Two Separate LEDs

Two separate LEDs - i.e. a green and a red LED - can be used to signal the high or low state of a GPIO.

Schematics

An output GPIO can both sink and source current. That’s why two simple LED can be used to indicate its state.

• High: current flows through the LED connected to GND.
• Low: current flows from the positive voltage source to the GPIO (which becomes effectively GND), and illuminates the LED connected in reverse connection.

This “trick” works because GPIO voltages are always below the LED reverse breakdown voltage. You cannot use this circuit with voltages higher than roughly 5V, or else you would risk destroying the LED connected in wrong direction.

Identifying Anode And Cathode

Here’s a quick refresher on LEDs and where to find their cathode (-) and anode (+):

Adjustments

Typical signal LEDs require approximately 10mA to light up in decent brightness. At 3.3V, this requires a current limiting resistor of 90 ohms for a typical green LED, and 130 ohms for a typical red red LED:

PS> Get-LedResistor -Current 10 -OperatingVoltage 3.3 -Color red, green


Required Resistor (Ohm) : 130
Operating Voltage (V)   : 3.3
Led Current (mA)        : 10
Led Voltage (V)         : 2
Led Color               : red

Required Resistor (Ohm) : 90
Operating Voltage (V)   : 3.3
Led Current (mA)        : 10
Led Voltage (V)         : 2.4
Led Color               : green

WARNING: LED Forward Voltage was guessed from color and can be completely different. Use at own risk.

To use the PowerShell cmdlet Get-LedResistor, install the donelandtools module from the Microsoft PowerShell Gallery: Install-Module -Name DoneLandTools -Scope CurrentUser -Force.

Make up your mind which LED should turn on when. The circuit cannot be changed later.

Testing

Let’s first test the schematics. For this, connect 3.3V and GND to the rails of your breadboard.

Then, connect one of the GPIOs that you use in your configuration (i.e. GPIO4) to both the cathode of the green LED and the anode of the red LED.

Connect the 90 ohms resistor to the anode of your green LED, and connect the other end of the resistor to 3.3V.

Likewise, connect the 130 ohms resistor to the cathode of your red LED, and connect the other end of the resistor to GND.

Now, when you change the switch in your Home Assistant dashboard that represents the GPIO you wired up, the LEDs should indicate the current switch status.

Smart Signalling On Perf Board

If you’d like to use the LEDs in a project like the Remote-Control Powerstrip, you may want to solder the circuitry to some perfboard.

Here is an example, placing two LED pairs (capable of signalling the state of two GPIOs) on one perfboard.

Place LEDs In Opposite Direction

Make sure you place the green and the red LED in opposite orientation onto the perfboard:

Next, ensure that both LED align with the perfboard and are not tilted:

Finally, bend the LED legs all the way to the sides so the LEDs are fixed and won’t slide out when you solder them to the perfboard.

Adding Resistors

Identify the side of the LEDs that will be connected to the GPIO: that’s the cathode (shorter leg) of the green LED and the anode (longer leg) of the red LED.

On the opposite side of the LEDs, solder the resistors to the LEDs:

Do this on both sides.

Then trim off the legs of the LEDs. Do Not trim off any part of the resistors.

Add Connections For GPIOs

Connect the other end of each LED pair with a wire, connecting the red led anode and the green led cathode. This provides you with two wires that later can be connected to the two GPIOs that you want to monitor.

Add Connections For Power Supply

Connect the two resistors that come from the red LEDs somewhere in the middle of the perfboard. Where the resistors connect will be the place where you later connect GND.

Do the same with the two resistors that come from the green LEDs: where these connect will later be the supply point for 3.3V.

Since the wires will cross over each other at some point, you may want to put the resistors that come from the green LEDs in red heat shrink. Note how one resistor is covered in heat shrink in the picture below:

Finally, add the power supply cables: connect a red wire to the junction point of the two green LED resistors, and add a black wire to the junction point where the two red LED resistors connect:

Testing

Once you have finished the perfboard, you can perform a first test: connect 3.3V and GND to your red and black wire. Next, connect one of the GPIO wires to GND, then to 3.3V. The appropriate LED should turn on.

When that works, it’s time to add the perfboard to your test setup: connect two wires to two of the GPIOs you use in this project (i.e. GPIO13 and GPIO14).

Connect the perfboard cables for 3.3V and GND to your breadboard power rail, then connect the two GPIO wires to the two GPIOs.

The two LED pairs on your perfboard should immediately start to signal the GPIO state, and when you go to your Home Assistant test dashboard and change the switches, then the LEDs on your perfboard should reflect these changes.

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