hi A45,
Have you decided to abandon the original circuit showing a Solar charger with a 6v SLA battery idea.?
A 6v SLA battery should not be discharged below 5.4v else it could be damaged.
E

 

I also notice the discussion about the voltage used. Before I get too deep into design I need a firm voltage to work from.

 

Wendy,
You want 53 LEDs? Just curious, why the odd number (and a prime to boot)? Are you trying to create a pattern?
Yes it is a pattern for house numbers.
Ericgibbs, & Wendy,
Yes I want to use 12 volts.

 

Also I'm using the circuit with a Mosfet, and Not the transistor Wendy! See post #15.

 

I'm assuming white LEDs, if not white or blue let me know, it does affect the design.

By switching voltages you have radically changed the design. The MOSFET has to be a normal unit, not a logic level unit. This is actually good, as almost any MOSFET will work.

 

OK, here is what I came up with on the merge.



It is pretty preliminary, I would test it first before committing to it. The LEDs will not be at full brightness, but they would probably last all night.

I would consider adding a shut down for low battery voltage. I will be glad to add one, but I wanted to get this part done.

If you would like to see how I would do it with a constant current source I would be happy to draw that up too. It would suck the battery dry pretty quickly though, since there would be 18 LED legs, each drawing 20ma for a total of 0.36A.

While I show TLC 555, they are just CMOS 555 chips, a 7555 would work fine too. You could even merge both chips into a dual 555 called a 7556.

 

Hi Everyone,
I'm thinking of making this circuit for a project for a friend new letter box, to illuminate with Leds over letterbox numbers.
I want to make this circuit in 12 volts version.
I will upgrade the following:
Solar Panel to 12 volts @ 10 Watts http://www.rpc.com.au/catalog/cnpv-12volt-solar-panel-10watt-p-2701.html
Battery 12 volts @ 15Ah
I will change the BD140 for TIP32 as the TIP32 is to energised a 12 volt 3 Amp relay, as I'm running more Leds then below.
Do need to beef up the circuit anywhere else.
Any help would be greatly appreciated

The parallel combination of 8 LEDs is 8x 20mA - if your BD140 can handle that current, it will do so if you modify for 12V.

You need to calculate new values for each LEDs resistor - subtract the sum of the Vf of however many LEDs you put in series from the value of supply voltage, take the amount that's left and calculate the value of resistor to drop that voltage at 20mA.

Looks to me like everything else will be happy with 12V.

 

The most per leg you can get with 3.6V Vf LEDs is 3 LEDs, so that would be a total of 18 LED legs.

With a joule thief arrangement the least you can use per leg is 4 LEDs, which is why I used what I did.

 

The most per leg you can get with 3.6V Vf LEDs is 3 LEDs, so that would be a total of 18 LED legs.

With a joule thief arrangement the least you can use per leg is 4 LEDs, which is why I used what I did.

The TS wants to convert to 12V - at the stated 3.6V per LED, you can just about get away with 3. That comes to 10.8V, which is the absolute minimum discharged voltage of a lead acid battery (many texts advise not discharging below 11.5V).

In addition; the stated 3.6V seems a tad high - about 3.4V would seem more like it.

 

The real problem is Vf variability. It is going to be hard to come up with a current limiting resistor that is predictable, which is why I was talking a constant current solution. Then there is the issue of battery voltage dropping as it discharges.

The joule thief solution doesn't care about Vf or battery voltages.

 

Wow thanks Wendy,
OK here some answers.
1./ Leds are white, read below:

• 100% Brand new and high quality
• Ultra-Bright 5mm White LED Light Lamp Diodes
• Emitted Color: White
• Lens Color: Water Clear
• Viewing Angle: 20 ~ 25 Degree
• Absolute Maximum Ratings (Ta=25°C)
• Max Power Dissipation: 80mw
• Max Continuous Forward Current: 24mA
• Max Peak Forward Current: 75mA
• Luminous Intensity: 15000mcd
• Forward Voltage (V): 1.6-2.1V
• Reverse Voltage: 5~6V
• Diameter: 5mm
  
  
• 2./ Is the Mosfet a negative Mosfet?
• 3./ What is CR1 what the spec on this TVS diode? can I use this:http://www.jaycar.com.au/PRODUCTS/A...iodes/UF4003-1A-200V-Ultrafast-Diode/p/ZR1034
• 4./ Also I've pre-arrange my schematic leds board for the previous layout (eg: 17 x 3, 1 x 2) in led's as I'm using 17 smd resistor at 68 ohms, and one 270 ohm smd resistor.

 

you are going to find your currents between 17ma to 27ma. This is the problrm when pushing the limits with LEDs and power supplies. It probably won't matter in your application though.

Yes, it is a N-Channel MOSFET. Very generic.

I have a tutorial on LEDs in general.

LEDs, 555s, Flashers, and Light Chasers

 

Sorry I made a mistake, leds layout as follows:

 

hi A45,
The LED V drop of 2.1V, is too low.
Most of the White types I have used are around 3v thru 3.3v

E

 

Well I'm only going by what the seller on Ebay specified! Must be 3.1 Volts!!

 

Forward Voltage (V): 1.6-2.1V

Unless there's a whole new breed of white LEDs out there that I haven't heard about I think that's a typo in the spec .
As Eric says, they're normally >3V.

Edit: If you have one of those LEDs to hand, connect it in series with a 9V battery and a 470 Ohm resistor and measure the voltage across the LED. Post the result.

 

Well I'm only going by what the seller on Ebay specified! Must be 3.1 Volts!!

If you have them on the bench, use a suitable limiting resistor with an LED for say 20mA and measure the forward drop, 'W' will need to know the actual value.
Try about 6 LED's to get an average value

 

Waiting on delivery, will have in the next 2 weeks.
I will do test when they arrive, and let Wendy know. Thanks guy's.

 

Hi Everyone,
Sorry for not replying any sooner, just received the Led's this morning.
Just finished testing 6 Led's all read 3.1 volts with 470 ohms resistor in series with led on 9 volts and check voltage with a digital meter.
Cheers Drew

 

hi A45,
Have you decided to run the LED's at 20mA rather than 10mA.

Are these the Amazon/Chinese LED's, they are bright at 10mA.
E