Remote repeater - bit more power to led output

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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.



Bob Latham
Stourbridge, West Midlands

Re: Remote repeater - bit more power to led output
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I found this picture, is that your setup?

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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.


Re: Remote repeater - bit more power to led output

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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.


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

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.




Bob Latham
Stourbridge, West Midlands

Re: Remote repeater - bit more power to led output
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Shit happens. good that you sorted it out.

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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.


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