Arduino - How to light an led on a digital pin with the least amount of current.

tried using a current limiting resistor?

Leds in this app are configured at 2v and 20 ma....so 12v-2v=10v....10v÷20ma=500 ohms. Hope that helps

Thank you

How many milliamps are being drawn at the base of the transistor?

Change led current limiting resistor to 470 Ohm for reason stated above. However, if you construct this circuit, LED’s these days are very bright you could easily get away with 10mA. As you obviously realise the real issue is the current taken from the 5V digital side and you can reduce this considerably. The transistor gain default on EC is 100 but in real life you are more likely to have a gain of, say, 300. Now, for a collector current of 20mA (for your led) you need a base current of 20mA/300. The base resistor will have a volt drop of 5-0.7=4.3V. Therefore base resistor =(4.3x300)/.02=64.5 kOhms, so use nearest preferred value of 62 kOhms.

Don’t forget to increase the transistor gain to 300. This reduces the 5V current from >4mA down to <100uA.

Finally, I would add a 1 MOhm resistor from transistor base to ground. This is to stop the transistor base ‘floating’ when the switch is in the open position.

Obviously you can recalculate the base resistor value according to the gain of the transistor you choose to use in a real life circuit.

I hope this helps.

Wow thank you Robert, quick question how do increase the transistor gain to 300? I am new at electronics.

Robert Kidd assisted with the circuit modification, problem solved.

I see you sorted it but you need to make the base resistor 62000 Ohm, you have 62 Ohm.

You’re very welcome. Pleased to have helped.

Oh, just noticed, the 1MOhm resistor should go directly to the transistor base.

What transistor would you recommend in real life situation Robert?

Any general purpose / switching transistor would do. 2N3904, ZTX108, BC547 are a few examples but almost any npn transistor will do for the quite low currents and voltages you are dealing with. Incidentally, you will see that transistors can have quite a wide range of gain, say 100-500, so for your purposes just assume you’re getting the lowest and use a base resistor value of, say, 10kOhm. This will still only draw less than 0.5mA from your 5V supply.

Thank you again