hi,
im trying to do a simple 8x8 one colour led matrix, but with some 12v led strip light.
i have the wiring done, but can't get my head around the usage when attached to the 12v lights.
if i run 12v into the arduino, it'll be destroyed, so ill use the output of the arduino to trip a transistor and use it like a switch.
but the other end (column side / negative) i need to use the arduino to send the excess voltage to gnd.
is the attached image correct?
The PNP will not turn off.
A PNP is on when its base is less positive (more negative) than its emitter.
The Arduino outputs 0 and 5V, both of which are less than 12V.
With its emitter at +12 it won't be truly off till the base is > 11V.
Abu need to replace the Ono for then on and have it flow to ground?
Sorry, this forum doesn't work well on iPhone! So this looks ok if I change the pnp for an npn?
No.
now i know i can't just run the 12v from the leds back into the arduino, that will definately cause some damage, but then how do i redirect the bulk of the voltage to the ground without using a pnp (wrong type) and not an npn?
Connect + of LED strip to +12.
Connect - of LED strip to NPN collector. NPN emitter to Gnd. Arduino pin to 180 ohm resistor to NPN base.
If you have a bunch of strips, then use ULN2803 as NPN buffer for Arduino pins, or as buffer for 74HC595 shift register, or use TPIC6B585 shift register with high current drive outputs.
Most strip lights have built in current limit resistors.
That certainly takes care of the current path to Gnd, but I think that he wants to be able to strobe the positive (high) side, too.
That's correct mr pancake. I feel that the transistors need resistors on the base and the collector to base but my dummies guide to electronics doesn't state how to work them Out, or because of the low voltage is this not needed?
What's the skinny on these "LED strip lights"?
Got a link with the details?
Anyway -- MIC4427 "MOSFET drivers" can be used for this sort of thing.
they're the pretty standard ones available on the bay-of-e.
LED type: SMD3528
Color: Cool white, gold yellow(Warm White), Sky blue, Grass green, Fire red
LED Quantity: 300leds / 5metre
View angle: 120°
Working input Voltage: 12VDC
Working power: 4.8W/M
Working Current/meter: 0.35-0.4A
Working Temperature: -20 to 50°
Luminous Flux: 180-240 Lumens/Meter
Protection Rate: IP65 (waterproof)
Size: L500cm (5M) xw0.8CM Xt0.30cm
Can be cut per unit (3 LED)
they are 3 smd3528 white leds in series, with an smd 131 resistor, and then wired so that each 3 led sequence is in parallel to each other. ive broken each -ve connector, laid out horizontally 8 sections (of 3 led) and then comnected all the -ves vertically.]
quite simple, and the 12v supply should give a good bright output.
then test run quite happily on a 9v pp3 battery, which means i can't really destroy the arduino whilst i'm testing!
i have taken some production photos, not that anyone would be interested in this basic project! lol
CrossRoads:
Arduino pin to 180 ohm resistor to NPN base.
thanks, but why 180 ohm?
Limits the current from arduino pin into the base of the transistor.
Vbe of a NPN transistor is usually about 1 diode drop, or 0.7V.
(5V - 0.7)/.024mA = 180 ohms.
Looking into this, and at the through hole version tc4426, it looks just like what the resistor transistor would do.
Would the Arduino plug directly into these, and then the LEDs directly on the output of this?
How would the column / negative side switch with these?
I think you should stay with the 4427 (it's non-inverting) that I mentioned.
Right, the Arduino outputs connected to the INA or INB will switch the corresponding OUTA and OUTB. VS is the +12V (that'll be available at the OUTx pins.)
For path to Ground, you could stay with the transistor circuit. Or you could use a ULN2803, no resistors necessary between their inputs and Arduino outputs (they're set up in the IC.) You didn't give me a current spec (don't leave me to go look up this stuff on ebay, just put up a link, preferably the manufacturer's own, OK?) - the ULN2803 has a considerable VCE(sat) at higher currents.
"SMD 131" == 130? ?
I think that 3 White LEDs in series is about 12V of VF.
I'm thinking that this will require a supply > 12V (15V ?) to do right.
So, as you have access to a string of these, then:
what's the voltage across the LEDs and what's the current at the intensity you find satisfactory?
I am using LED strips with same 3528 LEDs, resistors measure 150 ohm.
They light up nicely with 9V, and really bright with 12V.
No ammeter?
ok, this is getting pretty stupid now.
ive changed the transistors on the anodes for the uln2803 and put current limiting resistors on the cathodes...
this chip has just taken most of the electronics out of it!
i still cannot get any of the lights to light up whatsoever...
well, i lie, if i touch one of the leds bases, then they light up a tiny bit, so i think that means that there is some power going to the leds, and that its the grounding thats causing the issue.
on a side note, is there a site that shows just the arduinos mathmatical formulas.
i can get my head around
for(int i = 0; i<8; i++)
i just don't get the ++ part. i know that's what it needs to add calculations, but i haven't found it explained anywhere.
thanks!
kelvinmead:
ok, this is getting pretty stupid now.ive changed the transistors on the anodes for the uln2803 and put current limiting resistors on the cathodes...
this chip has just taken most of the electronics out of it!
i still cannot get any of the lights to light up whatsoever...
Wrong-O**!**
The ULN2803 should provide the path to ground (for the cathodes.)
kelvinmead:
i just don't get the ++ part. i know that's what it needs to add calculations, but i haven't found it explained anywhere.
i ++;
is C for "increment i"
.
blink of course as its a current sink chip...
And with the maths question I'm looking for a more rounded page to not have to ask millions of questions! 
now i have a mental issue... its wired up and flashing, so that's a massive bonus!
but i've only got it flashing by wiring it the wrong way!
i shouldn't be connecting the +12v to the output of the uln2803! unless i've blown the chip...