qmk_firmware/docs/feature_backlight.md

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# Backlighting :id=backlighting
Many keyboards support backlit keys by way of individual LEDs placed through or underneath the keyswitches. This feature is distinct from both the [RGB underglow](feature_rgblight.md) and [RGB matrix](feature_rgb_matrix.md) features as it usually allows for only a single colour per switch, though you can obviously install multiple different single coloured LEDs on a keyboard.
QMK is able to control the brightness of these LEDs by switching them on and off rapidly in a certain ratio, a technique known as *Pulse Width Modulation*, or PWM. By altering the duty cycle of the PWM signal, it creates the illusion of dimming.
The MCU can only supply so much current to its GPIO pins. Instead of powering the backlight directly from the MCU, the backlight pin is connected to a transistor or MOSFET that switches the power to the LEDs.
Most keyboards have backlighting enabled by default if they support it, but if it is not working for you, check that your `rules.mk` includes the following:
```make
BACKLIGHT_ENABLE = yes
```
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## Keycodes :id=keycodes
Once enabled, the following keycodes below can be used to change the backlight level.
| Key | Aliases | Description |
|---------------------------------|-----------|-------------------------------------|
| `QK_BACKLIGHT_TOGGLE` | `BL_TOGG` | Turn the backlight on or off |
| `QK_BACKLIGHT_STEP` | `BL_STEP` | Cycle through backlight levels |
| `QK_BACKLIGHT_ON` | `BL_ON` | Set the backlight to max brightness |
| `QK_BACKLIGHT_OFF` | `BL_OFF` | Turn the backlight off |
| `QK_BACKLIGHT_UP` | `BL_UP` | Increase the backlight level |
| `QK_BACKLIGHT_DOWN` | `BL_DOWN` | Decrease the backlight level |
| `QK_BACKLIGHT_TOGGLE_BREATHING` | `BL_BRTG` | Toggle backlight breathing |
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## Functions :id=functions
These functions can be used to change the backlighting in custom code:
|Function |Description |
|------------------------|--------------------------------------------|
|`backlight_toggle()` |Turn the backlight on or off |
|`backlight_enable()` |Turn the backlight on |
|`backlight_disable()` |Turn the backlight off |
|`backlight_step()` |Cycle through backlight levels |
|`backlight_increase()` |Increase the backlight level |
|`backlight_decrease()` |Decrease the backlight level |
|`backlight_level(x)` |Sets the backlight level to specified level |
|`get_backlight_level()` |Return the current backlight level |
|`is_backlight_enabled()`|Return whether the backlight is currently on|
If backlight breathing is enabled (see below), the following functions are also available:
|Function |Description |
|---------------------|--------------------------------------|
|`breathing_toggle()` |Turn the backlight breathing on or off|
|`breathing_enable()` |Turns on backlight breathing |
|`breathing_disable()`|Turns off backlight breathing |
## Configuration :id=configuration
To select which driver to use, configure your `rules.mk` with the following:
```make
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BACKLIGHT_DRIVER = software
```
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Valid driver values are `pwm`, `software`, `custom` or `no`. See below for help on individual drivers.
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To configure the backlighting, `#define` these in your `config.h`:
2021 May 29 Breaking Changes Update (#13034) * Add Per Key functionality for AutoShift (#11536) * LED Matrix: Reactive effect buffers & advanced indicators (#12588) * [Keyboard] kint36: switch to sym_eager_pk debouncing (#12626) * [Keyboard] kint2pp: reduce input latency by ≈10ms (#12625) * LED Matrix: Split (#12633) * [CI] Format code according to conventions (#12650) * feat: infinite timeout for leader key (#6580) * feat: implement leader_no_timeout logic * docs(leader_key): infinite leader timeout docs * Format code according to conventions (#12680) * Update ADC driver for STM32F1xx, STM32F3xx, STM32F4xx (#12403) * Fix default ADC_RESOLUTION for ADCv3 (and ADCv4) Recent ChibiOS update removed ADC_CFGR1_RES_10BIT from the ADCv3 headers (that macro should not have been there, because ADCv3 has CFGR instead of CFGR1). Fix the default value for ADC_RESOLUTION to use ADC_CFGR_RES_10BITS if it is defined (that name is used for ADCv3 and ADCv4). * Update ADC docs to match the