add documentation for component sn74hc595 (#637)

* add documentation for component sn74hc595

* fix references to pins

* add image and entry in index.rst

* add full size image

components/sn74hc595.rst

@@ -0,0 +1,62 @@
+SN74HC595 I/O Expander (shift register)
+=======================================
+
+.. seo::
+    :description: Instructions for setting up SN74HC595 shift registers as digital port expanders in ESPHome.
+
+The SN74HC595 component allows you to use SN74HC595 shift registers as I/O expanders
+(`datasheet <http://www.ti.com/lit/ds/symlink/sn74hc595.pdf>`__,
+`SparkFun`_) in ESPHome. It uses 3 wires (optionally 4) for communication.
+
+Once configured, you can use any of the 8 pins for your projects. Up-to 4 shift registers can be daisy-chained
+to provide more pins, without using more GPIO pins on the controller.
+
+Use of the OE pin is optional. If used, the pin should be pulled up externally.
+
+.. figure:: images/sn74hc595-full.jpg
+    :align: center
+    :width: 80.0%
+
+    SN74HC595 I/O Expander (shift register).
+
+.. _SparkFun: https://www.sparkfun.com/products/13699
+
+.. code-block:: yaml
+
+    # Example configuration entry
+    sn74hc595:
+      - id: 'sn74hc595_hub'
+        data_pin: D5
+        clock_pin: D8
+        latch_pin: D7
+        oe_pin: D6
+        sr_count: 2
+
+    # Individual outputs
+    switch:
+      - platform: gpio
+        name: "SN74HC595 Pin #0"
+        pin:
+          sn74hc595: sn74hc595_hub
+          # Use pin number 0
+          number: 0
+          inverted: False
+
+Configuration variables:
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+- **id** (**Required**, :ref:`config-id`): The id to use for this SN74HC595 component.
+- **data_pin** (**Required**, :ref:`Pin Schema <config-pin_schema>`): Pin connected to SN74HC595 SER input.
+- **clock_pin** (**Required**, :ref:`Pin Schema <config-pin_schema>`): Pin connected to SN74HC595 SRCLK pin
+- **latch_pin** (**Required**, :ref:`Pin Schema <config-pin_schema>`): Pin connected to SN74HC595 RCLK pin
+- **oe_pin** (*Optional*, :ref:`Pin Schema <config-pin_schema>`): Pin connected to SN74HC595 OE pin
+- **sr_count** (*Optional*, int): Number of daisy-chained shift registers, up-to 4. Defaults to ``1``.
+
+See Also
+--------
+
+- :doc:`switch/gpio`
+- :doc:`binary_sensor/gpio`
+- `Serial to Parallel Shifting-Out with a 74HC595 <https://www.arduino.cc/en/tutorial/ShiftOut>`
+- :apiref:`sn74hc595/sn74hc595.h`
+- :ghedit:`Edit`

index.rst

@@ -332,6 +332,7 @@ Misc Components
     MCP230XX I/O Expander, components/mcp230xx, mcp230xx.svg
     MCP3008 8-Channel 10-Bit A/D Converter, components/mcp3008, mcp3008.png
     SX1509 I/O Expander, components/sx1509, sx1509.jpg
+    SN74HC595 I/O Expander, components/sn74hc595, sn74hc595.jpg
     SIM800L, components/sim800l, sim800l.jpg
     DFPlayer, components/dfplayer, dfplayer.svg
     Captive Portal, components/captive_portal, wifi-strength-alert-outline.svg