(Solar) Powering an Si4713 FM Radio Transmitter

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(Solar) Powering an Si4713 FM Radio Transmitter

Post by rors »

Hello,

I am setting up a self-contained Arduino project that will broadcast radio for a few hours per day for 2 months, and I am trying to figure out the best way to power this.

I'll be working with the Adafruit Music Maker shield, linked to the Adafruit Si4713 FM Radio Transmitter. I will be programming it to play 30 minutes of audio, nine times per day (every hour, on the hour, from 9am until 5pm).

Can anyone give me some advice on how to determine the power needs of this project? Could I simply power it for that time with something like a car battery? Or if I need a solar panel, any advice on how to estimate what size will be adequate?

Thanks very much,
Rory

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Re: (Solar) Powering an Si4713 FM Radio Transmitter

Post by pdh »

You'd have to measure it to be sure -- set the system up to run from wall power and measure the power that it consumes; then you can decide how best to power it stand-alone.

That said, if you're willing to use a small lead-acid battery, that would almost certainly be enough to get you through a day. You wouldn't need a car battery -- a UB12220 wheelchair battery would probably do the job. Those are rated for 22 amp-hours at 12 volts; they're reasonably cheap and readily available. They're about 1/4 the size and weight of a car battery.

For longest life you shouldn't draw all 22 amp-hours before recharging, but even if you only used 10 amp-hours that would be 120 watt-hours, which would be enough to power a 15-watt project for 8 hours. You'll also need a voltage converter to regulate the 12-volt power down to 5 volts. You'll lose some power in the conversion, but even so I bet 15 watts will be more than enough.

Re-charging is the next issue -- simplest would be a charger that plugs into a wall socket, solar is more complex. You'll definitely have to measure your power usage ahead of time in order to design a good solar-powered system. But as a first cut a UB12220 that's recharged from mains power at the end of the day would probably do the job.

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Re: (Solar) Powering an Si4713 FM Radio Transmitter

Post by rors »

Thanks very much for the reply. My main question is: how can I power the Arduino (with peripherals) off a battery that is being recharged by a solar cell for about 2 months without any intervention?

I was about to purchase this Solar LiPoly charger (https://learn.adafruit.com/usb-dc-and-s ... r/overview) but then I realized that appears designed to work with a 3.7V battery, but Arduino requires 5V.

Do you think it would work to wire the output of that charger into this booster? (https://www.adafruit.com/product/2465)

Or perhaps even to get only the booster and simply replace the microUSB input to that by wiring up a solar panel? (For ex, https://www.adafruit.com/product/1525)

Thanks very much.

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Re: (Solar) Powering an Si4713 FM Radio Transmitter

Post by pdh »

You can't wire the panel directly to the PowerBoost -- the output of the panel will vary wildly depending on the amount of incoming sunlight, so you wouldn't get reliable power coming from the PowerBoost. The basic design for systems like this is that your project runs from batteries (that way the project always sees good, clean, reliable power), and your solar panel is connected to a controller that recharges the batteries when the sun is shining.

To decide what battery and panel and controller you need, you have to know your power requirements. How much power (in watts) does your project use during the 4.5 hours each day that it's playing? How much power does it use when it's not playing, during the other 19.5 hours?

Your location and time of year both matter too, since those things affect the amount of incoming sunlight that you have. You'll need more solar panel capacity and a bigger battery to do this in Cleveland as opposed to LA, or to do it in December as opposed to doing it in June.

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Re: (Solar) Powering an Si4713 FM Radio Transmitter

Post by rors »

Thanks very much for the help. I'm still trying to find the proper battery & solar charger solution for this project.

Its Arduino based, so running at 5V i believe? and i'm estimating that it will be drawing about 500mA.

My latest idea is to try using this:
https://www.BANNED.de/de/solar-ladegera ... 26996.html
(i'm currently in Germany.)

Any thoughts about whether this would work well? It claims to output 5V with a capacity of 5000mAh. And i believe it says the charge current is 2100mA.

If those specs are correct then shouldn't this be an ideal solution for this project?

Thanks!

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Re: (Solar) Powering an Si4713 FM Radio Transmitter

Post by pdh »

I'm not fluent in German, but are you sure that part is putting out 2100 mA? It has a 5000 mAH battery, but it looks like the panel puts out only 140 mA. The specs seem to say that the solar cell can provide 0.7 watts of power at 5 volts; and since watts = volts * amps, the numbers do work out to 140 mA.

If you're using 500 mA for 9.5 hours each day, that's 4750 mAH per day (assuming zero current draw when you're not broadcasting). So a fully-charged 5000 mAH battery would last about a day, and you have to have a panel that can collect enough energy each day to re-charge it. If the panel is really putting out only 140 mA, it won't be able to get close to that, even under ideal conditions.

