telephone busy light is not right.....

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Externet
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Re: telephone busy light is not right.....

Post by Externet » Sat Mar 15, 2003 6:40 pm

Hello.
The correct way to implement a telco line-in-use indicator is using the line sense relay part number 7278-RL available at MarlinJones and other places for less than $3.<p>Any other approach loads the telco line beyond the regulations.<p>The relay has to be installed in series to the telco line before the branching point; it works with as many paralleled branches as you want.<p>The relay can turn on leds or anything located at any location desired; the spare two outer wires in a telephone cable can be used to route the signal if preferred.<p>Miguel
- Abolish the deciBel ! -

gadgeteer
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Re: telephone busy light is not right.....

Post by gadgeteer » Mon Mar 17, 2003 10:05 pm

There's no reason why the zener current can't be significantly reduced; of course, too low and you get into the diode's "knee". But I don't see why it can't work reliably with zener currents of a few tenths of a milliamp. The other transistors can easily compensate for the low zener current. <p>Then again, why not a comparator, and a low-current voltage ref? Isn't there a cmos comparator? (BTW, I placed two high-power zener diodes across my phone line, cathodes connected together --- each zener is ~150 volts. Any static or lightning-induced spike over 151 volts will break over the zener and get clamped. Definitely a good idea if you're using anything cmos on the phone line. Do a resister-capacitor combo on your "off-hook-detector" to compensate for spikes too fast for the zener...

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dacflyer
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Re: telephone busy light is not right.....

Post by dacflyer » Sat Mar 29, 2003 10:05 pm

thanx all... all is well now...the last post be greg is all i needed... i have made 4,and they all work great...no line loading either...and the batteries will last forever..<p>thanks again greg...<p>and gadgeteer... gregs design is good enough for me... i belive in the KISS theory....
KEEP IT SIMPLE STUPID as they say...not saying your stupid tho..lol but i just like simple...
thanks a lot guys... consider this topic closed...

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