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Solar powered low power radio, part two

Over the last few months I've been experimenting with solar powered low power radio and it's been quite an education.
     My location in the rainy Pacific Northwest, on the Canadian Border, made the project even more interesting. Luxuriously long summer daylight and brutally short winter days, unpredictable cloud cover and several weeks of wildfire smoke probably present far more challenges than most of our readers will encounter.
     The first effort, titled Quick and simple FM radio station goes solar, used junk box items to investigate the concept. After connecting the apparatus and handing the power duties over to the sun, the first discovery made was that of flawed thinking.
      Yes, I thought the solar panel delivered about the same amount of current as the apparatus used but what about recharging the battery? Duh!
     When the battery went dead after about 10 days of operation, I realized I had created a solar assisted low power radio station. In other words, while the sun was out the station ran on solar power but after the sun went down the battery took over. The following day the solar panel took over again but didn't generate enough power to run the station and recharge the battery. A simple oversight caused by my enthusiasm to get the project started.
     I ran the station in solar assisted mode for a few weeks because this is still a pretty cool concept. Solar assisted is somewhat like a hybrid-powered automobile. I developed a schedule of recharging the battery with the wall wart charger once every seven days to demonstrate that even a solar assisted operation was better than being tied to a power outlet. For example, if I had two battery packs I could have done a weekly swap out and maintained remote operations in perpetuity.
     After I found out what it would take to deliver a fully solar independent operation, the 'solar assisted' concept became even more appealing but more about that in a future post. 
     Realizing that I needed far more solar power for this to be practical, I decided to get serious about the math.
     First, when I placed my VOM between the power source and the load, I discovered I had significantly underestimated the current draw of the radio station. The Scosche transmitter, MP3 player and attendant voltage management modules didn’t draw the 125 mA I had estimated. Instead, I measured over 220 mA or almost twice what I estimated.
     This also meant that the solar assisted mode never fully carrier the burden. The solar panel merely  slowed the rate at which the battery discharged.
     In my original post I postulated a second 1.5 watt panel might help but a better understanding of the power demands of even the very low power transmitter being used rendered that assumption ridiculous.
     The good news was that when I went looking for more power, I discovered that plummeting solar prices had brought down the cost of a ten watt panel to below the cost of the 1.5 watt panel I purchased a few years ago.
     Ohm's law tells us that a 10 watt panel at 12 VDC can provide 800 mA of power to the system and that was encouraging. With about 8 hours of darkness during the summer, I needed 8 X .220 or 1.76 (rounded up to 2) amps of reserve power for overnight operation. 800 mA of solar power would easily provide the 220 mA I needed to run the station and and an additional .580 mA to charge a battery.
     If my system could deliver 220 mA to the micro radio station and .580 mA to the battery, four hours of charging should give me the overnight reserve I needed. With 16 hours of daylight, I should have more than enough power for full solar operation.
     With renewed confidence in my approach, I ordered an ECO-WORTHY brand 10 Watt Solar Panel and a PowMr 10A Solar Charge Controller (a 1.5 Watt panel doesn't need a controller in a high demand circuit but a 10 Watt panel definitely needs controlling) from Amazon (total cost ~$25.00 with sales tax) and grabbed another surplus battery from my junk box, a 10 Ah Sealed Lead Acid (SLA) battery
     I retired my old power plant and moved the new system into service and waited for the results.
     To my delight, this second version of the operation proved to be a winner. Week after week, the MP3 player played, the transmitter transmitted and power was abundant.
     So, is that the end of the story? Would I be content with my little hacked Scosche transmitter and it's one block range? Stay tuned...
     "Experimental broadcasting for a better tomorrow!" It's not just a saying, it's a way of life! 

AM transmitter power amplifier from an old CRT monitor?

Uber-hacker WA2ISE tells us "If you still have an old CRT VGA computer monitor in the closet or basement. you could use the video amplifier CRT driver circuit as an RF amplifier."
     This rather amazing discovery, along with many other items of low power radio goodness can be found on WA2ISE's websites, found on www.wa2ise.com.
     WA2ISE's site links to some long dead Geocities 
sites, brought back to life flying the new .ws top level domain. 
     If you have any dead Geocities links, try swapping the .com for .ws on the same URL.
     Amongst the many interesting links in those old sites are:
    Tomaz's Schematics Collection featuring several FM transmitters and support circuits like simple mixers.
    50mW AM transmitter circuit (old non-secure url, some browsers won't display this image), nice low parts count experiential transmitter.
     AM tube microtransmitter discussion, a nice example of a low power AM tube transmitter circuit  and a discussion of using a CRT IC as an amplifier, referred to at the opening of this post.
     Exploring the web looking for low power stuff can bring a sad reminder that many well-intentioned people put lots of great radio info on line and we lose a little every day to expired domains and failed hosts.

Evan's AM Tube Transmitter Circuits

Hobbyist posts his learning experiences experimenting with AM tube transmitter designs and passes the savings along to you!
     Certified Electronic Technician Evan gives us six updated AM tube transmitter designs so tube enthusiasts get six more choice designs to build.
Image credit: Evan's AM Transmitter Circuits
     Evan's designs range from a simple one tube unit with about a dozen components to a five tube rework of a design from the late Fred Nachbaur.
     A visit to Evan's site brings you:
One Tube Transmitter!
Based on the Antique Electronic Supplies kit.
Two Tube Transmitter!
Now it's crystal controlled.
Three Tube Transmitter!
Has a selectable preamp, solid state or tube.
Fred Nachbaur's Goldberg!
A -revisit- with new circuit updates
A Linear 'power' Transmitter
A simple performer
Tuning Eye - Transmitter
A -revisit- adding an audio indicator tube
     Tube enthusiasts will find more than just schematics, as Evan takes us through his circuit analysis and optimization process and shares his mistakes and lessons learned throughout his site.
     So get your tube on - click on over to Evan's site and fill up on tubular goodness.

Three simple AM transmitter plans for Part 15 broadcasting

Back in the not-so-distant past the primary mode of transmitter diaspora was electronics magazines articles by circuit gurus like Rudolph Graf, William Sheets and Doug DeMaw.
     If you missed an issue with a great circuit, you could always order reprints for $2.00.
     The bad, and somewhat ironic, news is that the publishers of electronics magazines couldn't adapt to the new electronic publishing environment brought about by the medium we are using now and all those storied publications have perished.
     The good news is that an enterprising person has resurrected all of those dead issues in a delightful site called American Radio History
     Having been an avid reader of our dearly departed physical media, your faithful editor was able to take you back in time to bring back some great little Part 15 AM transmitter projects featured in that chosen medium of yesteryear.         
     So with that in mind, please to be enjoying three low power AM transmitter circuits from 80s and 90s electronics magazines.
     Parts lists included but you'll likely need to do some substitutions - more's the fun!

Doug Demaw's "Mini Broadcaster," with five 2N2222 transistors in this intermediate level project, Monitoring Times, December 1996

Doug Demaw's "Transmitter Basics," featuring a low parts count for this intermediate level project, Monitoring Times, October 1988

Joseph Smolski's "Build a 1930's transmitter," A tube powered mini-transmitter with fewer than 20 components for this beginner level project, Popular Electronics, November 1991
     
     So grab some of the new lead-free solder, heat up the iron and make some radio! Lots of articles on this very blog about antennas you can use to extend range.