I'm hoping this is the right Forum for these questions as It's the only one that Looks remotely close. I know almost nothing about this stuff but am having some fun trying to make a small project.
I'm trying to make sound monitor to listen to the wildlife in the woods, lots of Birds and other noise makers, I had a idea to use a small FM Transmitter that was from eBay, I went through a few until I came up with one that seems to do the job, (small spy type transmitter on FM) I have it hooked up in a small box with 2 'AA' Batteries and a small solar cell, it's been on for 10 days and the batteries are still at 1.2v each so that works, I can post a pic tomorrow
here are the specs
Output Power: 100mW Equivalent Noise: >=30dB Power Supply Voltage: DC 3.0V-5.0V Operating Current: 35mA Sound Track: LINE/USB channel (stereo); MIC channel (single channel)
Problem I'm having is with the on board Electret mic, it picks up everything, people far away, dogs barking and Airplanes, I'm curious as to a way to change the mic, I only need a small radius like 30-40 ft around the unit, it has a 3.5 Audio input and the Android type USB on the board, I'm not looking for more power or transmitting range as it works fine at about a 100 ft unless I need more juice to power a mic, I'm not looking to broadcast 300 ft and will keep the antenna down so it won't go farther and the Feds stay away, I could change the Batteries and cells to match whats needed, maybe a small powered mic but also do I need a way to filter out the other noise? I'm also not stuck on this method but have 20 bucks in a working unit other than the background noise so any ideas I would welcome, maybe there is a crystal alternative or? trying to do this as thrifty as possible. I do want the solar as I never have to change batteries just move some snow once in awhile so probably no more than 9 volts total as the cell would get larger and trying to keep this as small as possible
I know you will need more info but don't quite know what to give
I'm not sure how to help with the mic other than to possibly unsolder it from the board and then run some wire between the electret element and the board so that you have some ability to manipulate the microphone better. Maybe then add a foam cover over the mic to help with wind and cut down on distant pickup. Make sure to take note of the mics soldered polarity if you do this, they do have a positive and negative. Could always experiment with enclosures for the mic, like make a cone or something to focus the sound pickup from top to bottom or near-field/directional.
For power I'd try those solar lights that are super cheap which contain the AA rechargeable cell in it. Disconnect the LED and in place connect your transmitters power. It might be enough to keep it running for a while but most of them only turn on at night after charging all day so the circuit would need fiddling to get daytime activity. Might even be better off using a large rechargeable battery pack or make your own out of a bunch of 18650 cells like those in laptop battery bricks, wired in parallel would give enough oomph to run for days or weeks. Those usually run around 3.7VDC which is in the ballpark of your needs.
Giving me ideas now for a future fun project and being on regular broadcast FM is convenient since any radio can pick it up. I'd definitely use one of the lowest FM frequencies, maybe even 87.7 or 87.9MHz. Just at the edge of the dials end. There are some radios that cover even lower for the Japanese FM band down to 70MHz that could be used if more privacy is warranted. Cheap cordless microphones made for the 70MHz band like those used for kids karaoke machines come to mind which I actually do have one of and runs on a single AA battery..
I am using 2 of those cells wired together and get almost 5 volts kicking out in daylight and 2 AA rechargeable batteries, that part seems fine and has been on for almost 2 weeks now, I do have a diode in place so the cells don't suck the batteries at night
I have small powered mic coming tomorrow that I'm going to try, hopefully it will work and I can start tweaking it a bit as this works great but to much background noise, I would love to have a stand alone transmitter and receiver that's not on the FM band but have no idea where to look and keeping it small is the issue, I'm going to look into the lower bands like you said but will have to find a different transmitter as this only does 80 something to 108 so maybe a Karaoke mic and the receiver, I'll look into that also
Any ideas on a different type of frequency or stand alone option would be appreciated, maybe something that could be adjusted to take some of the noise out, I attached a picture of what I have, it's just in a container to try it out but you can get the idea, cells are on the lid if I can get just right I'll get a container of some kind made to put it in that I can just hang here and there in the back yard
I also removed the boards from the light and wired the solar cells in tandem to get the volts, I have no idea Of the amps but they keep 2 2000mah batteries going about 2 weeks now
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