Solar Powered Radio Conversion

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About: There's always a better way, just need to keep recycling those brainwaves!

I bought this radio a few years ago for listening to the morning radio in the bathroom.

It runs on 2 AA alkaline batteries which last about 2 months, with half an hours daily use. I then switched to rechargeable AA Nimh batteries, which lasted about 6 weeks per charge.

I eventually got fed up with manually recharging and replacing the batteries, so decided to try and convert it to solar power.

Step 1: Design

The destined bathroom had some natural light, but I didn't particularly want a mini solar panel obstructing the window pane.

I did however have a west facing external wall, on which I could mount a mini solar panel. The solar power could then be fed to the bathroom via run of twin cable through a hole in the wall.

The radio holds two AA Nimh batteries, totalling 2.4v (1.2v each). I had a spare 12v solar panel and wanted to make use of it in this project. So I decided to mount my 12v solar panel on a vertical fascia roof Panel.

In the vertical position, it can easily generate several milliamps even on a dull day. In summer it gets direct sun for 1-2 hours in the evening. As the charging current is low (in the order of tens of milliamps), the batteries should be able to cope with any overvoltage at such low currents. The battery load also limits any overvoltage being passed to the circuit board.

To prevent discharge overnight, a one-way diode will be added in series to the solar positive feed wire. This also drops the solar output voltage by 0.6 volts.

Step 2: Parts and Tools

A solar panel. I had a spare 12v solar panel which I used, but a 3v or 6v would better suit this project.

Twin core wire from panel to radio.
Diode 1n4001 or any other salvaged diode
2 AA Nimh batteries
Dc male and female connector (optional)

Tools
Soldering iron and solder
Voltmeter (optional)
Drill if using male/female connectors.


Step 3: Assembly

Solder the solar panel , wire and diode as shown in the design schematic. You can either connect the feed direct to the battery terminals (externally) or via male/female connectors (internally).

The diode is soldered between the solar positive feed and the batteries..Ensure diode polarity is correct by checking you get a voltage after the one way diode.

This radio had an existing dc supply socket, but I did not have a matching male plug, so added a new dc socket and plug.

Remove batteries to identify positive and negative terminals to which to attach the solar negative and solar positive (after diode) feed wires.

Reinsert the rechargeable batteries.

I mounted the solar panel to the fascia board using silicon as adhesive. Siliicon was also dabbed onto the solar panel wire terminals to prevent corrosion.

Step 4: Results

I have now been using my solar powered radio for over a year in UK weather conditions.

The radio gets used daily for half an hour and has never required a top up charge, or change of batteries.

On a bright day the solar panel generates 13v at 60 milliamps. On a dull day the solar panel generates 11v at 6 milliamps. So voltage is pretty much constant, but current generated varies.

The charged battery voltage after a sunny day shows 1.44v and being able to supply a short circuit load of up to 6 amps. The radio itself only consumes 0.1 amps. So I should easily be able to run for several hours every day. A 3v solar cell would have been better suited to this application, but I didnt fancy getting up a ladder again (if it aint broke dont fix it :-)

Goal achieved, no more battery swaps, no more landfill waste, and the radio now runs on the sun.

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