8 Input Shift Registers and Pulse Dialing

On the Arduino I have only a limited amount of IO pins, 20 in all. This would probably be enough pins if I just wanted 4 inputs for call sensing, 2 for pulse and off hook detection and the rest to operate the relays but I want to add a DTMF transceiver and to connect to an outside line so I'm going to need quite a few more. Enter stage right the 74HC165 shift in register for the inputs. This has 8 inputs which will be configured as per the map below...

#define ShIn_L1_OH_Pin 128 // call sense line 1

#define ShIn_L2_OH_Pin 64  // line 2

#define ShIn_L3_OH_Pin 32

#define ShIn_L4_OH_Pin 16

#define ShIn_OH_Pin 8 // source phone off hook and used for the pulse dial train

#define Dest_Phone_OH_Pin 4 // call answered, ie destination phone off hook

//#define Call_Progress_Pin 2 // not used

#define External_Ring_Pin 1  // incoming call from outside exchange

#define SHIN_CLEAR 0  // used to check no inputs are active

With the shift register IC at the top right, below shows the proposed setup on a bread board so the design can be tested. Also laid out the board is the dial pulse detection circuitry. I have soldered in a 600 ohm isolation transformer so I can plumb in a phone and have taken 36v power from the temporary Dupont connector on the Veroboard. The optocoupler is an 4N26 and is will eventually be connected to one of the inputs on the shift register to feeding to the Arduino to convert the pulse train to a digital number when a destination line is dialed. 

The pulse detection circuitry as added to the Veroboard and operation confirmed by an LED that blinks to the pulse train as the number is dialed. The next step will be to connect it to the shift register input. There is quite an interesting explanation of capturing pulse dialing with an Arduino here, including some simple coding.


A close up shows the 1w 1k resistor and capacitors feeding the Zener diode and resistor necessary to produce the voltage drop needed to trigger the optocoupler. The other side of the isolation transformer  will eventually be used to feed in the call progress tones to the caller. The output from the exchange side of transformer will eventually feed into the source phone relay matrix. Where a track has been cut I find it useful to mark it on the other side of the Veroboard with an indelible pen. Still missing is the bridge/call sense relay but I have shorted the connection so I can take the outputs of the four way optocoupler into the shift in register and so check which line receiver has been lifted.

Comments

Post a Comment

Popular posts from this blog

12 Boxing it up

Image
So now I have a working exchange consisting of four lines, numbered 122,222,322 and 422. I made the numbers three digits just for the sake of it really. My first exchange used 11 as the post-fix so I used 2 this time just because. If the extensions are not dialed as above then it will be a false number and a fast busy tone is played to the handset. Here it is boxed up and mounted on the wall. The segment display is used to describe call progress but when the exchange is idling it displays random characters generated every second just to show it's still alive. The red button resets the Arduino and the two black lumps on top are the fuses for the DC feed and the AC ringing. Above it are the junction boxes and batteries used by the Plan 7. Removing the lid shows how the exchange fits in the box. The large single Veroboard shows the exchange itself, so from top to bottom, the powers supply caps and VR's, to the left the call sensing, in the middle the isolations transformers for th...
Read more

11 The Relays do the Exchanging

Image
All that's left to do now is add the relays that will connect each source to each destination phone . As I have four lines I will need a set of eight solid state relays arranged so that one of each of the four source lines can be linked to one of the four destinations line via the bridge relay. I've been using the  Panasonic AQW210EH  throughout with the exception of the ringing relay. As in the bottom left(ish) of the picture, I take four feeds from each of the phone lines and arrange two 18 pin sockets (empty in the picture) to hold the four eight pin relays. At the time the picture was taken I've added copper links to two of the relays so I can dial and link my two phones in either direction, either as source or destination. A more detailed picture shows more copper links added, so we have a relay between each phone and the source and destination lines, each running down a copper strip of the Veroboard, to the right of relays. Each relay will be actuated by a link...
Read more

1 The Beginning

Image
The telephone system has always fascinated me. A couple of my earliest memories about phones date back to the early 1970's, when I was about 9 or 10. My mother came from quite a large family and we used to travel up from London to Somerset a couple of times a year to visit them. On one such visit during a hot summer out roaming the countryside with one of my Uncles, who was maybe five or six years older than me, we came across a derelict coal mine. We ventured in and found what I guess must have been a few Series 300 Bakelite phones and decided to liberated them. Back at his home we took them apart and I remember seeing the governor wheel in the dial mechanism along with a set of driver cogs, fascinated by the way it rotated when the dial was turned. My memory is a bit hazy here but I have a recollection that my Uncle somehow connected two of the phones together with a battery and we talked to each other over the phones. Hours of fun. After a few days with my Mothers family we woul...
Read more