When the arcade is completely built, I want it to have a button somewhere on the cabinet that acts as a power button. That is, pressing the button will cause the Raspberry Pi to boot, the TV to turn on, the marquee to light up, and the speaker amplifier to have power. Pressing the same button again will cause the Pi to shut down, and everything else to lose power.

Step 1: Toggle Pi between running and halt

I thought I was going to have to use a latch circuit, because I was under the impression that the only way to make a Raspberry Pi boot after a shutdown was to remove and restore power. But then I found out (by finding this: https://www.instructables.com/Raspberry-Pi-Power-Button/) that shorting pins 5 and 6 on the GPIO header will boot the Pi from a halt state. And then, to make this same connection cause a shutdown while it is running, you only have to add one line to /boot/config.txt:

dtoverlay=gpio-shutdown

Step 2: Use the Pi to control power to other components

So I can connect a momentary pushbutton to pins 5 and 6 and have a power button that does what I want, at least for the Pi. I can then use the Pi to control a relay that will control the power to everything else. Since everything ultimately is connected to AC power, I will use the relay to interrupt the AC line to everything but the Pi. That relay is already installed on the back of the control panel.

I connected the 5V pin on the relay module to pin 2 on the GPIO header, the ground to pin 9, and the control to pin 3 (GPIO 2). Then I wrote a python script called ‘turnOnPeriph.py’, which looks like this:

export RPi.GPIO as GPIO
import sys
GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)
GPIO.setup(2,GPIO.OUT)
GPIO.output(2,GPIO.LOW)

Then I added the following to /opt/retropie/configs/all/autostart.sh

# Turn on the relay that powers all peripherals
python /home/pi/bin/turnOnPeriph.py

It turns out that the relay is high-asserted, and the default state of GPIO pins is high, so the relay is activated as soon as the Pi has power. Once the autostart script executes, the relay is deactivated. This means that I will have to connect the AC line to the normally closed contact instead of the normally open. The only weirdness this will cause is that the marquee may flash briefly when the unit is plugged in or unplugged.

Step 3: Force Pi to load HDMI

So now we have a power button for the Pi, and the Pi switching on power to the rest of the system. But there’s more work to be done. By default, the Pi tries to detect a monitor on HDMI at boot, and if it doesn’t, it will not load HDMI, and there will be no output even if you connect a monitor later. The reason for this is that the Pi gets the resolution and refresh rate settings from the connected device and loads HDMI with those settings. This is a problem if we are using the Pi to turn on power to the TV.

Fixing this is a two-step process. First, add (or just uncomment if it’s already there) this line to /boot/config.txt:

hdmi_force_hotplug=1

This will cause the Pi to load HDMI regardless of whether a device is connected. Since the Pi won’t be able to get the resolution information, the second step is to tell it what settings to use. This is done by running raspi-config, navigating to “advanced settings,” then to “resolution,” and then selecting the option that corresponds to your output device. Of course, you have to know what the correct settings are. You can do this by executing the two following commands:

tvservice -d <file>
edidparser <file>

My TV is advertised as “720p”, but its native resolution is actually 1366×768. The refresh rate is 60Hz, so the correct setting is HDMI_DMT_1366x768_60

Step 4: Use HDMI-CEC to turn on the TV

Now the Pi will correctly load HDMI, and once the TV is powered and turned on, we will have our display working. Which leads to the next issue: when you plug in a TV (effectively what I’m doing with the relay), it does not automatically turn on.

Fortunately, the TV I bought (along with 99.9% of TVs made in the last decade or so) supports HDMI-CEC, or HDMI Consumer Electronics Control. I had to go into the menu on the TV to turn this on. Then CEC has to be installed on the Pi. That was easy enough to do following the steps provided by PiMyLifeUp (https://pimylifeup.com/raspberrypi-hdmi-cec/). With this installed, I can add the command to turn on the TV to my autostart script, and it should be ready to go.

However, the TV isn’t ready to receive CEC commands the instant it has power, so I need to be sure that it’s ready before I send the command, or it won’t work. If you query the power status of an HDMI-CEC device that is unplugged, the result will be ‘unknown.’ I added a while loop checking for status so that I know when the CEC module is responding. Here is my updated autostart.sh:

# Turn on the relay that powers all peripherals
python /home/pi/bin/turnOnPeriph.py

# Wait until the TV  has booted to  send 'on' signal
status='unknown'
while [[ $status == *"unknown"* ]]
do
  status=`echo 'pow 0.0.0.0' | cec-client -s -d 1`
done
echo 'on 0.0.0.0' | cec-client -s -d 1

emulationstation #auto

That should be all the setup required for the power button. All that’s left is the wiring!