How can one vary the resistance of a circuit using a microcontroller? [closed]

Question

I am trying to have a microcontroller light up a few LED's, all LED's have a forward voltage of about 3V. The issue is that my power supply(which in this mock-up is 9V) will push too much voltage if less than 3 LED's are on at one time, my question then is how can vary the resistance of the circuit or change the circuit so that when the microcontroller changes the number of LED's currently on the circuit doesn't fry?

On the real microcontroller there are extra I/O pins so if those need to be utilized feel free.

^{simulate this circuit – Schematic created using CircuitLab}

Edit: I should also add that I am newer to electronics and may need more explanation on more complex ideas

From the comments made it seems I've made some errors in my assumptions it seems that the diodes would actually be in parallel in the original schematic, this leads me to ask, if I simply changed the power supply to a 3V supply, would the circuit work as I wanted it to? I suspect no, simply because that seems too easy

Answer 1

It will be really difficult to run the LED's with just a switch like a BJT or mosfet with PWM. You need at least an RC circuit to control the current with a switch, because even if you do have a 'switch' it would only be a short time before the voltage would exceed the maximum rating of the LED and you would risk buring it out. So the best thing to do would be to use a current limiter such as a resistor at minimum, then use a BJT or mosfet controlled by the microprocessor.

Here is an idea below, but instead of the BJT being driven by the 555, you could use the GPIO from the microprocessor

Answer 2

Firstly, do not use a series connection for the LEDs. Much too difficult to control if LEDS need to be individually switchable.

Second, since you have unused IO pins, configure one IO pin per LED as output, and use the pin to drive the gate of a MOSFET that is connected in series with an LED and current-limiting resistor (R= 9/LED current). Connect each of the series-connected MOSFET+resistor+LED legs in parallel on the 9V supply. Now you have a individually controllable LEDS, as many as you wish.

EDIT to add longish reply to question in comment:

I assumed you wanted to be able to turn the individual LEDS on or off independently of the others. If that is a correct assumption, then you require one microcontroller output per LED to implement independent turn on/off for each LED (i.e. individually switchable).

I further assumed that independent on/off control of the each LED only was your objective. If dimming is also your objective, then individual LED dimming independent of the other LEDs is also possible with some more complex SW control. You would need ot write some code for the microcontroller that turns each LED on and off rapidly (more than 60 times per second), with the "on" duration being somewhere between 0% (LED off) and 100% (LED full on) of the on/off cycle.

A 3V supply would not really avoid the MOSFET resistor for two reasons: 1) you need the MOSFET to allow the microcontroller to turn the LED on/off, since most microcontroller do not have enough drive to power an LED without such arrangement. 2) a current limiting resister prevents the current from exceeding the LED's safe operating current/power and vapourizing within a short time. Typical LEDS have a maximum rated forward current of 20mA, so the limiting resister needs to be about (9-2)V/20mA = 350 $ \Omega $. Or with a 3V supply (3-2)V/20mA = 50 $ \Omega $.