Starting a project, solar powered night light

[Smoke_Maker]

I want to start a project but which area should I use for advise?? I have purchased my last wall plug-in night light What I want to build is a solar power low voltage LED night light. What do you say is this the right forum or what??

[sofaspud]

Sure! Probably the first step would be to determine how much light and run time you want, and choose the LED(s). White LEDs will brighten the path, but red will be easier on sleepy eyes.

[haklesup]

I'd stay right here. All the other new Forum sections are for specific columns in the 2 magazines. Unless you want to start a project featured in the magazine, this still seems to be the best section to post in.

Based on the heavy magazine content added, I expect they will be directing readers to the forums more prominently than before. If so, it will take a few months to kick in and the readers to get used to posting in the forum instead of writing letters directly to the editor. I hope it works, It would be nice to see some fresh avitars in here.

[VernGraner]

I'd stay right here. All the other new Forum sections are for specific columns in the 2 magazines. Unless you want to start a project featured in the magazine, this still seems to be the best section to post in.

Actually, we do have a section devoted to reader projects right here:

• Reader Projects
  If you've built a project from the magazine, or have something fun and interesting to share, show it off here!

Vern

[Smoke_Maker]

I'd stay right here.

OK, so I will stay here to work out the project and then take it to the READERS PROJECT when it is finished.

Specs. are in the works. I want this to be durable, long lasting, and trouble free, not a quick easy project.

12 lamps, make each one a individual unit, or 2 one unit, or 3 or all lamps one unit as in solar panel and battery. ? Scalability

4 watts, that is the current one now, I think 3 watts Will work just fine.

Ni cad, Ni Mi, lithium??

24 hour capacity in case of a cloudy day, no 110v back-up needed.

[sofaspud]

Hmmm... is there something more about this application we should know? You say, "24 hour capacity in case of a cloudy day, no 110v back-up needed." Do you need a nightlight on a cloudy day? You can read the newspaper at 3AM with 12 LEDs. I was thinking this was a hallway/foyer/path to the bathroom-type nightlight.
IMO a sealed lead-acid is the way to go BTW. But we'll cross that bridge when we get to it.

[haklesup]

What you propose sounds like an improved yard light. The basic principal is that the solar panel charges a battery during the day while the lamp is off and at some point switches over to delivering power to the load.

For starters, with the LED lamps you plan to use, determine the current needed to light them. Lets say 1.2V, 3A (3.2W). To keep that lamp bright for 8 hours you will need a battery with 24Ah, thats a pretty big battery. As for battery chemestry, a LiIon is probably smaller but probably more expensive too. NiMh is a good candidate, NiCd would probably not have enough capacity and Lead Acid is also a good choice. Lead Acid will reduce voltage gradually as it discharges wheres the LiIon or NiMh will maintain voltage longer and go flat more abruptly at the end

Bells and whistles come if you want to keep the lamp brightness constant until the battery cannot run the circuit any longer or if you don't mind it dimming near the end of the night. For that you would need to consider an LED driver IC.

Central to the project is the curcuit that decides when to switch from charge/Day mode to Night Light mode.

As for the solar cell, you will need to select one that can fully charge your selected cell in 6 hours (so you get good performance even in the dead of winter.

As an engineering practice, you need to see the range of each of these components of the design, weigh the tradeoffs and select a set of parameters and components that will come close to the perfect lamp without becoming too expensive or too unweildy.

I personally would start by examining how solar path lighting works. It sounds like you want to do the same thing but with a brighter light and longer run time. The shortcut project would be to just upgrade a path lamp with a bigger battery, bigger solar collector and bigger lamp.

[haklesup]

Actually, we do have a section devoted to reader projects right here:


Reader Projects
If you've built a project from the magazine, or have something fun and interesting to share, show it off here!

I guess I missed the "or have something fun and interesting to share" part and focused on the first clause

If that's the intent, much of what is in General would fit in that new section. Many posts are about starting, designing, debugging or showing off a project in some way. The description sort of implies that the projects are complete though. If you want most project traffic in that area, you might want to tweek the description a bit. FWIW

[Smoke_Maker]

Hmmm... is there something more about this application we should know? You say, "24 hour capacity in case of a cloudy day, no 110v back-up needed." Do you need a nightlight on a cloudy day? You can read the newspaper at 3AM with 12 LEDs. I was thinking this was a hallway/foyer/path to the bathroom-type nightlight.
IMO a sealed lead-acid is the way to go BTW. But we'll cross that bridge when we get to it.

