Automating HDMI input switching

I recently wrote about automating TV power control using IR LEDs. This enabled me to turn a TV and amplifier on and off with Amazon Echo and as part of other automation flows. However, with a cable TV box, Kodi media player, PlayStation 3 and Google Cast I still needed to choose the TV’s input source manually. I’d like to be able to say “Alexa, turn on cable”, or “Alexa, turn on Google Cast” and have it not only turn on the TV and amp as it can now, but also switch to the correct input.

One option would be to take advantage of automatic HDMI switching driven from a source coming online – but with things like the Cast and Kodi player being always-on this made this a no-go.

Another option was to use an external HDMI switcher box with an IR remote control and blast IR commands at this in the same way as for the TV itself. In fact this was my original plan and I carefully selected a HDMI switcher that was not only getting decent reviews, but also had a remote control button per channel (simply having a “next input” cycle button was not suitable because I’d have no way to know when I got to the input I wanted). However when I received the switcher I thought I’d try something different, just for fun.

Hardware

This particular switcher has an LED for each input, showing which is selected, and a single push button to cycle through the inputs. Having opened up the box by prising off the plastic feet and unscrewing the 4 small screws underneath I took a good look at the circuit board. I observed that the LEDs were wired with the cathodes connected to ground, therefore the anodes would be at a positive voltage when the LED was lit – this is good enough to directly drive a Raspberry Pi GPIO input pin in pull-up mode. By connecting all 5 LED anodes to GPIO pins I could easily tell which input was selected. The push button was also connected to ground, suggesting it was being used in a pull-up fashion. I took a gamble that I could get away with driving this directly from a Raspberry Pi GPIO output without any further interfacing. To make life even easier all of the LED and switch pins were through-hole soldered into the switcher’s PCB (i.e. not surface mount like most of the board) meaning small wires could be fairly easily soldered onto them.

hdmiwiring

Even more conveniently the arrangement of the box, although very compact, had plenty of room to bring in a 9 core cable (5 LEDs, 1 switch, 1 ground and two unused wires) leading to a neat appearance. To route the cable out I drilled a 5mm hole close to the IR socket on the edge of the box and routed the wires over the top side of the PCB.

On the Raspberry Pi end I simply chose 6 unused GPIO pins and connected the LEDs and switch input as well as connecting ground on the HDMI box to a ground pin on the GPIO header.

Software

I wanted to use the same Raspberry Pi running LibreELEC and Kodi that I was already using to drive the IR emitters. This meant extending the existing Kodi add-on with the capability to read the HDMI switcher’s LED status and drive its push button. The general strategy was to have an incoming MQTT message set the desired input number (1-5) and then send button push pulses until the associated LED lit up. One potential gotcha here is that this switcher skips over inputs that do not have an active HDMI source, therefore if the MQTT message requests an input with either nothing connected, or with a source that’s not switched on, the button pushing could go on forever. To avoid this I limited the number of button push attempts per MQTT message to 10.

The prerequisite was to add the “Script – Raspberry Pi Tools” Kodi add-on (search the registry from the Kodi add-on manager UI) to add the GPIO Python library.

The code is pretty simple. At start of day I set up GPIO – the first 5 pins are the inputs from the switcher’s LEDs and the final pin is the output to the push button:

sys.path.append('/storage/.kodi/addons/virtual.rpi-tools/lib')
import RPi.GPIO as GPIO

# Setup pins for HDMI switcher control
GPIO.setmode(GPIO.BCM)
GPIO.setup(21, GPIO.IN, pull_up_down = GPIO.PUD_UP) 
GPIO.setup(8, GPIO.IN, pull_up_down = GPIO.PUD_UP) 
GPIO.setup(16, GPIO.IN, pull_up_down = GPIO.PUD_UP) 
GPIO.setup(12, GPIO.IN, pull_up_down = GPIO.PUD_UP) 
GPIO.setup(7, GPIO.IN, pull_up_down = GPIO.PUD_UP)
GPIO.setup(20, GPIO.OUT)
GPIO.output(20, 1)

And initialise some global variables to record the desired input number and the count of button push attempts:

hdmiinput = 1
hdmiattempts = -1

In the MQTT message handler I added another sub-topic case:

            elif ll[2] == "HDMI":
                try:
                    hdmiinput = int(msg.payload)
                    hdmiattempts = 0
                except:
                    pass

