Best way to wire a bushel of LEDs?

[Peter_wadley]

Ok,

Ive wired the letters up.. included are some pictures of the project (i'm modifying a terminator poster)

The brightness of each letter is very close .. i chose to vary the resistance for each batch of LEDs .. keeping current of each around 22ma...

Now I would like to implement the PIC to control the letters..

I am having trouble wiring this..

I was going to have the PIC to control NPN transistors which would supply 24 volts to the each letter..

The LEDs are wired in series.. so to turn on a letter I must supply positive voltage, which goes through the resistor, then through the batch of LEDs... which then goes to a common GROUND connection which is located on the back of the poster.. (each letter shares a common ground)

The problem is.. how do i wire the transistor to supply 24 volts to the one red wire coming out of each letter?

I understand the load for transistors should be in series .. from the +V to the collector pin..

I am very confused about what to do.

Thanks for any help

 

[ronjodu]

Very cool.

Very cool project. You could add some really cool effects with a MAX chip like weegee suggested. My favorite is the MAX6957 (SPI version).
I have 3 cascaded together driving 4-7segments with DP and 2-10 segment bargraphs. Only 3 pins from the pic micro are used. Looking forward to updates on your progress...

 

[Peter_wadley]

Hey,

Not trying to nag..

but im still trying to figure out how I can wire this ...


I would use the PIC to directly source the LEDs.. but 24v through a PIC = smokey!

Can someone just let me know if its possible to use a transistor without wiring in series.

The project is kind of at a stand still now.. taking up my workbench..

thanks for any help!

 

[weegee]

sorry, but im now lost. if your just lighting the leds, and not doing anything fancy - why do you need the pic? it's just consuming board space and using un-neccisary parts.

anyway - suppy + to the annode of your led string via suitable resistor, and connect the common cathode to the collector of your transistor. connect the emmitter to gnd, and the base to your pic pin(via current limiting resistor). i think thats right

 

[justDIY]

you want an NPN transistor if you're switching the low side (-) of your led array, or a PNP transistor if you'd rather switch the high side (+)

it doesn't matter how you have the leds wired, as long as you can break the circuit somewhere to stick in your transistor.

http://www.kpsec.freeuk.com/trancirc.htm

 

[weegee]

oops, forgot that - my description was for an NPN transistor - sorry.

 

[Peter_wadley]

Ok

Yeah i thought i would need to go in series like your saying.

Its just that I have wired all the cathode commonly on one wires ...

I guess for each letter I will need a positive wire and a ground wire.. then put it in series with the transistor..

thanks

 

[blueroomelectronics]

Something like this
**broken link removed**

 

[justDIY]

Its just that I have wired all the cathode commonly on one wires ...

if all your leds are wired in parallel, I hope you're not still using the 24v supply... you'll be wasting almost all of power in the resistors ... you'll have a heater that also happens to be a light up sign.

 

[Peter_wadley]

I have it wired like this.

Would this be considered parallel?

Each letter has its own, different set of resistors.

 

[justDIY]

Peter,

That wiring looks fine ... your transistor would interrupt the circuit where after all the led strings are tied together, before they connect to the negative lead of the power supply