LED Short Course
There are a few numbers you need to select an LED. First is its forward voltage drop, usually between 1.2 and 3.5V, depending on the color. Next is the forward current. After that, its brightness usually expressed in mcd. Last is its physical description - size and mounting.
LEDs can be wired in series or parallel. Another needed bit of information, LEDs have polarity. They have a plus side (anode) and a negative side (cathode) and must be wired in the correct direction or they don't light.
If you wire the LEDs in series (the cathode of one to the anode of the next) the forward voltage drops add up and each sees the same current. So, if you put 10 LEDs in series that have a 1.5V forward voltage and each draws 2mA, you need to supply 15V at 2mA. The same current flows through each so that must match or you will get uneven brightness or damage LEDs. If you put them in parallel (all the anodes connected together and all the cathodes connected together) they all see the same voltage but the currents can be different. If the LEDs I described, you would need a 1.5V power supply with 20mA of current for your 10 LEDs.
How do you control the current, you might ask? The simple way is with a resistor in series with the LED. The calculation goes like this. If you have a 4.5V supply (3 batteries in series) and an LED that drops 1.5V, the resistor must drop 3V. If you want the LED to have 2mA flowing through it the resistor must be: R = V/I = 3V/.002A (that was 2 milliamps) or 1500 ohms (you can be off by about 10% with no harm done). Power is lost by the resistor (shorter battery life) so it is better to get the battery to match the LED voltage pretty closely if you can.
So, to get to your string of LEDs problem, if you wire 50 LEDs in series the required voltage is very high - 50 x 1.5 is 75V. You want them in parallel. Unless there is a separate current limiting resistor for each (which could be true if you buy a pre-made string) you want them all to require the same voltage and current. You can get tricky and use both parallel and series wiring to avoid the current limiting resistor entirely (3 1.7V LEDs in series will connect to 5V with no resistor needed. Then you could put those groups of 3 LEDs in parallel).
To make a long, flexible string, there is a lot of wiring, soldering and heat shrink. If you can find the string that meets your needs pre-made you should buy them. Usually, they use a flexible ribbon with copper that connects them all and can be cut. Avoid the ones that are individually controllable until you are ready to design a microprocessor into the circuit to control them or use their controller.
LEDs can be wired in series or parallel. Another needed bit of information, LEDs have polarity. They have a plus side (anode) and a negative side (cathode) and must be wired in the correct direction or they don't light.
If you wire the LEDs in series (the cathode of one to the anode of the next) the forward voltage drops add up and each sees the same current. So, if you put 10 LEDs in series that have a 1.5V forward voltage and each draws 2mA, you need to supply 15V at 2mA. The same current flows through each so that must match or you will get uneven brightness or damage LEDs. If you put them in parallel (all the anodes connected together and all the cathodes connected together) they all see the same voltage but the currents can be different. If the LEDs I described, you would need a 1.5V power supply with 20mA of current for your 10 LEDs.
How do you control the current, you might ask? The simple way is with a resistor in series with the LED. The calculation goes like this. If you have a 4.5V supply (3 batteries in series) and an LED that drops 1.5V, the resistor must drop 3V. If you want the LED to have 2mA flowing through it the resistor must be: R = V/I = 3V/.002A (that was 2 milliamps) or 1500 ohms (you can be off by about 10% with no harm done). Power is lost by the resistor (shorter battery life) so it is better to get the battery to match the LED voltage pretty closely if you can.
So, to get to your string of LEDs problem, if you wire 50 LEDs in series the required voltage is very high - 50 x 1.5 is 75V. You want them in parallel. Unless there is a separate current limiting resistor for each (which could be true if you buy a pre-made string) you want them all to require the same voltage and current. You can get tricky and use both parallel and series wiring to avoid the current limiting resistor entirely (3 1.7V LEDs in series will connect to 5V with no resistor needed. Then you could put those groups of 3 LEDs in parallel).
To make a long, flexible string, there is a lot of wiring, soldering and heat shrink. If you can find the string that meets your needs pre-made you should buy them. Usually, they use a flexible ribbon with copper that connects them all and can be cut. Avoid the ones that are individually controllable until you are ready to design a microprocessor into the circuit to control them or use their controller.