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Remote repeater - bit more power to led output

Hi,

I'd be grateful for a little advice.

I use a tvLINKplus device for repeating the magic eye remote signals which are fed back through the UHF aerial cable to my satellite box.

It works OK but the output LEDs have to be fitted in exactly the right place on whatever kit you're using or it doesn't work and even then, it's not as reliable as it could be. I Would also like to drive

3 leds for 3 devices.

So I think I could do with a bit more drive current.

I don't have the diagram of the tvLINKplus but by placing resistive loads on the output it looks like there is the equivalent of a 2,200 ohm resistor in series with the output.

I get a series of pulses which modulate a 38Khz carrier.

The pulses with no load are only +350mV When loaded with the original 2 leds this drops to 77mV

I was thinking of using an emitter follower circuit and connecting the base of the NPN transistor to the output of the tvLINK. I then realised that the transistor will need 0.6v base-emitter to switch it on and I've only got 350mV.

I need a simple circuit to give me a bit more drive, 77mV at 0.12mA is very small indeed to drive leds I was hoping to double it and make it 3 leds.

Any advice much appreciated.

Thanks.

Bob.

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Bob Latham 
Stourbridge, West Midlands
Reply to
Bob Latham
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I found this picture, is that your setup?

formatting link

I am of the opinion that an infrared LED won't get out of bed for less than 1000mV - how are you making these measurements?

Are you seeing DC voltage on the coax going into the TVLINK that connects to the eye?

It should be possible to drive a silicon transistor in common emitter mode from the LED drive and use that to drive several LEDs (or to drive a single LED that floods the whole room, so long as there is no leakage path back to the remote end)

it might be easy to tap into the DC supply to the tvLINKplus to run that, or it might be difficult.

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  Jasen.
Reply to
Jasen Betts

Wrongly.

After reading your post this morning I started to think the results I saw needed checking.

Another nagging worry was the fluke showing roughly 80% of what the scope said. If it was a square wave it should should 50% but it was bursts of square waves so should be much less than 50%.

I checked the scope, set to calibrate - yes. x10 turned off - yes.

Noticed the scope was set to 50mV /div. So I got a 1.5v battery and connected it to the probe expecting the display to vanish off the top. It didn't, it went up 2.7 divisions.

What!

Looked at the probe, the only thing on it is: Tektronix PR19C TKG 88/MK3

I tried another probe, and you'll not be surprised to learn it read

10 times higher.

So yes, didn't know that probe divided by 10, in my defence I purchased from a former employer in 1993, I think I've used it twice since. The probe does not indicate it divides by 10.

So, it looks like I have 3.5v of square wave output with no load which I could feed into an emitter follower to up the current available.

Talking of current, all my measurements yesterday were done by seeing the voltage across known resistors WITH THE SAME PROBE so they are all 10:1 out as well.

With that info, do you think I could get away with using say a BC108 as emiiter follower, base connected to tvLINK output, collector to

+ve supply emitter to limit resistor and then leds.

I believe the two leds I have at the moment are wired in series, vert hard to prove without destroying the led cable. Can I get 3 leds to work with a bit more power?

Any thoughts now you have more reasonable information to go on?

Once again, thanks for your help.

Thanks.

Cheers,

Bob.

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Bob Latham 
Stourbridge, West Midlands
Reply to
Bob Latham

Shit happens. good that you sorted it out.

BC108 is an antique, but yeah it should work.

emitter follower should work, but common emitter could be worth experimenting with too - you get more current into the base so potentially more gain. but 20mA into an LED will probably flood the whole rack if you aim it at the back wall or the ceiling.

--
  Jasen.
Reply to
Jasen Betts

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