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.