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 Forum index » DIY Hardware and Software » Stickney Synthyards
Solar charged rock mics
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PHOBoS



Joined: Jan 14, 2010
Posts: 1698
Location: Moon Base
Audio files: 346

PostPosted: Tue Apr 15, 2014 6:01 pm    Post subject: Solar charged rock mics
Subject description: might not be what you think,.
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For a couple of years I used a binaural mic made from a polystyrene head and some electret mics, attached to a battery powered preamp. I had a
couple of problems with it, sometimes I got a loud humming which miraculously dissapeared after prying out the mics and putting them back
in the head (which caused some damage to it), and there was always some low humming that I needed to filter out. I got a lot of distortion
when it was windy and blowing straight into the mics, setting up was sometimes a bit of a hassle and I had to replace batteries all the time.
The last time I tried to use it, it didn't work at all, probably because I left it outside too long.

So for some time I had been thinking of making some new mics that are solar powered/charged which I can leave outside. So when I need them I
just plug the cable in (inside) and I'm ready to go. We had some nice sunny weather a few weeks back and I used it to fix some of my solar
powered LED lights, checking which worked, replacing some batteries etc. I also have some lights in the shape of a rock and while I was looking at
them I got the idea of using those for the microphones.

The LED's are placed inside of a reflector so I removed the LEDs and glued in some electret mics instead. I then cut some pieces of foam to act
as a windscreen and covered them with a piece of nylon stocking, tied together at the back with a cabletie. This should protect the mics against
dirt and to some degree rain.

I also needed a preamp but only have a low voltage to work with. The rock lights have a battery compartment for two penlight batteries wired in
parallel, and one of the batteries was actually a fake one (looks like a battery but is completely empty inside Confused ). The original solar panels only
gave a very low voltage (I think it was just a bit over 1.5V) so I replaced those with some slightly larger ones that have a max voltage of 4.5V and
current of 50mA in full sunlight. I cut one of the battery contacts in half so I can use 2 rechargeable penlights in series. so that'll give me about 2.4V.
I then searched the web for a simple low voltage transistor preamp and found this schematic here

Posted Image, might have been reduced in size. Click Image to view fullscreen.
note: I added an extra resistor to power the electret mics

I tested it on breadboard and it worked quite well, so I build two of them on perfboard and hotglued them on top of the battery compartments.
I drilled a small hole in each rock for the audiocables, which are soldered directly to the PCB, and used 3mm neoprene LED supports to make it
waterproof.


rock mic parts.jpg
 Description:
reflector with mic glued in, piece of foam and nylon stocking.
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rock mic parts.jpg



rock mic complete.jpg
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parts of the mic put together
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rock mic complete.jpg



rock mic preamp - 01.jpg
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microphone preamp
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rock mic preamp - 01.jpg



solar charged rock mics.jpg
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Solar charged rock mics
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solar charged rock mics.jpg



_________________
"My perf, it's full of holes!"
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PHOBoS



Joined: Jan 14, 2010
Posts: 1698
Location: Moon Base
Audio files: 346

PostPosted: Tue Apr 15, 2014 6:26 pm    Post subject: rock mic test recording Reply with quote  Mark this post and the followings unread

Here's a test recording. Although the mics have a preamp they still need a lot of amplification (which is not unusual with outside recordings) and the mixer
I used adds a lot of noise to that, so I filtered some of the high end out. I also streamed it over the internet and mixed it back in to create a long delay. Cool


PHOBoS - Theo utsi dein si de.mp3
 Description:
outside recording with solar charged rock mics (Apr 07, 2014)

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 Filename:  PHOBoS - Theo utsi dein si de.mp3
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PHOBoS



Joined: Jan 14, 2010
Posts: 1698
Location: Moon Base
Audio files: 346

PostPosted: Tue Apr 15, 2014 6:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

after some testing one of the mics all of a sudden started to hum which I managed to get rid of by grounding the reflector the mic is glued to.
(I glued a cable to it with aluminium tape) Not sure what exactly caused that but it's gone now. And then the other mic started doing the same thing which
I fixed in the same manner. I also noticed that one mic has a louder ouput signal (maybe the batteries have a higher voltage ?!) and it also produces a
very low subsound, much lower from the previous ground hum. So I have to check that but because it so low it's very easy to filter out (I'm more
concerned about power consumption, but so far it seems to work fine).

