the shell is the ground

[joeldotc]

1) The pins on the back of the 3.5 plugs are tiny.  I bought some others that have a better separation of leads (meant for a break out box) but the collar of the plug is much shorter, about 2mm vs 5mm so I'm trying to see what I can do with it.
2) Discussion in the A10 thread that 12v battery power may be the cause of some issues so I was maybe looking at doing a 9v and 12v.  I typically run CA14s which, to my horrible ears, seem to like the higher voltage when using a SP 12v box.

When I built my BB, I just stuck the resistor/capacitor leads through the little holes on the 3.5mm plug pins then soldered/snipped the excess. If you're planning to build it using some extra cable, you should be able to stick the wire through those holes then solder as well. You can see what mine looked like under my post about cloning the UBB.

If you're worried about providing too much voltage, you can always adjust (increase) the resistance to get to your desired voltage.

[joeldotc]

In relation to your own attempt, I've been thinking about the project and if 3D printing could come in handy. I'm thinking if there ends up being a standardized parts lift for people, including the fob, then I could model a harness for the internals to make it easier for others.
Something I'll have a think on further in the near future.

This would be awesome - an open source design for a BB that gives a nice clean package.

On another note, I'm still trying to find a source to build a nice DIY stealthy pre-amp but I can't find any good designs and I'm not smart enough to design one myself haha!

[mrfender]

The SonyInsider page that many people have referenced is no longer up but I was able to find the schematic that the author had drawn up.  I'll just leave it here for reference.

[one8ung]

I have saved the Sony Insider thread: https://we.tl/t-rrqjY0tCFb

[shadowfax1007]

I have saved the Sony Insider thread: https://we.tl/t-rrqjY0tCFb

Uh, Anti-virus is coming up with a Trojan alert for that zip file. Have you scanned your PC recently for threats?

[one8ung]

I have saved the Sony Insider thread: https://we.tl/t-rrqjY0tCFb

Uh, Anti-virus is coming up with a Trojan alert for that zip file. Have you scanned your PC recently for threats?

Does anyone else also get this message?

I've scanned my computer several times but can't find anything!

[mrfender]

I have saved the Sony Insider thread: https://we.tl/t-rrqjY0tCFb

Uh, Anti-virus is coming up with a Trojan alert for that zip file. Have you scanned your PC recently for threats?

Does anyone else also get this message?

I've scanned my computer several times but can't find anything!

No warnings for me when I downloaded it. (Windows 11/Opera browser)

[shadowfax1007]

I've redownloaded it twice and now I'm not getting a warning at all.
Maybe a false positive on the first download....but it also doesn't make sense either. How bizarre.

[mrfender]

I found some 3.5mm rubber plugs on Amazon.  Looks to help make the holes a little less obvious.

No comments on my terrible Dremel skillz.

[adrianf74]

I found some 3.5mm rubber plugs on Amazon.  Looks to help make the holes a little less obvious.

No comments on my terrible Dremel skillz.

Nicely done.  Looks pretty decent to me.  Looks like a key fob that had some wear with some buttons on the side, really. 

[audBall]

I found some 3.5mm rubber plugs on Amazon.  Looks to help make the holes a little less obvious.

I use these to plug off inputs on some of my recorders, especially the dual 3.5mm inputs that allow optical inputs as well. Peace of mind with dust and all that.