Some time ago my brother and I made a pulse to tone (DTMF) converter for my old GNT-A F68 telephone, but after I replaced my microphone from an old unreliable carbon type to a modern electronic type my pulse to tone converter stopped working.
The problem was that the 12F675 PIC I used needs a 5V power supply to run properly and I only had 4.5-4.7V in the best case. I then searched for a PIC that would operate at a lower supply voltage and found the 12F1501 which can run properly from 2.5V. The next task was to lower the power consumption when the pulse to tone converter is idle and waiting for you call a number because the power consumption of the pulse to tone converter put a load on the IP telephone line and dampen the speaking/listening sound level significantly. The 12F1501 has a clever solution for this problem as the internal processor clock can be lowered to 31Khz during idle state and raised to 8Mhz when processor calculation power is needed for generating DTMF tones. As a result, I now have a current consumption of 1-2mA in the operational state and 0,16mA in the idle state. Running on an internal clock also means that I can remove the crystal and the two accompanying capacitors. I then looked closer at the circuit and how others have implemented dial pulse to DTMF converters and I found that I was also able to simplify the circuit quite a lot, take a look at my schematic in the electronic files.
When I decided to try to update my old grey GNT-A F68 with a British GPO BT 21A microphone I experienced a problem as the GPO BT 21A use less current than the Danish Danavox 4900-00 version, this resulted in a higher line voltage (above 6 Volt) causing the protection 5,1 Volt Zener diode to limit voltage and the dial pulse to DTMF circuit stopped working.
This time the problem is much simpler to fix by adding a 78L02 ACLP 2,6 Volt voltage regulator, a 27 Volt protection Zener diode and a 100nF decoupling capacitor in front of the existing pulse to DTMF circuit.
As a bonus, the GPO BT 21A actually function better than the Danavox 4900-00 and is giving a higher sound level for the person you are having a (telephone) conversation with.
So the conclusion if you want to build this circuit is to start with the version with the 78L02 ACLP 2,6 Volt voltage regulator and short circuit the voltage regulator if dial pulse to DTMF circuit doesn't work.
Update: We have updated schematic, component list and PCB layout files. A good tip is to buy components for both above and below 5V telephoneline versions!
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