Batsocks - Controlling Two LEDs per uC pin - Two LEDs in a 3 volt circuit

Controlling Two LEDs per uC pin

2. Two LEDs in a 3 volt circuit

In the following examples, the following assumptions are made:

A button cell is used as the power supply.
Vcc is 3v.
A 'standard' red LED is used.

These assumptions are to simplify the circuits down to their bare-bones. The reasoning for these assumptions is as follows;

A button cell will only supply a few milliamps. That means I can ignore current limiting resistors for the moment (hopefully without too much tutting going on).
'Standard' red LEDs have a V_F of around 1.9v. That's useful for reasons that will (hopefully) become clear...

This first circuit will not light either of the LEDs.

Circuit Diagram 2.1

"Why not? It's exactly the same as Circuit 1.2, there's just an extra LED in series."

The reason is because each LED needs to drop 1.9v. 2 lots of 1.9v is more than the 3v supply, hence the LEDs don't light.

Note: There is not actually a single threshold voltage at which an LED will suddenly light. A few microamps will flow at considerably below the normal lit forward voltage, and if viewed in low lighting conditions may be seen to glow. Some LEDs have quite a sharp curve where they start to let current flow. Others, noticeably modern 'high efficiency' or similarly named LEDs, have a very long curve and may glow well below their stated V_F. The amount of glow acceptable will depend upon the application. You'll need to read the LED's datasheets carefully and empirically test the LEDs. For the moment, however, we're going to continue with the assumption that the LEDs don't light at all in this circuit.

"So why have your just shown me a circuit that doesn't work?"

Because we can be connect a pin of a friendly uC in-between these two LEDs and do something exciting.

"What will that do?"

Well, it depends on how that pin is configured...

Case 1: The uC pin is set as an input.

In this case, neither LED will light. On AVRs, an input pin is 'high impedance', meaning it lets no current flow through the pin. That means that we can pretend that the pin isn't there and the circuit behaves as per diagram 2.1.

Note: AVRs can have pull-up resistors on input pins. These can be thought of as a 30-50k resistor between the pin and Vcc. This can be ignored in this application as the tiny current flowing through this is unlikely to cause the LED to glow. If it does, the pull-ups can be turned off for that pin.

Case 2: The uC pin is set as an output with a high state (e.g. 3v).

In this case, the circuit can be considered to be as follows:

Two LEDs in series with a 'high' uC pin in the centre

Circuit Diagram 2.2

Which means that LED_A lights up. LED_B is not lit, as the voltage differential across it is zero, i.e. both sides of LED_B are at 3v.

Case 3: The uC pin is set as an output with a low state (e.g. 0v).

In this case, the circuit is equivalent to the following:

Two LEDs in series with a 'low' uC pin in the centre

Circuit Diagram 2.3

Which means that LED_B lights up. LED_A is not lit, as the voltage differential across it is zero, i.e. both sides of LED_A are at Gnd.

Summary of pin states and which LEDs are lit:

Pin state	Hi-Z (input)	0v (ouptut, low)	3v (output, high)
LED_A	off	off	on
LED_B	off	on	off

Table 2.1 : Pin/LED states

"What's that Hi-Z business? Where's that come from?"

It's just some terminology used in TriState logic that most uC's i/o pins employ. As usual, wikipedia is the lazy-article-writer's friend. When using AVRs, it essentially it means that the pin is set as an input.

"Whoa! Hang on! What if we want both LEDs on?"

Those of you still awake may have noticed that none of the states in table 2.1 have both LED_A and LED_B lit at the same time?! So what do you do if your nuclear reactor console needs to show that BOTH cores are going critical?

It can't be done.

No, seriously; it can't be done.
You can't light both LEDs at the same time with this circuit

But... we can cheat...

If you switch between lighting LED_A and LED_B very fast, then they'll both look as if they're on. This might sound really nasty, but trust me, it works well. It's the sort of thing that microcontrollers are really good at.

e.g.


	{
	start:
		set pin 1 to HIGH ;
		set pin 1 to LOW ;
		// now do it over and over again
		goto start ;
	}

I'll walk through some more useful (and hopefully more elegant) code to do this later on...

Summary so far:

We can independently control 2 LEDs from a single pin.
It works with a supply voltage of 3v.

"But all my circuits run off 5 volts. What happens if I use this with a 5v supply?"

Oh good question. Good timing. That's exactly what I'm covering next...