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Sensing when TV is set to standby mode.

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alphacat

New Member
Hey,

I'd like to build a circuit which senses when the TV is set to standby mode and when the TV is back On.

So I thought of building a circuit which senses the current drawn by the TV.
I encountered a problem and i dont know how to overcome it.

I want this circuit to be able to detect standby mode of all TVs.
I noticed that the standby current drawn by new TVs is much lower than that drawn by old TVs.

So there could be a situation where a large old TV's standby current is about the same as the one drawn by a small new TV.
in such case, my circuit will fail to detect the standby mode of one of these TVs.

I know that I can add a self-calibration circuit, which will allow my circuit to learn the standby current and On current, which are drawn by each TV my circuit is connected to.

Is it possible to build such circuit without the self-calibration feature?

Thanks alot.
 
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Hero999

Banned
I don't think there's any way of self calibrating, not unless you use a microcontroller and are a software wizard anyway.

You can buy extension leads which cut of other appliances when it detects that the current drawn by the master socket drops below a certain level. These extension leads always have a pot so you can manually adjust the threshold current.
 

alphacat

New Member
The pot which you suggested is an additional way for calibrating the circuit.
Another way is using a chip which already has a calibration capability.

I believe that i'm capable of building such circuit which will include a calibration feature.

The thing is that i'd rather avoid this stage and have my circuit compatible as is for any TV - new or old.
Is it possible?
 
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Mr RB

Well-Known Member
Juts put a little trimpot on the back of the product. Maybe even a 2 position switch instead of a trimpot ("large tv"/"small tv").

Time for some homework, get one of those $20 mains power meters and measure standby and operating currents for lots of TV's, old ones and newer ones etc.

There are already commercial devices that detect when your TV goes into standby and power down all your other equipment, you will probably be competing with cheap Taiwan gizmo makers for sales.
 
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Hero999

Banned
Another idea is to intercept the control signal from the TV remote. This idea was the bases of a commercial product pitched to investors on the Dragon's Den, UK TV programme.

The weakness are that it needs setting up, i.e. your device needs to learn the code for the remote's off button and it won't work if the user uses the off button on the TV.
 

alphacat

New Member
Thanks for these ideas guys.

I guess that sensing the current alone isnt enough to detect standby mode of all kinds of TVs.
An additional feature such as what you suggested is probably needed.
 
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blueroomelectronics

Well-Known Member
I wouldn't think it would be that difficult to record the min / max current and fuzzy logic the on / off states. TVs will have a large on / off current difference.
 

alphacat

New Member
I wouldn't think it would be that difficult to record the min / max current and fuzzy logic the on / off states. TVs will have a large on / off current difference.

Well, it ain't difficult at all.

But i could never test the SB/On current of all the TVs.
What i'm afraid of is that somewhere out there there are
TV A: large, old TV.
TV B: small. new TV.

and that:
I_SB (A) ≈ I_On (B).

isn't such case possible?
 
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Hero999

Banned
With the old CRT style TV detecting whether the TV is working or not would be trivial: just sniff for the horizontal scan frequency. Unfortunately, this won't work for LCD or plasma screen.

I was going to suggest sniffing for the IF but this won't work if it's connected to a cable box via a SCART cable.

Maybe you could sniff for the vidio signal?

There must be some signal that all TVs produce which is easilly detectable.
 

crutschow

Well-Known Member
Most Helpful Member
Why can't you just detect the on-current, which is large, and not worry about the standby current? I would think that any TV's on-current, even a small efficient one, is larger than the standby current of an old, large TV.
 

kchriste

New Member
Forum Supporter
What I would detect would be the ΔI of the TV. Forget about absolutes. ie: A fivefold increase (Or more) could mean the TV has just turned on. A fivefold decrease (Or more) could mean the TV has just turned off.
 

RCinFLA

Well-Known Member
I would guess a 10 to 20 watts (its associated line current) barrier would alway satisfy the on/off detection.

I don't think there are any TV's that draw less then 20 watts when on. (unless you are talking about a small battery powered portable).

Use a current sense transformer.
 

Mr RB

Well-Known Member
Why can't you just detect the on-current, which is large, and not worry about the standby current? I would think that any TV's on-current, even a small efficient one, is larger than the standby current of an old, large TV.

The standby current should be very constant. The "on" current varies largely with screen content, on a large CRT TV the on current for a white screen might be double that with a dark screen.

I think Blueroom had the best suggestion so far. Just get the PIC to autodetect the lowest stable current (ie the standby current), then add a small % safety margin and save that value to eeprom. Fully automatic.

Or my idea just using a 2 position selector switch, allows a cheaper circuit maybe even using discrete parts (no PIC needed). A commercial product only needs to be good for 95% of customers, it's silly to try to anticipate EVERY possible customer at the cost of complexity and reduced profit per unit.
 

crutschow

Well-Known Member
Most Helpful Member
The standby current should be very constant. The "on" current varies largely with screen content, on a large CRT TV the on current for a white screen might be double that with a dark screen.
I still don't see why you can't just use a comparator to detect a threshold above a certain minimum power (say 10W which should be well above any standby power) to determine that the TV is on. Any variability above that has no effect on the detection.
 
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