i need a little help making this circuit (attached you will find the schematics) work. i am able to control the positive side of the LED's but i need to control the ground. i'm not sure how to mate it work i have tried using 2 ULN2003a but not sure it will work. any help is appreciated TIA
For simplicity I would recommend using WS2812 LEDs instead. Tons of info here.
need a little help making this circuit (attached you will find the schematics) work.
No you need a lot of help.
A ULN2003 will only switch to ground, that circuit is totally screwed. You need to connect the output of the shift register to the ULN2003's input. Then connect the output to the LED's resistor, with the other end of the resistor to the cathode. Then connect the anode to a positive supply.
You also need decoupling capacitors on all your chips.
You have connected the common on the ULN2003 to +ve, this is only to engage the flyback diodes when switching inductive loads, so leave it unconnected. You have no current limiting diodes in series with the LEDs, and their are still no capacitors.
But otherwise much better.
Oooh, the forum action of dropping Replys is getting pretty annoying!
0.1uF (100nF) from each shift register Vcc pin to Gnd is needed for stable operation.
Current limit resistors are needed to not burn out your LEDs are critical also. Determine them mathematically:
(Vs - Vce - Vf)/current = resistor value
So if your supply was 9V, Vce across the ULN2003 output was 0.7V (typical for a low current load), Vf was say 2.5V (typical for Red LED for example), and you wanted 12mA of current, then applying Ohms law for voltage, current, and resistance, V = IR:
(Vs - Vce - Vf)/I = R
(9V - 0.7V - 2.5V)/.012A = 483 ohm. 470 ohm is a standard value and will yield a little more current flow:
(9V - 0.7V - 2.5V)/470 ohm = 12.3mA
Determine Vf for your LEDs by measuring the voltage across each color with 5V and a 1K resistor.
Red is usually lower then Blue and Green, and perceived brightness may vary between the 3 of them at the same current.
hezrony96, I forgot to add that you have a nicely drawn schematic using Fritzing. It is so rare to see a schematic come out of that tool, let alone one that was layed out well with clear connections. Well done!
i added the capacitors (100nf), will also add add resistors, and i have disconcerted the COM of the ULN2003 from Vcc. would this circuit work?
A TPIC6B595 chip is the same as the 74HC595, but with mosfet outputs.
It could replace the 74HC595/ULN2003 combo.
Leo..
hezrony96:
i added the capacitors (100nf), will also add add resistors, and i have disconcerted the COM of the ULN2003 from Vcc. would this circuit work?
Yes it would stand a chance , I haven't spotted anything. The only slight worry is that you have polarity markings on the 0.1uF capacitors. These have to be ceramic, no other type will do. Ceramic capacitors are not polerised.