Circuit to count number of times phone is picked up

[Megamox]

I needed a circuit that would count the number of times a telephone on the line was picked up. I built the circuit below and fed it into a ripple counter with some logic for the output alphanumeric displays. But whenever i lift up the handset nothing happens!

Whenever a handset is picked up, line voltage is dropped to about 9v. The input to the circuit is from a diode bridge attached to the line. The 22n smoothes out any ring voltage (90V RMS). Have i missed something obvious?? is there a better way to do this??

Thanks in advance

 

[Optikon]

I needed a circuit that would count the number of times a telephone on the line was picked up. I built the circuit below and fed it into a ripple counter with some logic for the output alphanumeric displays. But whenever i lift up the handset nothing happens!

Whenever a handset is picked up, line voltage is dropped to about 9v. The input to the circuit is from a diode bridge attached to the line. The 22n smoothes out any ring voltage (90V RMS). Have i missed something obvious?? is there a better way to do this??

Thanks in advance

Strictly speaking, it is illegal for you to connect this circuit to the phone lines in the United States. Since you are in the UK, there might be a similar restriction.

The better way I wouold suggest is to buy a module that can be interfaced to the phone lines that has already been approved / listed by the FCC (having all the proper terminations and transient protections in place).

These modules can be legally used and provide digital signalling like on/off hook events, ring indicators and a duplex analog channel.

If you had one of these, then you can count hook events. Anyways, I dont recall who makes them off hand but I'll look it up if you wish. If you are building just one of these, you can get a free sample so you can have peace of mind.

 

[Megamox]

Thanks any info would be great

Yes we have restrictions here too, im wondering if its possible for me to make my designs FCC compliant, do i have to submit them to them or something, or is it too horribly complicated. I guess i better take a look at the guidelines, currently im using the BT-SIN (British Telecom Supplier information notes).

 

[audioguru]

Hi Megamox,
EEK! A fuzzy Jpeg.

Your circuit had nothing to turn off the transistor. The 22nF would remain charged at the threshold of the transistor conducting slightly.
The 22nF wouldn't be a filter for 90V ringing anyway, it would just charge to the peak voltage.

I added a capacitor from the base to collector to try stopping the 20Hz ringing voltage from activating the transistor. It is very difficult to stop a frequency with only a single-pole filter. It results in very slow edges of the output pulse, requiring a Schmitt-trigger input to the clocked counter.

 

[Megamox]

ahh i never thought of using a capacitor in the feedback like that, ill try that thanks!

Well i was using the 22n to actually hold the supply from going below the 10V threshold because thats what triggers the transistor into turning off and sending a pulse. I figured if the handset was picked up after a ring, the top end of the diode would be about 9v or thereabouts, and the 22n being fully charged would try to hold the rail but would discharge pretty quickly meaning the transistor would turn off and the collector would jump up to 9v clocking the rest of the circuit.

I could use 2 CMOS not gates on the end for the schmitt triggers right?

Megamox

 

[Nigel Goodwin]

Yes we have restrictions here too, im wondering if its possible for me to make my designs FCC compliant, do i have to submit them to them or something, or is it too horribly complicated. I guess i better take a look at the guidelines, currently im using the BT-SIN (British Telecom Supplier information notes).

Restrictions in the UK are FAR! more strict than the USA, you're not allowed to connect ANYTHING to the UK phone network unless it's passed expensive testing procedures (tens of thousands of pounds?) - this effectively prevents any home made phone projects.

FCC compliance isn't of any use, that's only for the USA.

 

[audioguru]

Yeah, you have a good idea to rectify the 90V ringing to keep the transistor turned on.
All you need is a resistor to turn off the transistor and your circuit is fine.
The output diode isn't needed because the very low current going to -0.6V won't hurt Cmos.

 

[Megamox]

I suppose i could have attached a contact switch to the handset which would trigger every time the phone was lifted off the cradle, something a bit more low tech and legal.

Im curious about something, can i attach my circuitry to something that meets the requirement. If i took a phone apart and used some of its circuitry as an interface to my own, would that be legal? (assuming my design didnt affect the normal operation of the equipment or line, perhaps opto-coupled). Does sticking an inductor coil near the speaker of a telephone make it illegal to attach it back to the line?

Thanks

 

[Megamox]

Im always forgetting the pull down resistors, somewhere in my head absence of voltage = ground - duh. Thanks.

The circuit simulates fine too, i guess ill have to pick apart the connections and see whats up with it.