I think this unit is a combination battery / controller / solar panel; if I'm reading the specs properly, the battery can store 5000 mAH, the panel puts out 140 mA / 0.7 watts, and the controller can handle as much as 2100 mA. The description also says that it can charge the battery from USB; I wonder if that 2100 mA of maximum current is intended to come from USB rather than from the solar panel? Or maybe it's saying that it can output up to 2100 mA of current?

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Re: (Solar) Powering an Si4713 FM Radio Transmitter

Post by rors »

Thanks so much for all the help, pdh!

I'm afraid I made a mistake in my previous post! As best as I can tell right now, my current usage will range from 50mA to 150mA. Not sure where I got the other value from.

So if I'm doing 30 minutes at 150mA, 8 times per day (4 hours) and 50mA for the other 20 hours, I'll have:
.5 * 8 * 150 + 20 * 50 = 1600mAh per day
at an average current of 1600/24 = 66mA

This seems much more manageable. I believe the device I previously mentioned should work, however the store here is out-of-stock, and instead has this one:
https://www.BANNED.de/de/solar-ladegera ... 70363.html

Fortunately that seems to have even better specs. As far as I can tell, the battery is a maximum of 6000mAh, and they claim the solar cell will charge at 230mA (assuming that is max, ideal conditions, etc)

Does this seem more feasible to you? I realize that also I can lower the current load by doing less than 8 broadcasts per day. If I did only 4 broadcasts for example, then I estimate the device should have an average current load of 58mA and 1400mAh per day.

Thanks again very much for your help – in advice, and in the approach about how to reason about this example!

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Re: (Solar) Powering an Si4713 FM Radio Transmitter

Post by pdh »

Sounds like you're getting closer! :-)

So you need 1600 mAH per day, and the newer panel can put out 230 mA max. That means you'd need 7 hours of full, direct sun per day (1600 / 230 = roughly 6.96), and it assumes no conversion losses between the panel and the project.

I think you're getting closer to a suitable design, but unless you're in a very, very sunny area at the height of summer, you probably won't ever see 7 hours of full output from a solar panel in one day. Here in the northeast U.S., with panels in a fixed position -- i.e. not re-aiming them as the sun moves across the sky during the day -- I can generally get the equivalent of 4 or 5 hours of full sun per day on good summer days, but it's unusual to get much more than that.

There are several reasons for this -- fixed-position panels won't actually be aimed straight at the sun most of the time; and clouds interfere sometimes; and the output of solar cells drops as temperature increases. (The panels I use have a rating that says they lose 4.4% of their maximum power for every 10 degrees C over 25 C; and the panels will get hotter than ambient air temperature if they're in direct sun, sometimes much hotter.)

I typically see end-to-end efficiency between 70% and 80% (so figure 75% average) -- meaning that for every 100 mA that comes out of my panels, only about 75 mA makes it to the project -- the rest is used up by the electronic controls and voltage converters that sit between the solar cell and the project.

Taking all of that into account, I'd expect the optimistic / maximum real-world output of 230 mA panels to be roughly 230 * 5 * .8 = 920 mA per day -- so still probably not enough. It's worth noting that the 230 mA panel works out to 1.3 watts, which is small as solar panels go. (Just for comparison, Adafruit's smallest panel seems to be their 2-watt "medium" panel, and they offer other panels as big as 9 watts.)

The panels you're looking at have the advantage of being all-in-one solutions -- panel + nice big battery + control electronics all in one package, and they deliver 5 volts of output which is exactly what you need. Those are good features, but I think you're going to need a larger panel.

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Re: (Solar) Powering an Si4713 FM Radio Transmitter

Post by rors »

hm. OK, thanks for the reality check. Well I would simply go ahead and get the Adafruit panel + charger + battery. But my only concern is that the battery is only 3.7V and i'm not sure that will work with my Arduino, which I believe is expecting 5V.

Do you have any experience with this? The only thing that really appeals to me about these all-in-one solutions is that many seem to have 5V output. And I dont think any of the Adafruit components accommodate that.

So then I guess I could also get the Adafruit voltage booster?

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Re: (Solar) Powering an Si4713 FM Radio Transmitter

Post by pdh »

Right, the nice thing about the systems you were looking at is that you don't need to choose individual components, and you don't need a voltage converter. Those are significant advantages. But if you can't get one with a big enough panel, then they just won't work for you (unless you're willing to charge the battery from wall power overnight every few days -- even the smaller 5000 mAH battery is plenty big enough to give you several days of runtime, it's just the solar panel that's undersized).

If you do end up buying the individual components, consider over-specifying a bit -- that will help a lot on cloudy / rainy days. The cost structure helps -- for example, Adafruit's 6-watt panel puts out 3 times the power of their 2-watt panel, but costs only twice as much. Same thing for the battery -- a 2500 mAH battery costs 1.5 times what a 1200 mAH battery costs, but stores twice as much energy.

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