Limit Switches with LED's

[Robin Hewitt]

can you recommend a particular NPN transistor, and what is the Resistor rating for the one in front of your 5v mark.

I am a dinosaur when it comes to transistors, how about a BC547.

Assuming 20mA to drive the LED and a 1.2 Volt drop across it, about 190 Ohms for the resistor.

I drew it up to minimise the component count, when the transistor switches on the Volts it has to play with fall away, may need a third resistor so don't lay any PCB's until you have tried it.

[HankMcSpank]

Assuming 20mA to drive the LED and a 1.2 Volt drop across it, about 190 Ohms for the resistor.

Personally I'd have gone with a resistor for each LED (to get their respective brightnesses balanced right)...therefore I'd connect the common anode direct to 5V & then place a current limiting resistor in each of the the LED cathode legs (the fwd voltage of a red led is about 2V, whereas a green led is about 3.2V ...therefore to get the right level balanced you'll likely need different value resistors).

Also 20mA - depending on LED - is gonna be very bright, I'd have aimed for 10mA or less.

Edit: Actually the way Robin has drawn it, for the red led you'll get a drop across the tranny itself, which pretty much balances the fwd voltage difference out for the red & green LEDs, so you may get away with a 180R single resistor for both....a 180R resistor will yield about 10mA through the LEDs

[Robin Hewitt]

Personally I'd have gone with a resistor for each LED (to get their respective brightnesses balanced right)...

That would get around the Volts falling away when the transistor comes on problem nicely and let you tinker with individual brighteness. Good idea.