[mythtv-users] Power Relay for Frontends

[tortise]

On 21/10/2012 3:42 a.m., DaWorm wrote:
> Measure the current draw to your TV both when active and when off.  When
> you turn the TV off, you can detect the decrease in current and initiate
> shutdown on the frontent

I am not sure what advantage measuring drawn current offers over a USB 
rail that is either 5V or 0V?  Sounds like a lot of extra electronic 
wizardary / complexity to achieve the same input control?  (I agree it 
would be helpful when the monitor has no switched USB out.)

 > (oneshot to opto that shorts power switch
 > pins).

Touching the power switch momentarily, seemingly the equivalent, on my 
frontend does nothing. If it did tell the FE to gracefully shut down 
then another relay could probably be wired to achieve this.

While not all monitors / TV's give EDID responses, sensing the presence 
of the monitor via EDID (or somehow sensing the voltage on the USB bus?) 
via a cron job to gracefully turn off seems to me a good idea.

I need to read up on serial ports, however it may be a serial port can 
be recruited to sense the TV's 5V and use that in conjunction with CRON 
to cover all turn off cases (for USB switching monitors).  In that event 
the cron job would sense for 0V and when it got 0V that would trigger a 
graceful shutdown.

There may be an even more KISS elegant way than this to achieve a 
graceful power off?

 >Same for power on, the increase in current also triggers the
 > oneshot.

My proposed cheap set of relay and diodes in conjunction with the FE 
Bios achieves power on via the TV power remote.

 >The circuit itself wouldn't draw much power but would have to
 > be on all the time.

Certainly that in itself would be compatible, however the relay idea 
draws no power when off.  Only the TV standby power is then drawn.

I gave some thought to using a Raspberry Pi to do similar but that also 
seems to duck the KISS principle...

Measuring power use from these power supplies and also what is actually 
charged is complex and difficult to measure, because where the current 
is out of phase with the voltage (that is the case with the switch mode 
power supplies that are generally utilised in these fancy digital kit) 
measuring real power charged is not at all straight forward.  The cheap 
power meters available are likely to under estimate the true position.

The other scenario example I have is a Revo with a monitor and separate 
Amplifier connected. In that case the power to the monitor and amplifier 
could be even more simply controlled using a 5V Relay connected to the 
Revo's USB bus, that when the REVO is powered up it turns on the 
amplifier and monitor's mains power, and when the FE is turned off it 
also automatically turns them right off.  Can still do remote WOL of the 
REVO in that configuration.  Not the same as a monitor with an IR remote 
that was first described.