Mac keep going to sleep - lid closed, connected to external monitor and bluetooth mouse & keyboard

[striders]

According to Apple, MacBook Air/Pro/Retina support the closed-clamshell mode, which allows keeping the MacBook ON when the lid is closed. Link from Apple site

So I have my MacBook Air AND Pro (both Yosemite) setup like this:
1). Connected to Bluetooth Keyboard (Logitech K810) and Mouse (Anywhere MX2). These 2 peripherals can be used by both MacBook without re-pairing. I.e. Press F1 -> MBA, Press F2 -> MBPR.

2). Connected to an HDMI Hub with switch to allows 3 video source (MBA, PC, Xbox, etc.) to connect to 1 external monitor

I keep the MBPR lid open but for the MBA, I keep the lid closed (using Twleve South BookArc). The setup above work, BUT, the MBA keep going to sleep after 1 hour.

Is this because:
1). When I switch the peripherals bluetooth connection to the MBPR, the MBA show a message that Keyboard/Mouse connection is lost.

OR

2). When I switch the HDMI connection to the MBPR, the MBA revert back to single-display mode.

Any ways to keep the MBA awake with the above config?

 

[Weaselboy]

2). When I switch the HDMI connection to the MBPR, the MBA revert back to single-display mode.

Any ways to keep the MBA awake with the above config?

This is your problem. It requires a video signal to stay awake in clamshell mode. There are apps to keep it awake like NoSleep and InsomniaX. You might try one of those.

 

[striders]

This is your problem. It requires a video signal to stay awake in clamshell mode. There are apps to keep it awake like NoSleep and InsomniaX. You might try one of those.

Ah, OK. I thought it's the power adapter that's keeping it awake. I'll try InsomniaX.

 

[striders]

UPDATE:

I installed InsomniaX and set it to disable sleep when the lid is closed and ONLY when the MBA is connected to power. Rebooted to make sure the settings stick.

Closed the lid, connect to external display. Bluetooth mouse & keyboard connected. So far so good.

However, the MOMENT I switched the HDMI connection (I use a switch to share 1 monitor with 2 other machines), the MBA WILL go to suspend mode, then eventually off.

So looks like the on-while-lid-is-closed feature only work when there is continuous signal from the monitor?

 

[striders]

UPDATE2:

Googling for "HDMI Switcher Yosemite" seems to support my theory: Clamshell mode (lid closed but OSX running) ONLY work if there is CONTINUOUS video signal. Using HDMI switcher will kill the signal when switched.

InsomniaX or any other similar tool wouldn't work.