It is my girlfriends birthday soon and I thought it would be fun to make her an electronics based present. My idea is a circuit embedded in a bangle. Basically it involves a BPW34 mini solar cell which at peak outputs 50mA, a 3F capacitor and a red led. There will be a two way switch which either charges the capacitor or discharges it to an led. The switch will be triggered to discharge by a nearby magnet which I will carry and therefore be able to light up the led with a strength based on how long we've been away.

Here's a rough idea:

Will this work? I've figured I've got at least a week before the capacitor goes over its 2.7V limit but ideally there would be some way of stopping the charging at some point. Is there any way to do this? Thanks for any help!

Phaedo

 

Why do you expect the capacitor to reach 2.7V in a week?

What about during the nights; won't the capacitor discharge itself when the solar cell is not charging it?

I don't see that working. :\

 

The LED would be damaged by the uncontrolled current that would flow were it connected to a fully charged cap- you need a resistor to limit this current flow.

In general, the circuit would be rather disappointing, the LED forward voltage (1.8 V approx for a red LED) is a large portion of the peak voltage, as soon as the cap voltage dropped below this point, the LED would go out- leaving most of the available charge unused. Note that other LED colors might not light at all at 2.7 V

 

Are you really sure this is what your girlfriend would want most for her birthday?

 

Why do you expect the capacitor to reach 2.7V in a week?

What about during the nights; won't the capacitor discharge itself when the solar cell is not charging it?

I don't see that working. :\

True, I didn't think of it discharging at night. Is there any way to prevent that?

The LED would be damaged by the uncontrolled current that would flow were it connected to a fully charged cap- you need a resistor to limit this current flow.

In general, the circuit would be rather disappointing, the LED forward voltage (1.8 V approx for a red LED) is a large portion of the peak voltage, as soon as the cap voltage dropped below this point, the LED would go out- leaving most of the available charge unused. Note that other LED colors might not light at all at 2.7 V

Thanks, I'll be sure to add a resistor, should I also add a diode to protect the solar cell. This is the first time I've messed around with capacitors. I did choose red because I knew it has the longest wavelength/least energy to work. Ideally what would happen is that the led would emit a decreasing glow depending on how long it had been since the last discharge, it doesn't have to be spectacular. The capacitance can be changed.

Thanks for the help,
Phaedo

 

True, I didn't think of it discharging at night. Is there any way to prevent that?

Maybe if you also add a one or two small button cells...

 

Maybe if you also add a one or two small button cells...

What so that the solar cell charges the cells and they charge the capacitor? Surely I might as well just use 2V worth of button cells in series then and scrap the capacitor?

Edit: Or just to provide a tiny current to keep the capacitor from discharging?

 

First of all, you've waited too long. Go buy flowers and dinner and work on this for the next time.

Consider using the circuit from a solar landscape light. Replace the LED with the one you like, and use the capacitor as the battery. I think this will solve all the issues raised so far.

 

First of all, you've waited too long. Go buy flowers and dinner and work on this for the next time.

Consider using the circuit from a solar landscape light. Replace the LED with the one you like, and use the capacitor as the battery. I think this will solve all the issues raised so far.

Haha, maybe you are right! It was only going to be supplementary anyway, more for my amusement ! I am happy with a red LED. What do you mean 'use the capacitor as the battery'? Thanks though.

 

What do you mean 'use the capacitor as the battery'?

Exactly that. If you want to use the capacitor instead of a battery, go ahead and swap it in. Or just use the battery - it'll hold a lot more juice for a given size and weight.

 

Exactly that. If you want to use the capacitor instead of a battery, go ahead and swap it in. Or just use the battery - it'll hold a lot more juice for a given size and weight.

I started of with a capacitor in, but a battery is seeming like a better idea now. But I don't know of a battery that could power the led and be charged by the solar cell and is small enough!

 

It's just a matter of how small you need. See this example.

 

It's just a matter of how small you need. See this example.

The solar cell only outputs 350mV max as far I can work out though and that battery needs 2.8V-3.3V. Good find though!

 

Don't know what type of LED you are using, but regular ones need around 1.5V; so just one solar cell won't be enough.

I have a miniature torch that uses 3 button cells in series (in a tiny space) to light just one regular blue LED.

 

The solar light circuits all use a boost or "joule thief" circuit, so that only a single battery is needed.

 

The solar light circuits all use a boost or "joule thief" circuit, so that only a single battery is needed.

Do you know of an example schematic?

 

Take a look at this thread, and the other things it links to. You can also google for "joule thief" and you'll be overwhelmed.

 

It might be easier to use button cells, as mentioned before, and a 10M ohm or so resistor connected directly to the Capacitor, so it is always charging. Then, using the magnetically triggered switch you mentioned earlier, you can connect the LED (through a resistor, to regulate current) to the capacitor. The capacitor will always be charging, even when the LED is ON, but that's ok, as it'll be a very small trickle.

Another big flaw, however, is that if you only ever turn on the LED when you want to see how long you've been away, the capacitor will never have a chance to fully deplete, and it'll always be bright when you turn it on.

 

Just saw a video that might help you in your project. It's basically what you want to build, but without the timer. It uses 10 solar cells, a choke, a supercapacitor and a joule thief.

http://www.youtube.com/watch?v=ghB2irHIN8I

 

Just saw a video that might help you in your project. It's basically what you want to build, but without the timer. It uses 10 solar cells, a choke, a supercapacitor and a joule thief.

http://www.youtube.com/watch?v=ghB2irHIN8I

That's awesome thanks! That and a reed switch would basically do what I envisaged! I could use less solar cells to control how often you'd need to flip the switch from off to on.