Thanks for the help guys

 

[Nigel Goodwin]

Im curious about something, can i attach my circuitry to something that meets the requirement. If i took a phone apart and used some of its circuitry as an interface to my own, would that be legal? (assuming my design didnt affect the normal operation of the equipment or line, perhaps opto-coupled). Does sticking an inductor coil near the speaker of a telephone make it illegal to attach it back to the line?

AFAIK you're not allowed to make any direct connection?, an inductive 'sticker' on the outside of an old phone was fine (no direct connection), as are acoustic connections to the microphone and speaker (again, no direct connection).

If you disassemble an approved item and modify it in any way, then any approval is obviously null and void, and (legally) would require submitting for approval tests.

As far as I can remember?, possible penalties include jail time and UNLIMITED fines!.

 

[Megamox]

Yikes!! I think ill try an inductive/mechanical approach, nothing's worth going to jail for

 

[Nigel Goodwin]

Yikes!! I think ill try an inductive/mechanical approach, nothing's worth going to jail for

It's perhaps changed since the advent of independent telecomms companies?, but who knows?.

 

[chemelec]

All the Circuit I see presented here Draw Current and are Easily Detected by your Phone Company. 2 meg isn't High enough.
Once, I used one that used 20 Meg Resistance and it was Detected by them, here in Canada.

About the only kind of circuit that Is NOT Detectable is one that is Capacitave Coupled. The Phone lines are Full fo capacitance from wire to wire.

There are ways to do this. Sorry I don't have a circuit handy at the moment.

 

[Megamox]

I think i know why the circuit isnt working. From my measurements i noticed that if the input voltage was about 7v, the end of the zener was about 1v when it should have turned off the base of the transistor. Looking at the characteristic of the zener diode it shows that its reverse bias voltage is only maintained when a minimum current is flowing through it. So as the input voltage lowered, the end of the zener connected to the base would fall and turn off the transistor, this would gradually stop current through the zener, and its characteristic shows at less and less current its reverse voltage decreases. So this would mean the end of the zener voltage would start to rise and it would turn the transistor on again so that current would again flow, which would make the zener voltage again try to fall to maintain its zener value and try to turn off the transistor again etc etc. I had to take the input voltage down to about 5v to actually turn off the zener!!

This is the only way i can think to explain the measurements. Or am i completely wrong.

Megamox

 

[Nigel Goodwin]

I think i know why the circuit isnt working. From my measurements i noticed that if the input voltage was about 7v, the end of the zener was about 1v when it should have turned off the base of the transistor. Looking at the characteristic of the zener diode it shows that its reverse bias voltage is only maintained when a minimum current is flowing through it. So as the input voltage lowered, the end of the zener connected to the base would fall and turn off the transistor, this would gradually stop current through the zener, and its characteristic shows at less and less current its reverse voltage decreases. So this would mean the end of the zener voltage would start to rise and it would turn the transistor on again so that current would again flow, which would make the zener voltage again try to fall to maintain its zener value and try to turn off the transistor again etc etc. I had to take the input voltage down to about 5v to actually turn off the zener!!

This is the only way i can think to explain the measurements. Or am i completely wrong.

Sounds OK to me, zeners aren't a good choice for doing that. Bear in mind you're looking for a HUGE voltage drop, not just a volt or two. A simple potential divider would be enough to sense it.

As I recall, it's quite common to use an opto-coupler to sense the phone being off hook?.

 

[Megamox]

Just made an adjustment and added the divider. I dont see any problems with it, simulates okay too.
But yeah i think perhaps an LDR under the cradle would be much easier, lift up the phone, light triggers the LDR and much less hassle

Megamox

 

[Megamox]

Tested the circuit above, works fine
The 22n stops ringing from being counted, so whenever any handset is lifted the counter clocks up the count.
But am using an LDR now, just looks a little messy with wires from each phone trailing along the floor to the counter, oh well!

Megamox

 

[audioguru]

An LED/phototransistor optocoupler with a full-wave bridge rectifier and ringing filter placed in series with one telephone line wire where it branches to all phones would detect the current used by a phone when it goes off-hook. That's how telcos and PBX's do it. :lol:

 

[Megamox]

Ah so in series, whenever a phone is lifted, the circuit is completed and the opto coupler is activated. But this is still illegal to do right?

Megamox

 

[audioguru]

But this is still illegal to do right?

They won't detect it and I won't tell. :lol: