Layer LED Indicators Implementation Guide Needed

[TimoWielink]

I am looking for some help to see if it's possible to Implement simple LED indicators to show which layer is currently active on a custom keyboard.

Hardware Setup

• Board: nice_nano_v2
• LED Connections:
  • Layer 0 LED: P1.13 (GPIO1_13)
  • Layer 1 LED: P0.09 (GPIO0_09)
  • Layer 2 LED: P1.11 (GPIO1_11)

Current Implementation

LED Node Definition (.overlay)

layer_leds {
    compatible = "gpio-leds";
    layer0_led: layer0_led {
        gpios = <&gpio1 13 GPIO_ACTIVE_HIGH>;
        status = "okay";
    };
    layer1_led: layer1_led {
        gpios = <&gpio0 9 GPIO_ACTIVE_HIGH>;
        status = "okay";
    };
    layer2_led: layer2_led {
        gpios = <&gpio1 11 GPIO_ACTIVE_HIGH>;
        status = "okay";
    };
};

Behavior Attempt (.keymap)

behaviors {
    layer0_led_on: layer0_led_on {
        compatible = "zmk,behavior-output";
        #binding-cells = <0>;
        gpios = <&layer0_led>;
        output-high;
    };
    // ... similar definitions for other LEDs ...
};

macros {
    to_layer0: to_layer0 {
        compatible = "zmk,behavior-macro";
        #binding-cells = <0>;
        bindings = <&to DEFAULT>, <&layer0_led_on>, <&layer1_led_off>, <&layer2_led_off>;
    };
    // ... similar definitions for other layers ...
};

Configuration (.conf)

CONFIG_GPIO=y
CONFIG_LED=y
CONFIG_LED_GPIO=y
CONFIG_ZMK_MACRO=y

Current Error

devicetree error: binding controller <Node /behaviors/layer0_led_on in '/tmp/zmk-config/zephyr/misc/empty_file.c'> for <Node /macros/to_layer0 in '/tmp/zmk-config/zephyr/misc/empty_file.c'> lacks binding

Questions

1. What is the correct way to implement LED layer indicators in ZMK?
2. Is there a specific behavior type we should use for GPIO LED control?
3. Are there any example implementations we can reference?

Additional Context

We've tried various approaches including:

• zmk,behavior-gpio
• zmk,behavior-output
• zmk,behavior-led-control
• Direct GPIO references and LED node references

The goal is to have the corresponding LED turn on for the active layer while turning off the LEDs for other layers. Any guidance on the proper way to implement this functionality would be greatly appreciated!

[caksoylar]

What is the correct way to implement LED layer indicators in ZMK?

A small module is probably what I'd go for for maintainability, but see the answers below.

Is there a specific behavior type we should use for GPIO LED control?

• Backlight behavior can control GPIO LEDs (doesn't have to be PWM) but it doesn't support multiple different instances for different LEDs
• Underglow only works for smart LED strips, not GPIO-based LEDs
• External power control also works with toggling only one pin

So no, there is no good built-in solution to toggle multiple different GPIOs.

Are there any example implementations we can reference?

There is a community-based solution: @elpekenin's userspace has two functions that you can combine to achieve this:

• A layer callbacks feature that triggers a listed behavior on layer on/off (another alternative here)
• A behavior to control individual GPIOs called &gpio

The examples in the README combine the two to essentially create layer indicator LEDs.

As an aside, all the available behaviors are documented in the docs: https://zmk.dev/docs/keymaps/behaviors. The ones you tried don't exist, otherwise they'd be documented.

[TimoWielink]

@caksoylar Thank you! I was able to get something working with Elpekenin example.