In the docuemnt it says Forward Voltage 2.4 V IF=20mA
And Min. ---V Typ. 1.8V
From what I've understood I can't provide it with power from the Arduino pins, because they are 3.3V and 5V.
So my obvious solution is to put resistor infront of all the LEDs, but can this be solved by a clever circuit? Can I spread out one regular Pin (not normal 3.3V or 5V) to 2 LED lights.
3.3V/2 = 1.65V each LED.
Since it doesn't have a minimum volt, it should work? Am I on the right direction?
You can even connect several of these (common cathode) devices directly to an Arduino if you multiplex them. For example, if you want to drive 4 of these displays, you need 12 Arduino pins.
You also need 8 resistors (say 200 Ohms) for the leds and 4 transistors with base resistors.
arduino_x:
Voltage doesn't "go out" or "return"- that's more like a description of current, ie there's some motion.
But "absorb" is probably a good enough word for now, although "dropped" is usually used. So you have 5V across the led and resistor, of which 2.4 is dropped across the led you have to lose the other 2.6 across the resistor.
Seeing as the current has to go through both the resistor and the led, and we know we want 20mA (= 0.02A) through the led, that has to be the current through the resistor.
Ohm's Law tells us V=IR or R=V/I, so our resistor needs to be 2.6/0.02 = 130 ohms. That's not a standard value iirc, so you would go for the next biggest to play safe which I think is 150.
Ok, dropped. So the resistor can break down volt, but regular components can not? Or this is just a way to calculate?
Also I think I finally fully understood the "V=IR" now!
6v6gt:
You can even connect several of these (common cathode) devices directly to an Arduino if you multiplex them. For example, if you want to drive 4 of these displays, you need 12 Arduino pins.
You also need 8 resistors (say 200 Ohms) for the leds and 4 transistors with base resistors.
I didn't understand. Common cathode (=negative pin, the one that should be grounded?)
I need to look up what multiplexing is..
If I have potentiometer, can I let that one act as a resistor?
Arduino ---> Potentiometer ---> All 7 LED's?
hellophant:
I didn't understand. Common cathode (=negative pin, the one that should be grounded?)
I need to look up what multiplexing is..
If I have potentiometer, can I let that one act as a resistor?
Arduino ---> Potentiometer ---> All 7 LED's?
NO.
Using one series resistor on the common pin will give a constant amount of current that will be divided between all the LEDs. Their brightness will depend on how many are turned on at once. It looks bad for 1 to be significantly brighter than 8. If you want a consistent brightness for all segments, each segment needs it's own resistor.
On top of that, LEDs can be roughly approximated by a constant voltage load, and it's not a good idea to put constant voltage loads in parallel.
Clever use of PWM can compensate for the brightness differences between 1 and 8, which
allows a single resistor on the common terminal - this is feasible because the LEDs in one
display are manufactured together and are well matched and in the same thermal environment,
although generally you don't parallel LEDs.
Arduino outputs have about 30 ohms of resistance in their output transistors which
helps balance LEDs too.
Proper LED driving chips have constant current outputs so that variation in LEDs is not an issue
(including the very different forward voltages of different colours).
Using one series resistor on the common pin will give a constant amount of current that will be divided between all the LEDs. Their brightness will depend on how many are turned on at once. It looks bad for 1 to be significantly brighter than 8. If you want a consistent brightness for all segments, each segment needs it's own resistor.
On top of that, LEDs can be roughly approximated by a constant voltage load, and it's not a good idea to put constant voltage loads in parallel.
Alright got that! What is the case if my 7Seg display has a common anode(?) the pin that recieves current? Then I put resist on that, but the rest?
hellophant:
Alright got that! What is the case if my 7Seg display has a common anode(?) the pin that recieves current? Then I put resist on that, but the rest?
It does not matter which direction the current is going or which terminal of the LEDs is common. You put resistors on the pins that are NOT common, and connect the common pin straight to whichever power rail it's supposed to go to (GND for CC, V+ for CA).
hellophant:
Ok, dropped. So the resistor can break down volt, but regular components can not? Or this is just a way to calculate?
Also I think I finally fully understood the "V=IR" now!
Different components behave differently. For resistors, voltage and current have a proportional relationship with the resistance being the conversion constant as shown in Ohm's law.
An LED can be very roughly modeled as a constant voltage load, meaning that it will have the same voltage (called the "forward voltage" for a diode) across it no matter how much current is flowing through it. This is why you can't just hook 5V up to an LED with 2.2V forward voltage: the current draw will be far too high. You cannot have two constant voltage devices in parallel like that. Some way of limiting current is needed. A resistor is the easiest method, but higher-power ones will use an efficient switch-mode constant current supply.
It's important to remember that the "constant voltage load" is only an extremely crude approximation of a diode and is only accurate within certain limits. It's good enough for low current LEDs or small signal diodes, but if the current is reversed or the current gets too high the approximation will no longer be accurate.