I have to make a traffic light for a school project and I decided to use a PICAXE-20X2 microcontroller which has 8 output pins. I have 10 LEDs for the red, amber & green lights, 10 also for the red and green man, 10 for the walk and the same for the wait sign. I have connected the 10 for each pin in parallel and a maximum of 20 LEDs will be on at a time. I want to know if this will work?
I have a PIXAXE-20X2 and want to connect 65 LEDs as outputs! Is it possible
ledoptoelectronics
Related Solutions
"Most light" usually means 20mA. Here is the generic equation for an LED:
\$ R = {(V_S-V_f)\over I_f}\$
Red:
\$ {(12\text{V}-2.17\text{V})\over 20\text{mA}} = 491.5\Omega \$
Green:
\$ {(12\text{V}-3.11\text{V})\over 20\text{mA}} = 444.5\Omega \$
Blue:
\$ {(12\text{V}-3.07\text{V})\over 20\text{mA}} = 446.5\Omega \$
So we're talking 510ohm for red and 470ohm for green and blue, assuming your supply is 12V. Otherwise substitute the correct value in the equations and recalculate.
Using both NPN and PNP transistors is probably your best bet.
simulate this circuit – Schematic created using CircuitLab
Or this way is more elegant.
ADDITION
If you must drive this with a single pole switch, you should probably make the signal more active.
Best Answer
Since the maximum output current for a PICAXE pin is 20 mA, you will need a driver transistor between the PICAXE and each group of 10 LEDs.
Something like this:
simulate this circuit – Schematic created using CircuitLab
The value of the resistors in series with the LEDs should be selected to produce the desired current in the LEDs (not too close to the Absolute Maximum rating!)
The transistor can be any NPN type that can handle the total current of the 10 LEDs.
If you use a separate power supply for the LEDs, its ground must be connected to the transistor's emitter, and to the PICAXE ground.