DIY solar powered alarm clock

[Daox]

So, I'm in the market for a new alarm clock. My current one is about 10 years old and it has started making an annoying electrical buzzing noise like a bad florescent light. I originally thought it would be cool to get a solar rechargeable alarm clock! Clocks don't use much power and it would be nice to have one less thing drawing 1-3W continuously.

I went on amazon and started searching. I only found a few solar powered alarm clocks and none of them really met my needs (which are actually quite simple). I wanted an alarm clock that I can read at night, none of that pushing a button to light it up stuff since the alarm clock is slightly out of reach. None of the solar powered alarm clocks had this feature, arguably to save on battery life.

What is a guy to do if he can't buy what he wants? DIY of course!

So, I started looking for battery powered alarm clocks of which they are many. You can always add a solar panel to something with batteries. One feature I found while searching is an automatic back lighting feature. They have a light sensor on the clock and when it gets dark in the room it turns the back light on. This saves power during the day and extends battery life.

All I need to do is select a clock and then pick out a solar panel and build a small charge controller for it. Sounds like a fun little project, and I get to get my hands a little dirty with some solar stuff.

[Daox]

Here are a couple of the clocks I am looking at:

Amazon.com: Westclox LCD Alarm Clock 70045: Home & Kitchen

Amazon.com: Elgin 2-Inch LCD Multifunction Alarm with Smartlite: Home & Kitchen


Right now I'm leaning toward the 2nd one because it uses 3 AA batteries vs 3 AAA batteries and I wouldn't mind having a bit more battery life. However, even with the AAAs, since we're hooking up a solar panel, it should hopefully be a non-issue.

[Daox]

I also did some quick searching for a DIY charge controller for nimh batteries and found this:

Float Charging NiMH Cells [227] | General Electronics | Electronics

It uses a 12V supply which is a bit high. I was thinking a 5-6V solar panel would be fine. I haven't looked at it quite enough to see if it will just work with a lower voltage panel. Much more searching to do.

[hamsterpower]

Sounds like an entertaining project. First thought is to canibalize one (maybe two) solar path lights for the whole solar kit. I would think that would be enough. My wife has a travel alarm that only needs one AAA every two or three years, maybe longer.

[MetroMPG]

Interesting. Subscribed.

I'm partial to wind-up clocks, but they don't usually have backlights!

[Daox]

So, I've been doing some reading on nimh chemistry (which I do know some stuff about, but not charging specifically). I found this specific site quite informative:

How to charge Nickel Metal Hydride Batteries.

The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.

So, in order to keep the system super simple and easy all I need is to provide approximately 1.41V per cell and keep the current below C/10.

Lets start with the C/10 part. C/10 means capacity divided by 10. Most rechargable cells have a capacity rating in mAh or Ah. The batteries I have on hand are shown below. They are rated for 2200 mAh. So, if I keep the charging current below 2200/10 = 220 mA I don't need a charge controller chip. Since the clock should pull so little energy, this slow charge rate should fairly easily be able to keep up. So, whatever solar panel we select it needs to have a max output of 220 mA or less.





On to the voltage side of things. Max charge voltage should be about 1.41V per cell. The alarm clocks I've looked at both use 3 battery cells, so I'm looking for a max charge voltage of 3 x 1.41 = 4.23V.

Unfortunately, you can't just get a solar panel that puts out 4.23V. However, from what I've seen, 5V and 6V panels are readily available at the current levels we're talking about. So, we have to figure out a way to bleed off either .75V from a 5V panel, or 1.75V from a 6V panel.

To get this voltage drop is actually quite easy, and it is going to be done by another component we need for the solar setup anyway. This component is a diode. Its essentially a one way valve (check valve) for electronics. This prevents the solar panel from draining power from the batteries when there is no sun. A diode also has the added side effect of creating a voltage drop across it. It is similar to a plumbing system with a check valve in it. All check valves create a pressure drop across them because they cause a slight restriction. The same thing happens in an electrical circuit. There is always a voltage drop across a diode, and you can get diodes with different voltage drops.

For a 5V panel, this diode will drop the voltage by 0.75V bringing the total voltage down to a max of 4.25V.
BAT48 STMicroelectronics | 497-2512-1-ND | DigiKey

For a 6V panel, this diode will drop the voltage by 1.75V again bringing the voltage down to 4.25V
MUR1100ERLG ON Semiconductor | MUR1100ERLGOSCT-ND | DigiKey


So, I really just have to pick out a alarm clock, grab a solar panel off ebay, and then get a diode that will drop the voltage the correct amount. Really, its a pretty simple setup.

[Daox]

Alright, I went ahead and nabbed this solar panel:

New 5V 200mA 1W Monocrystalline Solar Panel Power Cell | eBay

Its rated for 5V and 200ma.

I also ordered the diode mentioned above for the 5V panel.

[Daox]

Just for kicks I decided to calculate the ROI on the 'solar investment' small as it may be. Thats not really the reason I'm doing this, but it I think the result is interesting.

First we have to figure out how much I spent on solarizing the clock. I'm not going to include the cost of the clock because I'd be buying a new one anyways. I'm also not going to include the rechargable battery cost because I'd be using these rechargable batteries anyways, just charging them with an AC powered battery charger. So, we really just have the cost of the solar panel and diode.

Solar panel: $7.99 shipped
Diode: $0.50 + $1.00 shipping

So, I have a solar investment of $9.49



Now, lets find out how much that old clock is costing me. Using my killawatt, I know my old clock pulls a continuous 3W of power (which is rediculous for what it actually does). So, lets calculate the cost.

3W * 24 hrs/day = 72 watt hours per day = 0.072 kWh per day

I pay an average of $0.17 per kWh (fees included so this is actual cost).

0.072 * 0.17 = $0.01224 per day = $0.37 per month = $4.47 per year

Wow, that dumb old alarm clock costs me just over 1 cent a day to sit there and show me the time.

Last calculation:

$9.49 investment / $0.01224 savings per day = 775 day ROI

So there we have it, a 775 day ROI. That is just a bit over two years for the solar stuff to pay for itself. That is probably the best ROI I've ever seen for a solar PV product. I honestly expected it to be more than that.

[Daox]

I came back last night from a weekend away from home to see a package at my front door, and inside was the alarm clock.






So, of course I threw some batteries into it and started to tinker around with it. The battery compartment should be easily modified to add a few wires for the solar charger.







So far I'm liking it. The backlighting works very slick. As soon as the lights go out it turns the backlight on, and when the lights come on it turns off. The backlight is a bit brighter than it needs to be. It does illuminate the room a little. Its not as bright as the picture suggests, but it does let off more light than my old clock. I might try to tweak this if I can figure out where the power to the backlight is, but its not essential.

I'll be doing some more tinkering with it soon to get it ready to add the solar panel to it. I'd ideally like to be able to just add a charging port/plug to it like a laptop or cell phone has so it can easily be unplugged and looks nice. We'll have to see how much room there is inside though.

I also got the diode for the solar panel a day earlier. The solar panel is still on its way over from China.

[MetroMPG]

So you plan to add an AC adapter so you can trickle charge it from the wall if needed?