I explained it wrong, 12 different lights for the house, hallway base board, kitchen, etc. and for the 24 hours I just want it to be overbuilt so in some unusual extremes like a another nuclear winter my lights will work for two or three days

[Smoke_Maker]

For starters, with the LED lamps you plan to use, determine the current needed to light them. Lets say 1.2V, 3A (3.2W). To keep that lamp bright for 8 hours you will need a battery with 24Ah, thats a pretty big battery. As for battery chemestry, a LiIon is probably smaller but probably more expensive too. NiMh is a good candidate, As for the solar cell, you will need to select one that can fully charge your selected cell in 6 hours (so you get good performance even in the dead of winter.

As an engineering practice, you need to see the range of each of these components of the design, weigh the trade offs and select a set of parameters and components that will come close to the perfect lamp without becoming too expensive or too unweildy.

I personally would start by examining how solar path lighting works. It sounds like you want to do the same thing but with a brighter light and longer run time.

Your right Hak, my cheap failing yard lights gave me the idea when I first put them in a year ago.

OK how did you come up with a 24Ah battery, now I'm learning something.

So I'm down to two chemistry's, LiIon and NiMh, I will do the research and get the cycle life of the batteries, I don't want to overcharge and undercharge the batteries, I'm thinking max charge should be about 85% to 90%, and discharge should be no lower than 30% (batteries don't die, they are murdered by bad design and stupid people)

I have done some hobby electronic work before but I can barely remember which end of the soldering iron to pick up, but it will come back quick.

[sofaspud]

3 amps times 8 hours is 24 amp/hours. That isn't truly realistic though. For example, if you want no less than 30% discharge then you would need a ~72Ah battery. That's for 1 night's use. You've chosen lithium-ion or nickel metal hydride cell technology. A quick look at Digi-Key showed some 12V 3.2Ah NIMH, with "Call" where the price should be........
Let's determine what our objectives are before we jump too far ahead.

[dyarker]

3A seems like A LOT of current for a dozen or two of LEDs!

[Smoke_Maker]

3 amps times 8 hours is 24 amp/hours. That isn't truly realistic though. For example, if you want no less than 30% discharge then you would need a ~72Ah battery. That's for 1 night's use. You've chosen lithium-ion or nickel metal hydride cell technology. A quick look at Digi-Key showed some 12V 3.2Ah NIMH, with "Call" where the price should be........
Let's determine what our objectives are before we jump too far ahead.

wooops, nothing is out not even lead acid ($ ). I looked last night and found NiMh to have 500 cycle life and LiIon to have 1000 cycle life, now thats full useful working range so short DOD (depth of discharge) will increase that number. With nothing in or out yet I would like to make things a little worse, the sun works on a 12 hour cycle. As Hak (I hope it's OK to shorten your name?) suggested the yard light scheme will work with the solar cell voltage turning on and off the light.

How many Led do I need for one light? I will have to find 1 watt light and a 2 W and 3W to see what is really needed, let me see what I can find. Wait a minute I have some white LED I purchased 1 year or so ago and I have a bench top power supply and a weekend.

[Smoke_Maker]

3A seems like A LOT of current for a dozen or two of LEDs!

You're right, the luminosity of LED and resistive lamps to current are different (this is good for me)

I pulled this off the Internet Yahoo answers

"The color and type determines the required voltage. This ranges from 1.8V for a certain red, through 2.4V for reds, oranges, greens, yellows. Violet, UV, blue and white go up to about 5V.

Low current LED's go down to 1mA at 1.8V (red) for their rated output, which is 1.8V * 0.001A = 2mW or 0.004W.

Most common LED's use 20mA at 2V or so, so about 40mW (0.04W).

Large LED's can use up to 5V and 0.6A, or 3W, and bigger ones are coming out all the time. The voltage pretty much stays the same, but the current goes up."

A quick look at digi-key
http://search.digikey.com/scripts/DkSea ... 76-1008-ND
600Ma at 13.2 volts

I will dig out the LED I have and get the data sheet. But right now lets assume 0.6A for 3 watts at 5 volts, .6Ax12hours = 7.2+(.25capicity) 1.8 = 9Ah per day x 2 days = 18Ah.

Large format LiIon is looking good but lead acid look even better. With a shallow discharge I will be able to get my 5+ years out of them, where LiIon will get 10++ years.

May be I will build some of each and let you know the results in 15 or20 years

I still need to know how many watts I want, so I will do the experiment tonight and see how many watts I am comfortable with.

[sofaspud]

You may want different amounts of light for the different areas. What you could do is get a 5 or 6 volt battery and a 100 ohm or larger potentiometer and test various LED output levels in those areas. Record the data to use in the design.