How to enable pulse dialling in a telephone ?

[reubenn]

I want to enable pulse dialling in a project that uses a landline telephone and a microcontroller.
1>Should i use a dedicated IC ? Or will a relay switch do ?

2>If i use pulse dialling technique, which relay should i use? A reed relay?

3>How many lines are actually there in a telephone line connection?
How should the relay be connected so that it can be controlled by the micocontroller ?

4> I found this project on the net :
**broken link removed**

Here, the author has used a dry reed relay.But doesn't a phone line have 4 wires internally? How is the relay connected ?

 

[FvM]

But doesn't a phone line have 4 wires internally?

No, just two. The shown circuit can basically work, but is misses a RC contact protection circuit. A device that is required to comply with telecom regulations would need additional functions as overvoltage protection, line current sense, dial tone detector etc.

 

[reubenn]

1> Isn't the diode across the reed relay providing sufficient protection ?
If not, what should be the additional circuitry involved for over voltage protection ?

2> Also, i didn't understand the modem-transformer part properly.How is it generating a 300 Hz tone ? Does the pin 9 have an oscillating output?
I tried simulating that part of the circuit in Proteus (circuit is in the attached zip, dont know whether the circuit is right). All i got was a steady dc supply at the o/p of the circuit.(Please correct the circuit if it is wrong).

3> Also, shouldn't a 300 Hz tone be generated everytime the reed relay is activated?
It's in series with the phone line, right?How is it that only after the number is dialled that the 300 Hz tone is generated? Some activity at the pin 9 again ?

 

[mvs sarma]

I want to enable pulse dialling in a project that uses a landline telephone and a microcontroller.
1>Should i use a dedicated IC ? Or will a relay switch do ?

2>If i use pulse dialling technique, which relay should i use? A reed relay?

3>How many lines are actually there in a telephone line connection?
How should the relay be connected so that it can be controlled by the micocontroller ?

4> I found this project on the net :
**broken link removed**

Here, the author has used a dry reed relay.But doesn't a phone line have 4 wires internally? How is the relay connected ?

most phone instruments have a shorting link, that selects pulase or tone, as we see in Indian context.
perhaps world over this is maintained. please check whichip is used for dialing out a number. once you see the data sheet of the concerned chip down loadable from www.alldatasheet.com , you get to know how the chip manages pulse.

in the phone PCB you get the P/T shorting link. once you remove that it falls back to pulse mode. even now, you can go to tone mode by initially pressing "*" followed by the number.
All the best.

Sarma

 

[FvM]

You are asking very basic things regarding telephone operation. I'm sure, that there are lots of tutorials on the internet. Cause I learned these things as a young school boy, I'm not aware of the best today's information.

The 300 Hz tone is an application specific signal of the said alarm device. It's apparently generated by the PIC software.

P.S.: The diode is connected to the relay coil. How should it protect the contact?

 

[mvs sarma]

You are asking very basic things regarding telephone operation. I'm sure, that there are lots of tutorials on the internet. Cause I learned these things as a young school boy, I'm not aware of the best today's information.

The 300 Hz tone is an application specific signal of the said alarm device. It's apparently generated by the PIC software.

P.S.: The diode is connected to the relay coil. How should it protect the contact?

Contact protection might need an RC series snubber across it, with 100Ohms and 0.1uf /100V disc cap

 

[reubenn]

i have a few queries :
1>Should i found out phone line current ? Is it safe to do so ? Current in this can be measured with a multimeter ?
2>How do i decide the reed relay rating ?
3> Is contact protection essential ?
Please help me regarding these doubts

 

[mvs sarma]

i have a few queries :
1>Should i found out phone line current ? Is it safe to do so ? Current in this can be measured with a multimeter ?
2>How do i decide the reed relay rating ?
3> Is contact protection essential ?
Please help me regarding these doubts

1. NOT needed as it would be a working phone.

2. any reed relay would work. you only go by the operating voltage for the coil.
If you need to operate using a NPN transistor with relay coil in its collector(with a diode in opposite direction across the coil) and other side of coil connected to 12V input voltage.
you can use 12V reed relay with a Normally Open(NO) contact. or use a change over contact and only use the NO portion. this would help you not to pump spikes into the 5V regulated section.

3. Contact protection helps to use the relay for long times instead of changing very frequently.

Sarma