Auto SCART switching

[The Master]

Hi, Im back on the video switcher project again. We have a TV which you can only turn on by pressing one of the chanel buttons. The power button will only turn it off. That means that for SCART connectors you have to press a chanel then hit the AV button. Im hoping to do away with the TV remote completely since our Sky remote (Satellite TV) has the functionality to control our TV too but it doesnt have the AV button on it. Our Sky box is capable of changing the TV to AV using the SCART lead though.

What i would like to do is have the SCART output from this video switcher box send the same signal to tell the TV to switch to AV mode even if you put it on one of the normal RF chanels. We dont use RF now so thats not a prob. Ive been reading a Wikipedia article (http://en.wikipedia.org/wiki/SCART) about it and near the bottom it appears that i need to output 12V to pin 8. Is that correct or is pin 8 just for the type of input?

One thing that article doesnt seem to mention is when to send the signal. Im under the impression that you must send the 12V signal as long as you want the TV to remain on the AV input and if you turn it off then i guess the TV will reset itself back to the normal chanel. When our TV turns on it doesnt change to the AV chanel if the signal is already being sent (i dont think).

Does the signal have to be a frequency or would a constant voltage do? I want to output this from a PIC chip (through transistor or relay etc). So could i just turn it on or do i need PWM?

Can someone confirm the correct pin and voltage before i go testing please. I know all TVs are different so ill have to experiment a little to get everything right for my TV

[derryck]

The Wiki page you linked to tells you all you need to know. ~12V on pin 8 to switch your TV to SCART input, ~6V to both switch to SCART and widescreen (if supported by TV). Pin 21 is ground for this input.

In your case I think you will have to reassert this input after you've switched your TV on, or otherwise it will remain on whatever channel you had to use to switch it on.

[The Master]

Thanx. I did think that simply putting 12V into pin 8 would work but i just wanted to be sure before i go dumping any kind of voltage into my mums TV. I think ill have to mess around to find the best way to reassert the input because all TVs seem to handle it differently

[Ioannis]

To be sure, put a limiting resistor before sending 12V and 10Amps into the SCART!

Ioannis

[The Master]

Good point. How many amps should i be putting in? I will probs be using a 6 or 12VA transformer so i think that translates to 0.5 to 1A

[Melanie]

AMPS into a SCART!!! Your mother's got a Soviet-era steam-driven TV?

1mA is probably overkill - a 10K Series Resistor should suffice.

[The Master]

By the way, that transformer is to power the entire circuit aswell. Our TV isnt too bad. Our last one was probably older than me though. Shes planning to get one of those flat screen TVs in the future so i best not break that one!

Thanx for the advice. Ive got pleanty of 10Ks laying around