I already made an extra preamp with some low noise NE5534 opamps so I don't have to add so much extra gain on my mixer and it also filters the
subsound out and some of the high frequency noise. I will post a schematic of that later but I want to put it in a box first.

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PHOBoS



Joined: Jan 14, 2010
Posts: 1698
Location: Moon Base
Audio files: 346

PostPosted: Tue Apr 15, 2014 7:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

ok here's the schematic. I added a voltage doubler to get plenty of headroom which runs at a high (inaudable) frequency. I'll probably just put
it in a small box that I keep inside powered by a wallwart but maybe I make it solar powered aswell. sunny


solar rock mics voltage doubler and preamp 2.gif
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voltage doubler and extra preamp
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solar rock mics voltage doubler and preamp 2.gif



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DES



Joined: Feb 28, 2003
Posts: 674
Location: Oxford, NJ
Audio files: 6

PostPosted: Tue Apr 15, 2014 7:33 pm    Post subject: Re: Solar charged rock mics
Subject description: might not be what you think,.
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Heh heh -rock lobster Laughing

I've got a set of glasses that have a clip on each side for a small electric condenser mic. The glasses were the black heavy 'nerdy' kind like Elvis Costello used to wear. I should dig those out and see if they work.

Nice work Phobos!

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PHOBoS



Joined: Jan 14, 2010
Posts: 1698
Location: Moon Base
Audio files: 346

PostPosted: Wed Apr 16, 2014 3:34 am    Post subject: Re: Solar charged rock mics
Subject description: might not be what you think,.
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Thanks DES Very Happy

DES wrote:
Heh heh -rock lobster Laughing

Laughing

_________________
"My perf, it's full of holes!"
http://phobos.000space.com/ http://www.acidtrash.com/ Stickney Synthyards
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PHOBoS



Joined: Jan 14, 2010
Posts: 1698
Location: Moon Base
Audio files: 346

PostPosted: Sun May 25, 2014 12:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
I also noticed that one mic has a louder ouput signal (maybe the batteries have a higher voltage ?!) and it also produces a
very low subsound, much lower from the previous ground hum. So I have to check that but because it so low it's very easy to filter out (I'm more
concerned about power consumption, but so far it seems to work fine)


I tried to figure out what the problem was. My first idea was that it might be caused by difference in hfe of the used transistors. (can be twice as
high for the BC549C). Although from what I understand of the schematic the feedback resistor should take care of that to some degree. So I
desoldered the transistors and measured the hfe of those (and the rest of the BC549C's I coud find) and taped them to a piece of paper with the
value written on the tape. The closest values I found were 691-693 and 685-688.
so for one mic I used hfe T1 = 685, hfe T2 = 691
and for the other one hfe T1 = 688, hfe T2 = 693

That didn't really help, although I got the idea that now the other one was louder but that must have been a mistake. So I started measuring
voltages; supply voltage was roughly the same for both, but then I measured the voltages over the electret mics, and I got 1.5V for on mic
(the loud one) and almost nothing for the other one. Turned out that I used a 47K resistor instead of a 4.7K to power the electret mic doh
I replaced it and the levels are a lot more equal now Very Happy.

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DES



Joined: Feb 28, 2003
Posts: 674
Location: Oxford, NJ
Audio files: 6

PostPosted: Sat May 31, 2014 8:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Good detective work!

Smile

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PHOBoS



Joined: Jan 14, 2010
Posts: 1698
Location: Moon Base
Audio files: 346

PostPosted: Sun Jun 01, 2014 4:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

posted another recording here. Cool
Used my mixer again not the extra preamp.

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