Phone in use circuit not working

I tested and built the following circuit.

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It all worked fine untill I tried to test it by calling into my phone. (the one test I didnt do before I soldered it all together)

What happens is taht the phone rings for a about 1/2 a ring then stops ringing. To my surprise when I life the phone up the line is alive eg. Its like the device has answered the phone.

I think that the circuit must be drawing too much power and then telling the phone that it has been answered, because the voltage has gone down.

I have had to replace the Transistors with NTE199, which according to the book are the same. I have also used a rectifier that is a 2 amp 400v SIP 2KBP04M-1. Im wondering if I used a different rectifier if that would change the results?

I recognize that your not suppse to take power from the phone co. but I would like to get this circuit working. Can anyone tell me why its not working. thanks.

Reply to
steve
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That sounds right. The phone company senses an off-hook state based on DC current passing through the local loop. So your explanation makes sense.

I think your part substitutions are correct. But I wonder if you have the bridge rectifier wired correctly - the phone lines connected to the ~ pins. Or, could that part be faulty?

If everything is wired correctly, try removing R4 from the circuit and see if ringing works properly then. If it does, then you may just need a higher value resistor for R1.

Actually, I don't much care for the circuit. During each cycle of the ring signal, the voltage on your circuit will go to zero. It won't go negative because of the rectifier, but it will go to zero. As it gets near zero, then again as it comes back up, Q2 will turn on for a bit, and current will flow, until the voltage gets high enough to turn on Q1, which will turn off Q2. But during that Q2-on period, maybe enough current will flow to signal an off-hook to the central office. Well, I'm clearly just guessing, but I find myself wanting to put a capacitor from the base of Q1 to ground.

Anyway, good luck.

Reply to
Peabody

Thank you for your comments.

Well I have wired the phone line to the middle pins on the SIP [ + | | - ] And + to plus and - to Neg of the diagram.

Thanks for the tips. I will try them.

By the way one thing. The diagram calls for R2 to be 33k. But a comment on the bottom says that the value should be the same as R1 3.3. So I made R2 3 .3. I asumed that the poster was right, because I know how scematics get sc rewed up some times. In your opinion should R2 be 3.3k or 33k. Maybe thats whats wrong? Regards.

Reply to
steve

Yes, I think that's right.

Well, using 33k will make the circuit draw less current, but the green LED probably will not light up enough to be visible. But of course you really don't need the green LED, just the red one.

My concern is that the circuit just draws too much current, and therefore shows as off-hook. I would try it at 33k, and take the green LED out (or just temporarily jumper across it, and see if it works. Beyond that, you could increase the resistance of R1, but then that will make the red LED dimmer.

There is another version of this circuit that uses MOSFETs instead of NPN transistors, and all but the LED resistor are in megohms, so it draws very little current when on-hook. That's because the MOSFETs switch based on voltage instead of current.

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Reply to
Peabody

ringing.

device has answered the phone.

phone that it has been answered, because the voltage has gone down.

are the same. I have also used a rectifier that is a 2 amp 400v SIP 2KBP04M-1.

like to get this circuit working. Can anyone tell me why its not working.

I think some one gave you some bad transistor numbers..

THe phone system in the US rings at around 100 Volts AC and on hook is around 50Volts. Off hook may give you around 10 volts and it depends on how many phones you have at one time off hook.

The 2N3392 can only handle 25Volts and it may work if you can insure that the unit will switch on in time to load it down. Further more, the other transistor will be sitting there with this 50v (on hook), leaking through the collector.

The NTE199 replacement isn't much better, it has a 70V limit at best and most likely will work find when on hook, but when it rings, you can expect some leaking to be taking place.

You first need to get HV transistors.

Try getting some 2N5550 transistors, or the 2N5551 which is a little higher.. Mouser.com has 2N5551 for 0.78 ech and they have over 3k in stock.

Jamie

Reply to
Jamie

That one would most likely work better, at least the transistors are rated at 200V, which is your biggest problem with the first circuit..

Also this circuit has much less load since it seems to only work when off hook.

Jamie

Reply to
Jamie

Here's one I used for years without problem. Similar to what Peabody posted, one less FET.

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I can't find the original URL. Mikek

Reply to
amdx

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I remember buying an off hook relay module for a phone line from radio shaft years ago, back in the old days, it never worked.

would never switch on. Also, they stopped selling them. Must of been a reason for it :)

Some times it is more sensible to buy ready made over making it.

