You can read about this player/transmitter configuration on this blog, here: Quick, simple FM radio station.
Like many of you who have tried the Scosche FMT4 FM transmitter, I soon realized that battery power wasn't going to be practical.
It was nice that the AAA batteries for the transmitter and portable MP3 player lasted over 3 days but the down side was that the batteries only lasted 3 days - not ideal for a 24/7-365 operation.
Also like many of those who have used the FMT4 for Part 15 broadcasting, I've added an old cell phone charger and voltage drop resistor to provide full time power - but that means you're tethered to an outlet.
Using this simple FM station model, plug in power isn't all bad. Most of us have noticed an occasional 110 V outlet in intriguing public places and adding the Simple FM Station to a location like that could be fun, all by itself.
Looks like I need to do a blog post on guerilla FM station placements!
To really liberate this guerilla radio operation, I wanted to give solar a try.
Looking through my junk box, I found some items that could give me the chance to try solar power in a low power radio configuration.
Item 1 was a solar battery maintainer, providing up to 22 volts and 120 MA, around 12 - 13 volts under load.
Item 2 was a very old portable jump start box that no longer had enough oomph to turn over a car engine.
After charging up the jump starter with the wall wart charger included with the starter, I was happy to discover it still held a charge - not enough to start a car but plenty to run the station.
With a solar panel and a means to store solar power, I then needed to match the power available to the needs of the player and transmitter.
My first effort was to use voltage drop resistor circuits to get my 1.5 V and 3 V sources for the player and transmitter. Unfortunately, I found that as the battery level dropped, my dropped voltage fell below the operating range of the connected devices.
The next option, Item 3, was to try a DROK DC-DC Step-down Voltage & Current Regulator Module from Amazon. Two were definitely overkill, as I could have used one to get down to 3 V and a voltage drop resistor to get down to 1.5 V but this is experimental broadcasting, so why not experiment?
I connected both regulator inputs to the 12 V source and then trimmed the output on one to 1.5 V and the second to 3 V for the player and transmitter, respectively.
The regulators created a bit of audible noise, barely perceptible when listening on an FM receiver, but the noise abated as the capacitors on the regulator boards charged up. The regulators have been quiet since shortly after being connected to power.
The MA consumption of the transmitter and player are estimated, based on the 700 or so MAH in a AAA battery. These devices ran about 70 - 80 hours continuously before the batteries went dead.
The 120 MA output for the solar panel is, of course, in full sunlight. Fortunately, I've been impressed with the output, measured by a VOM, even in less than ideal conditions. The panel is a no-name unit I bought online about 8 years ago but it seems to be working well. The stated output is 1.5 Watts.
I'm located not far from the Canadian border so we have long days from late February until early October. I might add a second 1.5 Watt panel during the winter.
So, how's it working?
So far, it's been encouraging. The solar panel has kept the battery charged enough to run the system for nearly two weeks.
Without any post processing, the audio quality is a bit weak but the station comes through A-OK when tuning an analog receiver. I have an old guitar compressor that helps quite a bit and that runs on a 9 V battery. The next upgrade might be to put the transmitter and player on one regulator, with a voltage drop resistor for the player, and connect the compressor to the second regulator.
True solar power aficionados might be asking "Where's the controller for the solar panel?" and the answer is that there isn't one. The input is pretty marginal and with the steady drain on the battery from the regulators, transmitter and player it seems that there should be adequate headroom.
If the battery does overcharge and become damaged, that's OK - it was headed for the junk pile anyway. I have two surplus 10 AH gel cells waiting to take over and I'll add a controller when I switch over.
When high and in the clear, one of these Scosche units with a hacked antenna can get out several hundred yards. I'm hoping I'll be able to finish testing and arrive at a final configuration that makes ideal placement possible, with the unit totally self contained and having no need to connect to power or audio.
Of course, this is merely a low cost demonstration project but a project that suggests some fascinating steps forward.
A little bigger solar panel, a properly sized controller and battery matched to an FM transmitter with a little more output power could be very interesting. Pair that with a low current draw audio source like a full feature MP3 player and the opportunities are multifold.
A more refined setup could really present some interesting opportunities for placement near a larger potential audience.
Experimental broadcasting for a better tomorrow!