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DIY key fob battery box

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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!  :cheers:

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.

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