Circuit for solar garden lamp

[bjarne]

Hi.

I'm working on a circuit for a solar lamp. I use a solar panel with 5.5Vp / 200 mA and 3,6 V battery. I found this circuit http://www.circuitdiagram.org/automatic-solar-garden-light-circuit.html useful.

I have assembled the circuit on a bread board and it works. However I measure about 2 V over the BC547 transistor(collector emitter) when the solarpanel is producing 0V. Meaning that the voltage for the LED is too low.

Is it possible to modify the circuit in order to get as low voltage over the transistor as possible? Basically I wish to use the transistor as switch to turn the led on when the solarpanel voltage is low. I'm not sure the transistor fully opens in that state.

Thanks in advance

Kind regards
Bjarne

 

[Joe G]

try thePhotocell Circuit, using a comparator. It might work better. **broken link removed**

 

[JimB]

At a quick look the circuit seems rather poor in several ways.

Without a complete redesign, you could try reducing the value of the 100k resistor, or changing the transistor for a BC547C which has a much higher gain.

JimB

 

[Joe G]

And I bet you tested it w/new batterys? The leds may not even light if the panel dosen't charge the batterys enough and/or it's cold outside. also do a search on garden lights on this forum, there has been quite a few topics on it, it will just take a bit of your time and you will learn alot more

 

[audioguru]

The BC547 is too weak to light 10 LEDs brightly. Its absolute maximum allowed collector current is only 100mA and it performs poorly above only 50mA.

It is wrong to connect LEDs directly in parallel like that unless you buy many, test them all then select ones with exactly the same forward voltage or one or two will hog all the current and quickly burn out while the others are not lighted, then the others will burn out.

A transistor as a switch needs a base current that is 1/10th its collector current as shown on every datasheet. For 10 LEDs, each at 15mA then the collector current is 150mA and the base current needs to be 15mA. Then the series base resistor must be .... HOLY SMOKES! The transistor is not a switch, it is an emitter-follower. No wonder it does not saturate.
It is a horrible circuit.

 

[bjarne]

Wow. Thanks for all answers

Joe G: Looks interesting. I might use something similar to circuit B. I need to use the solar panel as dark indicator. Say if the voltages drops below 0.5V the LED turns on. I do not know the operation of opamps so I have to look in to this. Is it possible to run a opamp with 3.6 V and lower(when batterys are drained)?

Searched with google and on this forum to learn more. With some luck. Many circuits out there and most uses a npn or pnp transistor for switching the LED on/off. My batterys are brand new.

JimB: Tried reducing down to 10k and it does help a little. However still a huge voltage drop 1.5-1.6 V. Haven't tried the BC547C - since I haven't got any. Using BC547B so far. Unsure if it's worth ordering some.

Audioguro: Oh sorry. I never intended to use more than 2 LED's. I follow our considerations regarding the transistors. I would prefer something thats either on or off.

In addition to the circuit I've added a simple "joule thief" circuit for powering white LED's. This works just fine and I intent to keep this - still needs a proper "dark detector" switch.

 

[audioguru]

The comparator in "circuit B" uses an LM339 quad comparator or an LM393 dual comparator, not an opamp.
The datasheets for these comparators show that their minimum supply is 2V. Their output current is too low to brightly light an LED.

 

[Joe G]

I would use something better as AG stated, something like a MCP6541 or MAX921. There are probably even better ones now

 

[bjarne]

Thanks again. It seems like there isn't any "easy way" to do this. With the modifications suggested by JimB and the joule thief circuit it works fairly. It's able to lid up two white led's reasonably bright. Therefore i'm going to stick with this.

 

[audioguru]

ONE of my solar garden lights has lasted for 3 years! Most of them lasted for only 6 months because the Chinese Ni-Cad battery cell rusted away. I replace the cheap Ni-Cad with an American Ni-MH cell that has a stainless steel case that does not rust.

Do you believe it? Some of the solar panels on my cheap Chinese solar garden lights got sunburned which caused them to fail. I have two with multi-color LEDs and I replace the discharged battery with a charged battery every three days.

 

[bjarne]

Sounds familiar. It is almost impossible to find a decent quality solar lamp in my area. Lots of cheap models and most likely useless all of them. Last one I bought died because of rain - flooded housing.

 

[audioguru]

I make sure that rain cannot get in my solar garden lights but condensation causes their cheap metal to rust away.
Even the wires on the LEDs rust away.

Sunlight damages the LDR that turns it on at night. Too much heat?

 

[NorthGuy]

My wife buys lots of them. Some just deteriorate within weeks when put outdoor. Some are good quality. Regardless, batteries stop working well when it's cold and certainly cannot survive Canadian winter. Therefre they only work from mid May to mid October, when it's plenty of light anyway.

They could be made better with bigger panels, brighter LEDs, and durable batteries, but who would pay $50+ for a garden light.

 

[4pyros]

Here is a nice Kit;
https://www.bakatronics.com/shop/item.aspx?itemid=586
$15 but mite last as long as your batteries, uses LED for sensor.

 

[bjarne]

I think that solar panel is way to small for powering 5 LED's. It would require many more sunny hours than available in my area.

 

[audioguru]

Some of my newest solar garden lights came with genuine Westinghouse (and you can be sure... if it's Westinghouse) lightweight AA size Ni-MH cells. But their capacity is only 350mAh which is about 1/7th the capacity of an AA Ni-MH cell I can buy. It says "made in China" in English, French and Spanish. It also says, "Do not swallow" do you believe it?
These cells lit the solar garden lights for only about 1.5 hours at dusk but were not lit at night. I replaced them with 2300mAh cells and they are lit ALL NIGHT LONG even in winter.