actually used resolution ADC driver for ChibiOS actually uses the 10-bit resolution by default (probably to match AVR); fix the documentation accordingly. Also add both ADC_CFGR_RES_10BITS and ADC_CFGR1_RES_10BIT constants (these names differ according to the ADC implementation in the particular MCU). * Fix pinToMux() for B12 and B13 on STM32F3xx Testing on STM32F303CCT6 revealed that the ADC mux values for B12 and B13 pins were wrong. * Add support for all possible analog pins on STM32F1xx Added ADC mux values for pins A0...A7, B0, B1, C0...C5 on STM32F1xx (they are the same at least for STM32F103x8 and larger F103 devices, and also F102, F105, F107 families). Actually tested on STM32F103C8T6 (therefore pins C0...C5 were not tested). Pins F6...F10, which are present on STM32F103x[C-G] in 144-pin packages, cannot be supported at the moment, because those pins are connected only to ADC3, but the ChibiOS ADC driver for STM32F1xx supports only ADC1. * Add support for all possible analog pins on STM32F4xx Added ADC mux values for pins A0...A7, B0, B1, C0...C5 and optionally F3...F10 (if STM32_ADC_USE_ADC3 is enabled). These mux values are apparently the same for all F4xx devices, except some smaller devices may not have ADC3. Actually tested on STM32F401CCU6, STM32F401CEU6, STM32F411CEU6 (using various WeAct “Blackpill” boards); only pins A0...A7, B0, B1 were tested. Pins F3...F10 are inside `#if STM32_ADC_USE_ADC3` because some devices which don't have ADC3 also don't have the GPIOF port, therefore the code which refers to Fx pins does not compile. * Fix STM32F3xx ADC mux table in documentation The ADC driver documentation had some errors in the mux table for STM32F3xx. Fix this table to match the datasheet and the actual code (mux settings for B12 and B13 were also tested on a real STM32F303CCT6 chip). * Add STM32F1xx ADC pins to the documentation * Add STM32F4xx ADC pins to the documentation * Add initial support for tinyuf2 bootloader (when hosted on F411 blackpill) (#12600) * Add support for jumping to tinyuf2 bootloader. Adds blackpill UF2 example. * Update flashing.md * Update chconf.h * Update config.h * Update halconf.h * Update mcuconf.h * eeprom driver: Refactor where eeprom driver initialisation (and EEPROM emulation initialisation) occurs to make it non-target-specific. (#12671) * Add support for MCU = STM32F446 (#12619) * Add support for MCU = STM32F446 * Update platforms/chibios/GENERIC_STM32_F446XE/configs/config.h * Restore mcuconf.h to the one used by RT-STM32F446RE-NUCLEO64 * stm32f446: update mcuconf.h and board.h for 16MHz operation, with USB enabled, and other peripherals disabled. * Format code according to conventions (#12682) * Format code according to conventions (#12687) * Add STM32L433 and L443 support (#12063) * initial L433 commit * change to XC * fix L433 * disable all peripherals * update system and peripheral clocks * 433 change * use its own board files * revert its own board files * l433 specific change * fix stm32l432xx define * remove duplicate #define * fix bootloader jump * move to L443xx and add i2c2, spi2, usart3 to mcuconf.h * move to L443 * move to L443 * fix sdmmc in mcuconf.h * include STM32L443 * add L443 * Include L443 in compatible microcontrollers * Include L443 in compatible microcontrollers * Update config bootloader jump description * Update ChibiOS define reasoning * Update quantum/mcu_selection.mk * fix git conflict * Updated Function96 with V2 files and removed chconf.h and halconf.h (#12613) * Fix bad PR merge for #6580. (#12721) * Change RGB/LED Matrix to use a simple define for USB suspend (#12697) * [CI] Format code according to conventions (#12731) * Fixing transport's led/rgb matrix suspend state logic (#12770) * [CI] Format code according to conventions (#12772) * Fix comment parsing (#12750) * Added OLED fade out support (#12086) * fix some references to bin/qmk that slipped in (#12832) * Resolve a number of warnings in `qmk generate-api` (#12833) * New command: qmk console (#12828) * stash poc * stash * tidy up implementation * Tidy up slightly for review * Tidy up slightly for review * Bodge environment to make tests pass * Refactor away from asyncio due to windows issues * Filter devices * align vid/pid printing * Add hidapi to the installers * start preparing for multiple hid_listeners * udev rules for hid_listen * refactor to move closer to end state * very basic implementation of the threaded model * refactor how vid/pid/index are supplied and parsed * windows improvements * read the report directly when usage page isn't available * add per-device colors, the choice to show names or numbers, and refactor * add timestamps * Add support for showing bootloaders * tweak the color for bootloaders * Align bootloader disconnect with connect color * add support for showing all bootloaders * fix the pyusb check * tweaks * fix exception * hide a stack trace behind -v * add --no-bootloaders option * add documentation for qmk console * Apply suggestions from code review * pyformat * clean up and flesh out KNOWN_BOOTLOADERS * Remove pointless SERIAL_LINK_ENABLE rules (#12846) * Make Swap Hands use PROGMEM (#12284) This converts the array that the Swap Hands feature uses to use PROGMEM, and to read from that array, as such. Since this array never changes at runtime, there is no reason to keep it in memory. Especially for AVR boards, as memory is a precious resource. * Fix another bin/qmk reference (#12856) * [Keymap] Turn OLED off on suspend in soundmonster keymap (#10419) * Fixup build errors on `develop` branch. (#12723) * LED Matrix: Effects! (#12651) * Fix syntax error when compiling for ARM (#12866) * Remove KEYMAP and LAYOUT_kc (#12160) * alias KEYMAP to LAYOUT * remove KEYMAP and LAYOUT_kc * Add setup, clone, and env to the list of commands we allow even with broken modules (#12868) * Rename `point_t` -> `led_point_t` (#12864) * [Keyboard] updated a vendor name / fixed minor keymap issues (#12881) * Add missing LED Matrix suspend code to suspend.c (#12878) * LED Matrix: Documentation (#12685) * Deprecate `send_unicode_hex_string()` (#12602) * Fix spelling mistake regarding LED Matrix in split_common. (#12888) * [Keymap] Fix QWERTY/DVORAK status output for kzar keymap (#12895) * Use milc.subcommand.config instead of qmk.cli.config (#12915) * Use milc.subcommand.config instead * pyformat * remove the config test * Add function to allow repeated blinking of one layer (#12237) * Implement function rgblight_blink_layer_repeat to allow repeated blinking of one layer at a time * Update doc * Rework rgblight blinking according to requested change * optimize storage * Fixup housekeeping from being invoked twice per loop. (#12933) * matrix: wait for row signal to go HIGH for every row (#12945) I noticed this discrepancy (last row of the matrix treated differently than the others) when optimizing the input latency of my keyboard controller, see also https://michael.stapelberg.ch/posts/2021-05-08-keyboard-input-latency-qmk-kinesis/ Before this commit, when tuning the delays I noticed ghost key presses when pressing the F2 key, which is on the last row of the keyboard matrix: the dead_grave key, which is on the first row of the keyboard matrix, would be incorrectly detected as pressed. After this commit, all keyboard matrix rows are interpreted correctly. I suspect that my setup is more susceptible to this nuance than others because I use GPIO_INPUT_PIN_DELAY=0 and hence don’t have another delay that might mask the problem. * ensure we do not conflict with existing keymap aliases (#12976) * Add support for up to 4 IS31FL3733 drivers (#12342) * Convert Encoder callbacks to be boolean functions (#12805) * [Keyboard] Fix Terrazzo build failure (#12977) * Do not hard set config in CPTC files (#11864) * [Keyboard] Corne - Remove legacy revision support (#12226) * [Keymap] Update to Drashna keymap and user code (based on develop) (#12936) * Add Full-duplex serial driver for ARM boards (#9842) * Document LED_MATRIX_FRAMEBUFFER_EFFECTS (#12987) * Backlight: add defines for default level and breathing state (#12560) * Add dire message about LUFA mass storage bootloader (#13014) * [Keyboard] Remove redundant legacy and common headers for crkbd (#13023) Was causing compiler errors on some systems. * Fix keyboards/keymaps for boolean encoder callback changes (#12985) * `backlight.c`: include `eeprom.h` (#13024) * Add changelog for 2021-05-29 Breaking Changes merge (#12939) * Add ChangeLog for 2021-05-29 Breaking Changes Merge: initial version * Add recent develop changes * Sort recent develop changes * Remove sections for ChibiOS changes per tzarc No ChibiOS changes this round. * Add and sort recent develop changes * add notes about keyboard moves/deletions * import changelog for PR 12172 Documents the change to BOOTMAGIC_ENABLE. * update section headings * re-sort changelog * add additional note regarding Bootmagic changes * remove changelog timestamp * update dates in main Breaking Changes docs * fix broken section anchors in previous changelogs * add link to backlight/eeprom patch to changelog * highlight some more changes * link PRs from section headers * Restore standard readme * run: qmk cformat --core-only