And in the main loop (which cycles every 0.5s):

        if (hdmiattempts > -1) and (hdmiattempts < 10):
            for i in range(4):
                hdminow = 0
                if GPIO.input(21): hdminow = 1
                if GPIO.input(8): hdminow = 2
                if GPIO.input(16): hdminow = 3
                if GPIO.input(12): hdminow = 4
                if GPIO.input(7): hdminow = 5
                log("HDMI switcher currently on %s, want %s" % (hdminow, hdmiinput))
                if hdminow == hdmiinput:
                    hdmiattempts = -1
                    break
                else:
                    log("Sending HDMI switch button trigger")
                    GPIO.output(20, 0)
                    time.sleep(0.1)
                    GPIO.output(20, 1)
                    time.sleep(0.1)
                    hdmiattempts = hdmiattempts + 1

The full Kodi add-on code can be found here. It’s quick and dirty, but it works.

Automation with Echo

So now I have a way to change HDMI input to the TV by sending a MQTT message such as “KODI/Lounge/HDMI=4“. To use this with Amazon Echo I extended my existing TV control with specific cases for each of the 4 inputs in use. The configuration for the Alexa smart home skill adapter (see Controlling custom lighting with Amazon Echo and a skill adapter for more on this) sends a single MQTT command into my home broker:

                {
                    "applianceId":"loungetvcable",
                    "manufacturerName":"James Bulpin",
                    "modelName":"LWRF",
                    "version":"v0.1",
                    "friendlyName":"Virgin Media",
                    "friendlyDescription":"Living room TV and amp on cable TV",
                    "isReachable":True,
                    "actions":[
                        "turnOn",
                        "turnOff"
                    ],
                    "additionalApplianceDetails":{
                        "mqttTopics":"compound/loungetvcable"
                    }
                },
                {
                    "applianceId":"loungetvcast",
                    "manufacturerName":"James Bulpin",
                    "modelName":"LWRF",
                    "version":"v0.1",
                    "friendlyName":"Chrome Cast",
                    "friendlyDescription":"Living room TV and amp on Google Cast",
                    "isReachable":True,
                    "actions":[
                        "turnOn",
                        "turnOff"
                    ],
                    "additionalApplianceDetails":{
                        "mqttTopics":"compound/loungetvcast"
                    }
                },
                {
                    "applianceId":"loungetvps3",
                    "manufacturerName":"James Bulpin",
                    "modelName":"LWRF",
                    "version":"v0.1",
                    "friendlyName":"PS3",
                    "friendlyDescription":"Living room TV and amp on PS3",
                    "isReachable":True,
                    "actions":[
                        "turnOn",
                        "turnOff"
                    ],
                    "additionalApplianceDetails":{
                        "mqttTopics":"compound/loungetvps3"
                    }
                },
                {
                    "applianceId":"loungetvkodi",
                    "manufacturerName":"James Bulpin",
                    "modelName":"LWRF",
                    "version":"v0.1",
                    "friendlyName":"Kodi",
                    "friendlyDescription":"Living room TV and amp on Kodi",
                    "isReachable":True,
                    "actions":[
                        "turnOn",
                        "turnOff"
                    ],
                    "additionalApplianceDetails":{
                        "mqttTopics":"compound/loungetvkodi"
                    }
                },

(See this gist for the context – these rules are just added to the list)

Rules in the rules engine then turn this into TV power control and in the “on” case, also sends the HDMI control message:

  case 'compound/loungetvcable':
    resources.mqtt.publish("KODI/Lounge/TV", message.toString());
    if (message.toString() == "on") {
      resources.mqtt.publish("KODI/Lounge/HDMI", "1");
    }
    break;
  case 'compound/loungetvcast':
    resources.mqtt.publish("KODI/Lounge/TV", message.toString());
    if (message.toString() == "on") {
      resources.mqtt.publish("KODI/Lounge/HDMI", "2");
    }
    break;
  case 'compound/loungetvps3':
    resources.mqtt.publish("KODI/Lounge/TV", message.toString());
    if (message.toString() == "on") {
      resources.mqtt.publish("KODI/Lounge/HDMI", "5");
    }
    break;
  case 'compound/loungetvkodi':
    resources.mqtt.publish("KODI/Lounge/TV", message.toString());
    if (message.toString() == "on") {
      resources.mqtt.publish("KODI/Lounge/HDMI", "4");
    }
    break;

In closing

By adding explicit input selection to the set of MQTT commands I can send I can now build more complex automation actions that not only turn things on or off but can also select inputs. This enables a single Echo command such as “Alexa, turn on Chrome Cast” to get all the necessary devices into the right state to achieve the desired outcome of the TV and amp being on and displaying the output of the Cast.

hdmisidebar

 

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