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Old 18th October 2011, 10:33 AM   #1
cliveb is offline cliveb  United Kingdom
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Default Drive a relay from SCART pin 8?

I wish to power on an external device when my DVD player is turned on. An obvious way to try and do this would be to use the 6V signal on pin 8 of the SCART output.

However, a typical relay with 6V coil pulls about 60mA, and even a reed relay pulls about 12mA at 6V. A solid state relay pulls about 15mA. I am led to believe that the current available on the SCART pin 8 would be 5mA at the very most.

I did wonder about using an optocoupler (eg. MOC3020), which draws about 10mA at 1.5V, which is 15mW. If the SCART can deliver 3mA at 6V, that would be 18mW, so perhaps that would work.

The only other solution I can think of would involve using a transistor to boost the current, but that would need a power supply which kind of defeats the object of only turning on the external power when the DVD starts up.

Any suggestions?
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Old 18th October 2011, 11:37 AM   #2
Mooly is offline Mooly  United Kingdom
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Not sure what you are trying to power but a couple of suggestions.

If you are confident in what you are doing you can use an unused scart pin (isolate it first in the DVD player) to supply power (withing the capabilities of the DVD PSU) to an external device. I did this a few years ago on a Sony TV to supply a headphone amp with power (12 volts DC at up to say 100 ma). You could use a switched rail in the player directly or arrange solid state switching that activates the output only when pin 8 is active.

If you want to switch mains then the opto isolated triac is good... I have used these. Why not just load pin 8 with a resistor and LED and see what the drive capability is. If the short circuit current of pin 8 is low then a series resistor may not be needed at all with the LED in the opto.

I used an MOC3020 here,
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Old 18th October 2011, 01:12 PM   #3
cliveb is offline cliveb  United Kingdom
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Many thanks for your reply.

Quote:
Originally Posted by Mooly View Post
Not sure what you are trying to power but a couple of suggestions.
The device is in fact just a simple opamp circuit to add a bit of gain to the audio output from the DVD player. I don't know why, but DVD players seem to have a significantly lower audio output voltage than other devices such as PVRs and VCRs, and our TV doesn't have quite enough gain to get a comfortable volume.

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If you are confident in what you are doing you can use an unused scart pin (isolate it first in the DVD player) to supply power (withing the capabilities of the DVD PSU) to an external device.
I hadn't thought about modifying the DVD player internally, but considering it's a cheap 40 player, I guess I can afford to risk it. In which case an obvious approach would be to simply install the opamps inside the player, where I can presumably easily find a suitable voltage rail.

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If you want to switch mains then the opto isolated triac is good... I have used these. Why not just load pin 8 with a resistor and LED and see what the drive capability is.
Good idea, I should have thought of that. I knew that the thing which needs driving inside the optocoupler is just a LED, but had failed to make the connection about testing the output.

But thanks to your first suggestion, I now realise that installing the gain stage internally is the elegant solution, so I will concentrate on that for now.
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Old 18th October 2011, 06:09 PM   #4
Mooly is offline Mooly  United Kingdom
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Knowing what you want to do there are more possibilities

Does the DVD player use opamps in the analogue stages and if so maybe it could be easily modded to give more gain without addiitional opamp stges. Perhaps just a case of altering or adding a resistor.

The TV !
It's possible that the TV audio input (scart) has series resistors in series with the audio feed to pad it down. This is something I altered on a Sony 25" TV of ours many years ago. Just a case of dropping the two resistors down a bit.
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Old 18th October 2011, 07:18 PM   #5
cliveb is offline cliveb  United Kingdom
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Quote:
Originally Posted by Mooly View Post
Does the DVD player use opamps in the analogue stages and if so maybe it could be easily modded to give more gain without addiitional opamp stges. Perhaps just a case of altering or adding a resistor.
Well I haven't opened it up yet, but it's a cheap modern player so I'm expecting to see loads of SMDs on the main board. I know my limits and don't go near SMDs.

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It's possible that the TV audio input (scart) has series resistors in series with the audio feed to pad it down. This is something I altered on a Sony 25" TV of ours many years ago. Just a case of dropping the two resistors down a bit.
I don't mind possibly blowing up a 40 DVD player with my tinkering, but no way am I going to fiddle around inside a 800 TV!
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Old 19th October 2011, 06:46 AM   #6
Mooly is offline Mooly  United Kingdom
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Fair enough...
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