Jamie

Reply to
Jamie

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Yes, and that has the capacitor I kept wanting to put into the first circuit. That should delay the turn-on of the FET enough to keep the LED off during ringing.

Reply to
Peabody

ringing.

device has answered the phone.

phone that it has been answered, because the voltage has gone down.

are the same. I have also used a rectifier that is a 2 amp 400v SIP 2KBP04M-1.

like to get this circuit working. Can anyone tell me why its not working.

I remember as a teenager, I was playing around with the phone line. I learned two things. One, if you hook the right resistance across the line, incoming calls get a "busy" signal, but you can still make outgoing calls. Two, when someone is calling you, you can get a jolt if you're holding both ends of the line.

So, when the phone rings, you get a 60V AC signal. Perhaps you need slightly more resistance in the line. Unless D2 is already too dim, I'd try changing R1 to a higher resistance.

Reply to
Daniel Pitts

ringing.

device has answered the phone.

phone that it has been answered, because the voltage has gone down.

book are the same. I have also used a rectifier that is a 2 amp 400v SIP

2KBP04M-1.

results?

would like to get this circuit working. Can anyone tell me why its not working.

It was 90 volts.

Reply to
Michael A. Terrell

Sticking your fingers across a voltage source was never a precise means of measuring voltage.

Michael

Reply to
Michael Black

33K looks right to me, if perhaps a little too low.

the green LED illuminates when the circuit is off-hook which is 40V or more. 33K only gets you ballpark 1.3mA, but much more than that and it'll start looking like off-hook.

If you want it to be brighter get a better LED or connect several LEDs in series.

--
?? 100% natural
Reply to
Jasen Betts

There is quite a long specification for telephone lines in the US. I have a copy, thanks to Don Bowey, when he dropped a copy over at my home back around 2003 or so. It represents an industry composite of various systems and includes envelopes of operation. The gist of it is, as Don wrote before coming over back then,

"The FCC R&R require a minimum of 5M of customer premises equipment loop resistance in the on-hook state. Also. at 220uA some Central Offices will declare a line fault and remove the line from service."

The basic idea of presenting 5MOhm total customer premises load (that is the TOTAL of all phones and connected equipment) when on-hook is incredibly hard to meet with an unpowered attachment that shows "in use" and "not in use" led lights.

I've also read that, "if you are in USA or canada Bellcore specs allow a max of 5micro amps at 48VDC in an on-hook state." That's 10MOhm, in effect, though I believe the 5MOhm for the entire premises still fits the spec.

The upshot here is that you aren't going to be seeing an LED lit continuously and still meet on-hook specifications. You can design a system to blink periodically, by drawing very little current onto a capacitor, which charges slowly up and is then discharged into an LED when the voltage rises to the trigger voltage. You can design that to meet specifications. But that's not what you've been given to build.

Jon

Reply to
Jon Kirwan

That's why I bought & built a Heathkit VTVM when I was 13.

Reply to
Michael A. Terrell

There are various sources of the specifications that the on/off hook threshold can be looked up, then it should be a few simple calculations to determine the circuit's current draw.

Its not uncommon to draw *some* current on hook - many phones maintain a supercap for various memory functions.

Assuming an LED doesn't violate the on hook current threshold, maybe a MOSFET (2N7000 etc) could be worked in as its voltage driven rather than needing base current.

Its worth bearing in mind; there's usually a zener or two downstream of the bridge rectifier in a phone (typically 12V) - but usually some series resistance as well, so the voltage off hook will settle a little higher.

Reply to
Ian Field

Apparently telcos sometimes apply test voltages upto 200V, but the normal operating voltage is considerably lower.

220V MOVs & gas discharge arrestors are commonplace in telecoms equipment, the last phone I stripped & traced had a 200V sidac.
Reply to
Ian Field

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I

With all the discount stores sprouting like weeds, there's many things I once would've built are now cheaper than the parts to buy.

Reply to
Ian Field

The ones I often find in phones are the MPSA42 & 92 - ones PNP & the other NPN.

Reply to
Ian Field

If you can use that 48V to charge a capacitor above 32V without violating the on hook current spec; you can make an LED flash very brightly periodically by dumping the cap into it with a DB3 diac.

You'd have to trial & error for how many uF to make the flash bright enough without blowing the LED.

Reply to
Ian Field

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