I'm looking to make a circuit as a small part of an A-Level DT project. I have soldering experience and have completed circuit kits such as amplifiers, but beyond that I have little experience and I am extremely inexperienced with actually devising circuits myself. The circuit I am aiming for is based upon one on this page, under the subheading "Adding a battery", with the only significant difference being that I would add a switch to control whether or not the LED's are receiving power. The basic components of the circuit are explained below the diagram on the link provided.

• http://www.evilmadscientist.com/2008/simple-solar-circuits/

The solar panels I have are 2 "3V 100mA 0.3W TruOpto OPL30A10101" Solar Modules, which I purchased on:

• http://www.rapidonline.com/Electric...0-3W-60x48x3mm-with-20cm-Flying-Leads-56-0124

I also have a 4.5V 150mA 900mAh NiCd cell that I would like to use as the rechargeable battery in the cell, but I can also purchase any additional or replacement cells if they would work better.

I'd like to have the 2 solar panels (Or one if that simplifies things drastically) functional, on top of an architectural model, with the battery hidden and the LED's wired up to the lighting in the house, so 2 or 3 LED's would be optimal, so that the solar panels would charge a battery and also LED's which could remain powered in the absence of the solar panels producing energy by using the stored energy in the battery. I don't know where to put things in series or parallel although I have done several years of Physics so I have some understanding of circuitry and the differences between the two types. I really have no idea about which resistors or diodes I would require, and I'm unsure as to whether the circuit depicted would even function with the battery powering the LED's because it seems like the current direction would be different when coming from the panels as opposed to coming from the Battery, and as LED's are diodes it would only work when coming from a single direction? I'm sure there must be a simple solution I do not know about, and apologies if this is too simple a problem to be posted about or if I should have tried to solve my problem from similar threads but i tried that to no avail. I'm more than happy to provide additional information and do the legwork if more information is required, any advice about different power sources or even completely different circuit schematics or advice in general would be greatly appreciated.

Thanks in advance for any responses and support
Dan

 

Your first post. Welcome to AAC!

as a small part of an A-Level DT project

Does that mean this is a school project for which you'll receive a grade?

The circuit I am aiming for is based upon one on this page, under the subheading "Adding a battery", with the only significant difference being that I would add a switch to control whether or not the LED's are receiving power.

Did you read the next section that discusses using the solar cells as a photo sensor?

 

Use a Pr4403 chip

http://forum.allaboutcircuits.com/threads/pr4403-led-driver-chip.118513/

 

Yes, it is part of a school project, but the overall project is an architectural model and the circuit is only required to be simple in order to demonstrate solar energy. I don't know anything about how to use chips or how to set them up, could you elaborate on how I'd use them?

Yeah I read all of the page that I linked, I didn't think using it as a photo sensor would be necessary but if that is required I could implement it, but all of the functionality that I'd like is the circuit shown with an additional switch that controls the LED's.
Thanks

 

Two 3V solar cells in series won't give you sufficient voltage to charge a 4.5V NiCd battery pack because you need a blocking diode to prevent discharge through the solar cells when they're not charging the batteries.

NiCd cell nominal voltage is 1.2V, so 4.5 isn't a valid voltage. If it's really 4.8V, charging problem is even worse.

 

I used a voltmeter across the battery and read 4.52V and it comprises of 4 cells so that means it is 4.8V then? In which case, which battery would be more suited and if named battery is to be used, how do I go about deciding on a suitable blocking diode, and where in the circuit would the diode be implemented?

 

Use 3 NiCd cells. Diode selection is not critical, anything with a current rating of at least 100mA. This is the circuit from the URL you provided.


You could use your 4 cell NiCd battery, but it would never fully charge.

 

Is there any specific issue with the battery never fully charging, other than..the wasted cell? Also, where could a switch be added to determine if the LED's receive power? As shown, adding a switch, even after the junction in the centre at the top, would prevent the battery from charging also? And would the 2 solar cells produce 6V when in series? Many thanks for the assistance so far, I'm asking as many questions as possible in an attempt to further my understanding

 

Is there any specific issue with the battery never fully charging, other than..the wasted cell?

Runtime and the fact that the cells will develop memory and never charge to full capacity.

Also, where could a switch be added to determine if the LED's receive power?