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|Define |Default |Description |
|-----------------------------|------------------|-----------------------------------------------------------------------------------------------------------------|
|`BACKLIGHT_PIN` |*Not defined* |The pin that controls the LED(s) |
|`BACKLIGHT_LEVELS` |`3` |The number of brightness levels (maximum 31 excluding off) |
|`BACKLIGHT_CAPS_LOCK` |*Not defined* |Enable Caps Lock indicator using backlight (for keyboards without dedicated LED) |
|`BACKLIGHT_BREATHING` |*Not defined* |Enable backlight breathing, if supported |
|`BREATHING_PERIOD` |`6` |The length of one backlight "breath" in seconds |
|`BACKLIGHT_ON_STATE` |`1` |The state of the backlight pin when the backlight is "on" - `1` for high, `0` for low |
|`BACKLIGHT_LIMIT_VAL` |`255` |The maximum duty cycle of the backlight -- `255` allows for full brightness, any lower will decrease the maximum.|
|`BACKLIGHT_DEFAULT_LEVEL` |`BACKLIGHT_LEVELS`|The default backlight level to use upon clearing the EEPROM |
|`BACKLIGHT_DEFAULT_BREATHING`|*Not defined* |Whether to enable backlight breathing upon clearing the EEPROM |
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Unless you are designing your own keyboard, you generally should not need to change the `BACKLIGHT_PIN` or `BACKLIGHT_ON_STATE`.
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### Backlight On State :id=backlight-on-state
Most backlight circuits are driven by an N-channel MOSFET or NPN transistor. This means that to turn the transistor *on* and light the LEDs, you must drive the backlight pin, connected to the gate or base, *high*.
Sometimes, however, a P-channel MOSFET, or a PNP transistor is used. In this case, when the transistor is on, the pin is driven *low* instead.
This functionality is configured at the keyboard level with the `BACKLIGHT_ON_STATE` define.
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### AVR Driver :id=avr-driver
The `pwm` driver is configured by default, however the equivalent setting within `rules.mk` would be:
```make
BACKLIGHT_DRIVER = pwm
```
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#### Caveats :id=avr-caveats
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On AVR boards, QMK automatically decides which driver to use according to the following table:
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|Backlight Pin|AT90USB64/128|AT90USB162|ATmega16/32U4|ATmega16/32U2|ATmega32A|ATmega328/P|
|-------------|-------------|----------|-------------|-------------|---------|-----------|
|`B1` | | | | | |Timer 1 |
|`B2` | | | | | |Timer 1 |
|`B5` |Timer 1 | |Timer 1 | | | |
|`B6` |Timer 1 | |Timer 1 | | | |
|`B7` |Timer 1 |Timer 1 |Timer 1 |Timer 1 | | |
|`C4` |Timer 3 | | | | | |
|`C5` |Timer 3 |Timer 1 | |Timer 1 | | |
|`C6` |Timer 3 |Timer 1 |Timer 3 |Timer 1 | | |
|`D4` | | | | |Timer 1 | |
|`D5` | | | | |Timer 1 | |
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All other pins will use timer-assisted software PWM:
|Audio Pin|Audio Timer|Software PWM Timer|
|---------|-----------|------------------|
|`C4` |Timer 3 |Timer 1 |
|`C5` |Timer 3 |Timer 1 |
|`C6` |Timer 3 |Timer 1 |
|`B5` |Timer 1 |Timer 3 |
|`B6` |Timer 1 |Timer 3 |
|`B7` |Timer 1 |Timer 3 |
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When both timers are in use for Audio, the backlight PWM cannot use a hardware timer, and will instead be triggered during the matrix scan. In this case, breathing is not supported, and the backlight might flicker, because the PWM computation may not be called with enough timing precision.
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#### Hardware PWM Implementation :id=hardware-pwm-implementation
When using the supported pins for backlighting, QMK will use a hardware timer configured to output a PWM signal. This timer will count up to `ICRx` (by default `0xFFFF`) before resetting to 0.
The desired brightness is calculated and stored in the `OCRxx` register. When the counter reaches this value, the backlight pin will go low, and is pulled high again when the counter resets.
In this way `OCRxx` essentially controls the duty cycle of the LEDs, and thus the brightness, where `0x0000` is completely off and `0xFFFF` is completely on.
