adsl filter with L.E.D indicator

Thread Starter

rnbguy

Joined Oct 8, 2008
10
Hi All,
I have a question, I am trying to build an adsl filter or passive device, which contains an L.E.D indicator if ADSL frequencies are detected on the line.
The device does not need to handshake or anything, just detect dsl frequencies (not just voltages)?

Am i asking for something from the future or is this possible given what we have today?
 

Wendy

Joined Mar 24, 2008
21,906
It is very possible, though I am weak on how to do it. You would need a external power supply, no taking power from the phone company to power this. Probably a simple high pass filter (like a DSL filter) with a signal detector would do it.
 

Thread Starter

rnbguy

Joined Oct 8, 2008
10
Hi there, thanks for the reply, thats actually light at the end of the tunnel.

the part im stuck on is, what can be used as the signal detector component?
 

Wendy

Joined Mar 24, 2008
21,906
An amplifier with a diode detector. The diode detector will convert the AC that is the signal into DC voltage, which could trigger a circuit that will light the LED.

Like I said, I'm weak on DSL theory (though I had DLS once). I am making a basic assumption that their is a DSL carrier wave. I feel comfortable making this assumption, but I've been wrong before.

Ever do any breadboarding or electronics?
 

Thread Starter

rnbguy

Joined Oct 8, 2008
10
the issue i see with that, is there are always voltages on the line, regardless if dsl is present or not.

so converting and amplifying the ac signals to dc to power up a led could be testing the wrong thing
 

Thread Starter

rnbguy

Joined Oct 8, 2008
10
what if i use a frequency to voltage converter which will generate a certain voltage based on frequency input, this can be followed by an L.E.D which is powered on once a certain voltage is applied (meaning the input frequency at the converter is high enough to be adsl frequencies)..?
 

Wendy

Joined Mar 24, 2008
21,906
Want me to draw a schematic? No guarentees it will work, but I'm always telling people a schematic is worth a thousand words.
 

crutschow

Joined Mar 14, 2008
23,500
It is very possible, though I am weak on how to do it. You would need a external power supply, no taking power from the phone company to power this. Probably a simple high pass filter (like a DSL filter) with a signal detector would do it.
A DSL filter is a low-pass filter used in series with each phone to block the DSL signal and prevent noise in the phones from interaction of the DSL signal with the phone electronics.

The DSL carrier is around one MHz so an RC high pass filter of about 500kHz with a 1MHz amplifier may allow detection of the DSL signal. Rectify that signal to give DC which can drive a DC amplifier to light the LED.

I think the signal is quite small so you may need a fair amount of AC gain to detect the signal. Circuit and line noise could be a problem.
 

t06afre

Joined May 11, 2009
5,934
If I understand your question correct. You are going to use the phone line as power for this. Not sure how smart this is.
 

crutschow

Joined Mar 14, 2008
23,500
Actually it is both. The low pass side goes to the phone, the high pass side goes to the modem.
The confusion is that we are apparently talking about two different types of DSL filters. The ones I've seen are connected in series with the phones and have one input and one (low-pass) output. There is no filter in the line going to the modem.
 

Wendy

Joined Mar 24, 2008
21,906
I've had DSL (FIOS now), the modem had the type of filter I described.

http://en.wikipedia.org/wiki/DSL_filter

A phone only needs the low pass filter as you describe.

I prefer my FIOS, since the router is part of the system. With DSL I had to provide my own. FIOS is supposed to be much faster, but I have trouble telling the difference due to my old slow computer.

My base assumption is the DSL side of the main filter is only the high frequencies, which is what I assume the OP is needing to use.

If I were to design a detector I would use Schottky diode due to their low voltage drops and make a simple AC/DC converter, feeding the results into a simple comparator. If the signal strength were too low I'd think about putting an amplifier in front to boost the signal. Normally I would use an op amp, but it sounds like the frequency is too high, so a transistor version it is.
 
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