Anywhere in the loop made up of the batteries, resistor, and LED.

And would the 2 solar cells produce 6V when in series?

Yes. In series, voltage adds. In parallel, current adds.

 

Use 3 NiCd cells. Diode selection is not critical, anything with a current rating of at least 100mA. This is the circuit from the URL you provided.
View attachment 96580

You could use your 4 cell NiCd battery, but it would never fully charge.

1N914 for a diode? May I suggest a 1N5817 Schottky diode. Less loss across the diode.
It was previously mentioned that the solar cells may fall short of giving the battery a full charge. No problem there. The battery will still charge to a sufficient level to demonstrate the concept.

 

1N914 for a diode? May I suggest a 1N5817 Schottky diode. Less loss across the diode.

Splitting hairs are we now???

It was previously mentioned that the solar cells may fall short of giving the battery a full charge. No problem there. The battery will still charge to a sufficient level to demonstrate the concept.

It's still useful to know so the OP won't get the impression that it's okay to do in other applications.

 

I will look into getting that diode, thank you. I'm not particularly bothered about not maximizing on the battery and I don't mind if it lowers it's potential as I'm unlikely to use it in any future project, but thank you for making me aware. Sorry for wanting reiteration, but you say the switch is okay to be placed anywhere in the loop on the right of the diagram? But if the switch is open, surely there is no complete circuit to allow the solar cell to charge the battery? At first glance i thought it could but does it not require that the LED be passed through initially before the current reaches battery? And so if the LED's are cut off how can it reach the battery. I'm almost certainly incorrect I'd just like to understand fully. Finally, would the 140 Ohm resistor still be suitable? In which case what rating LED's would I need and could I use two or three, or would the voltage be too low to support more than one? Thanks

 

I will look into getting that diode

It doesn't need to be 1N914, any diode that can handle 100mA would be sufficient. In a more robust design, you would derate from component maximums, but it isn't critical in this application.

you say the switch is okay to be placed anywhere in the loop on the right of the diagram? But if the switch is open, surely there is no complete circuit to allow the solar cell to charge the battery?

There is always a complete circuit for the battery to be charged when the solar cells are providing sufficient voltage.

At first glance i thought it could but does it not require that the LED be passed through initially before the current reaches battery?

The LED can be turned on by the battery or the solar cells. Which ever is providing the higher voltage will power the LED. The diode reduces solar cell voltage.

would the 140 Ohm resistor still be suitable?

It depends on the requirements for the light from the LED. If you're just trying to demonstrate on/off, you can operate the LED at a low current; which would extend battery life. If you want useful light, you need to operate at a higher current. A standard operating current for LEDs is 20mA. If you operate at a higher current, you get more light. Use Ohm's Law to calculate resistor value: R = V/I.

In which case what rating LED's would I need and could I use two or three, or would the voltage be too low to support more than one?

Again, it depends on your requirements. If it's a simple indicator, any LED operated from a few mA to 20mA is fine. Typical forward voltage is around 2V for red, green, orange, or yellow, with red typically being the lowest. Forward voltages for blue and white are higher.

 

How much voltage am I working with exactly? Should I use the 4.8 from the battery as a guide? Or the 6V from the cells? I'll decide upon the LED's I have available to me and calculate the resistor I need from there, but again in order to use the formula, which voltage do I use to calculate it? Thank you for all of the advice

 

How much voltage am I working with exactly? Should I use the 4.8 from the battery as a guide? Or the 6V from the cells?

The battery will limit the voltage to around 4.8V. To insure long LED life, choose a sane operating current and allow for the voltage to be 5.3V (full sunlight with the battery removed).

 

The battery voltage will be the relevant voltage. Once connected to the battery, the PV voltage will be drawn down to that.

What dl324 said!

 

I think I have all the information I need currently, I'll check back in a day or two when my components have arrived and i've had time to try assemble the circuit. In the mean time, thank you so much for all of your help, and special thanks to dl324 for tolerating my repeated stupidity. Thanks a lot
Dan

 

Splitting hairs are we now???
It's still useful to know so the OP won't get the impression that it's okay to do in other applications.

re: diode
Just looking to save every fraction of a volt possible since he is cutting charging voltage so close. True, this would not be a good idea for a commercial product.