I am trying to understand an already existing circuit shown below taken from http://boris0.blogspot.fr/2013/09/rotary-dial-for-digital-age.html.

The aim of this circuit for the right part is to produce pwm signal, to filter it and to send dtmf signal back to phone line.

I was asking me if the purpose of the transistor is to adapt impedance of microcontroller output which is between 20 and 50 kOhms and the line which is around 600 Ohms ? Is it doing a kind of sum of this signal with DC of line ? Is it really working like emitter-follower ? (here we are not only taking Vout but Vout + Vce) How to choose R2 value ?



1 Answer 1


The Plain Old Telephone Service (POTS) telephone system signalled dialling by connecting together the two phone line wires with a series of pulses. The rotary dial was pulled round to the required digit and released to slowly trawl back to the rest position. As it trawled back, it generated a series of loading pulses across the phone line, representing one pulse per digit dialled. So dialling 9 would generate 9 pulses on the way back to the rest position. This was the case for decades, long before the tone-pulse scheme of DTMF came along.

Have a read of www.britishtelephones.com/howtele.htm, it'll go over this in more detail.

The wires had around 50 V across them when unloaded and the phone dialling pulses loaded them. Your BJT is there to connect such a load to a line voltage far higher than your microcontroller could take and draw a current it couldn't handle.

The microcontroller supply comes from the line, using that simple Zener regulator. It's large capacitor is intended to hold up the microcontroller's supply while the line is dragged down by the dialling pulses.

  • \$\begingroup\$ Thanks you for your answer. For the first part concerning pulse to tone and the one concerning zener diode I already know that is why I did not took so much time on it. What I do not really understand is how transistor works. Is it really working like emitter-follower ? It is not like common emitter-follower schemas. How to choose proper R2 value ? ... \$\endgroup\$ Mar 22, 2017 at 21:25

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