The breathing effect is achieved by registering an interrupt handler for `TIMER1_OVF_vect` that is called whenever the counter resets, roughly 244 times per second.
In this handler, the value of an incrementing counter is mapped onto a precomputed brightness curve. To turn off breathing, the interrupt handler is simply disabled, and the brightness reset to the level stored in EEPROM.
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#### Timer Assisted PWM Implementation :id=timer-assisted-implementation
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When `BACKLIGHT_PIN` is not set to a hardware backlight pin, QMK will use a hardware timer configured to trigger software interrupts. This time will count up to `ICRx` (by default `0xFFFF`) before resetting to 0.
When resetting to 0, the CPU will fire an OVF (overflow) interrupt that will turn the LEDs on, starting the duty cycle.
The desired brightness is calculated and stored in the `OCRxx` register. When the counter reaches this value, the CPU will fire a Compare Output match interrupt, which will turn the LEDs off.
In this way `OCRxx` essentially controls the duty cycle of the LEDs, and thus the brightness, where `0x0000` is completely off and `0xFFFF` is completely on.
The breathing effect is the same as in the hardware PWM implementation.
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### ARM Driver :id=arm-configuration
While still in its early stages, ARM backlight support aims to eventually have feature parity with AVR. The `pwm` driver is configured by default, however the equivalent setting within `rules.mk` would be:
```make
BACKLIGHT_DRIVER = pwm
```
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#### ChibiOS Configuration :id=arm-configuration
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The following `#define`s apply only to ARM-based keyboards:
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|Define |Default|Description |
|-----------------------|-------|-----------------------------------|
|`BACKLIGHT_PWM_DRIVER` |`PWMD4`|The PWM driver to use |
|`BACKLIGHT_PWM_CHANNEL`|`3` |The PWM channel to use |
|`BACKLIGHT_PAL_MODE` |`2` |The pin alternative function to use|
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See the ST datasheet for your particular MCU to determine these values. Unless you are designing your own keyboard, you generally should not need to change them.
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#### Caveats :id=arm-caveats
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Currently only hardware PWM is supported, not timer assisted, and does not provide automatic configuration.
### Software PWM Driver :id=software-pwm-driver
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In this mode, PWM is "emulated" while running other keyboard tasks. It offers maximum hardware compatibility without extra platform configuration. The tradeoff is the backlight might jitter when the keyboard is busy. To enable, add this to your `rules.mk`:
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```make
BACKLIGHT_DRIVER = software
```
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#### Multiple Backlight Pins :id=multiple-backlight-pins
Most keyboards have only one backlight pin which controls all backlight LEDs (especially if the backlight is connected to a hardware PWM pin).
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In software PWM, it is possible to define multiple backlight pins, which will be turned on and off at the same time during the PWM duty cycle.
This feature allows to set, for instance, the Caps Lock LED's (or any other controllable LED) brightness at the same level as the other LEDs of the backlight. This is useful if you have mapped Control in place of Caps Lock and you need the Caps Lock LED to be part of the backlight instead of being activated when Caps Lock is on, as it is usually wired to a separate pin from the backlight.
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To activate multiple backlight pins, add something like this to your `config.h`, instead of `BACKLIGHT_PIN`:
```c
#define BACKLIGHT_PINS { F5, B2 }
```
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### Custom Driver :id=custom-driver
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If none of the above drivers apply to your board (for example, you are using a separate IC to control the backlight), you can implement a custom backlight driver using this simple API provided by QMK. To enable, add this to your `rules.mk`:
```make
BACKLIGHT_DRIVER = custom
```
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Then implement any of these hooks:
```c
void backlight_init_ports(void) {
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// Optional - runs on startup
// Usually you want to configure pins here
}
void backlight_set(uint8_t level) {
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// Optional - runs on level change
// Usually you want to respond to the new value
}
void backlight_task(void) {
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// Optional - runs periodically
// Note that this is called in the main keyboard loop,
// so long running actions here can cause performance issues
}
```
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## Example Schematic
In this typical example, the backlight LEDs are all connected in parallel towards an N-channel MOSFET. Its gate pin is wired to one of the microcontroller's GPIO pins through a 470Ω resistor to avoid ringing.
A pulldown resistor is also placed between the gate pin and ground to keep it at a defined state when it is not otherwise being driven by the MCU.
The values of these resistors are not critical - see [this Electronics StackExchange question](https://electronics.stackexchange.com/q/68748) for more information.
![Backlight example circuit](https://i.imgur.com/BmAvoUC.png)