Alpha

Folks, While browsing through my collection of Link circuits online the other day, I noticed an ommission of the Ring Trip cct on the Link A2B+1 DTMF design. Shown below the main cct is the RT cct, composed of the relay RTR/2 a diode and the AC voltage source (between 30 and 40 VAC is usually OK for local intercom ccts,) and the relay contacts. What should also be there, is a 2.2uF/250V capacitor (a large greencap/browncap or suitably voltaged MKT cap. This is because the relay itself will only trigger and operate on a DC circuit (such as a called party's handset going 'off-hook') while passing AC, and not operating. The 2u2 cap ensures that only AC current will bypass the relay coil, giving a more stable ring trip circuit which only operates when the relay coil senses DC current. Austin Hellier Downunder

Folks, the Link P design has been finalised and those of you who are waiting in telephone land can be assured that it will work (I made it work reliably only last week!) and once again follows the simple design philosophy of simplicity and "off the shelf" components - no special chips or fancy and expensive relays. The Link L will take a little longer to produce, as I always like to write my explanatory text while building and observing a working model. So when the Link P has been stripped off of the SK-10 panels, the Link L will take its place and go through the same test out procedure to ensuare that it will work reliably for those who need to build it. Austin Hellier Downunder

Folks, the transformer depicted in the diagrams is a simple audio style transformer, with a 1K centre tapped winding (for the phone handset side of the cct) and an 8 ohm winding, which is used to couple dial tone, ring tone and busy tone (where it's used) from IC1's output pin 5, to the caller. The pin number on IC1 where D3 is connected is pin 4 - pins 3 and 11 aren't shown either, and they form the voltage control pins for each half of the NE 556 dual timer chip. You can connect 0.1uF greencaps from each pin to ground for a smoother tone output and more stability in the circuit, especially if your power supply arrangements are a little rough. Connecting the buzzers inside older pulse dial handsets shouldn't be too much of a problem. You can use 2 sided tape to stick them to the inside of the base of each phone handset, and sonnect them via the phone's switch hooks. You may need to check the handset circuit diagram, or alternatively, trace it out on the pc board, to see where the AC ringer's blocking capacitor is. You will need to bypass it (use a jumper wire) as the DC signal from the Link's ringer circuitry won't pass trhough the blocking cap. Hope this answers all current questions for now. Austin Hellier Downunder

Hi folks, It's been a while since I last posted to all you hyperboreans out there in telephone land, but nevertheless, I've been tinkering (read 'Linkering') at my workbench and I've got 2 new link designs to share with you some time soon. The first one has been designated the "Link P" and the P stands for privacy. With the addition of a second bank of four relays (eg: for the Link 4+0 version) and some optocouplers, transistors etc., you can have total privacy on your Link around the house DTMF or pulse dialing intercoms. The ciruit allows for the first call to be established in total privacy, and if a second would be caller lifts their handset, they are returned 'busy tone' indicating that the Link P already has a call in progress. The second design, called the "Link L" is designed for those of you who may have only one 4 wire telco cable running through your entire house/property, and it may well be that this cable is either inaccessible, or it may not be practical to install additional cables for extra handsets, without pulling half the roof tiles off, or jack hammering hald the slab foundation up. While the Link L uses party line for its internal conversations (as a matter of simplicity, there's no privacy on internal or external calls) it works well in either pulse or tone mode, and can still be effectively connected to an outside telco line (providing that this is street legal in your locale!!!) Will upload circuits and text ASAP (my PC is off the air at the mo, but I can use op's until it's fixed) so keep your eyes peeled for these two new additions. Austin Hellier Downunder (but not out) 8) email: [email protected] 8)

Folks, terribly sorry to not have answered any of you for so long, but I was away from my normal surroundings for quite some time (not in jail or anything) but to answer a couple - no I don't know where you can get the MC 45436P IC from in the US or Canada. You can mail order them from DSE or Jaycar from Oz though if you're really stuck. As for the Link 4+0, its the same line circuit repeated 4 times, and just about any DTMF decoder chip will do, as long as it puts out a 1 of 16 code, and a flag or strobe for active outputs. The 1 of 16 decoder IC is a CD 4514B in all my DTMF designs, and its clo0ck runs from either a 3.57Mhz or 4.43Mhz crystal, in // with a 10MR resistor. Austin Hellier

Just use it the way I've described it to you up above in the first response to your question. I can't see why the transformer coupling won't work, whether it's the MT 8870 or the MC 45436 chip, or it's a transceiver (both ways) device. You may have to identify a pin on the chip which alternates between transmit and receive, in order to enable both functions (eg: this pin may have to be taken high to +5 volts for the chip to transmit, and then be taken low to 0 volts - for it to receive - something like that.) Look on the data sheet for an 'enable' pin (sometimes called the CE or chip enable pin.) Alpha

Find yourself a good tutorial on the NE 55 timer chip on the Net (there's a few good ones around). Basically, the 555 chip in its 'monostable' mode has what you are looking for. Pin 2, held to +ve rail via a 4.7k resistor, is switched momentarily to ground to turn the timer on. Then pin 4, which is held to ground by a 4.7k resistor is used to turn the timer off manually (use two momentary on push button switches for this task) but will time out all by itself, according to the R/C combination connected between +ve rail (resistor) and pins 6 & 7 and then from those same two pins to ground (capacitor). A good tutorial will show several wired up versions, including this one, and may even show a table of correct R/C combinations, to get close to the time out that you want, without all those messy calculations. Alpha

Have a look at The Link Telephone Intercom (posted as project number 13 under 'Telephone Related') this may give some clues as to the basics of telephony - it also uses a couple of opto couplers - the closest I could get to an optical PBX using off the shelf parts :-) Alpha

Have a look on the Net for the MT 8870A DTMF Transceiver by Mitel Corp. Look for a data and applications info sheet, this whould help yo a lot. Alpha

Hi there, connecting a DTMF device to an active phone line can be tricky. For starters, you've got arond 50 volts DC on the line when it's idle (phone is 'on hook') then you've got around 80 to 110 volts AC when the phone is ringing (interrupted) and around 5 to 12 volts when the phone is picked up 'off hook' to make or answer a call. Many DTMF decoder chips (eg: the MC 145436P by Motorola) uses only 5 volts DC to power it up. So your device must have at least one low voltage DC supply independent of the phone line for it to work safely (no house fires from bodgy circuitry!) and reliably. You can try connecting the DTMF chip via a 600 ohm/600 ohm transformer specially designed for phone line interfacing. One side of this transformer (known as the 'primary winding') will conect via 2 capacitors (around 0.22uf at 400 volts DC each - usually mains grade power filter caps set in epoxy, often called 'greencaps' or by the more modern named 'MKT' version). The other side of the transformer winding (known as the secondary winding) an be connected to the input of the DTMF chip via a smaller capacitor (a 0.1uF/100 volt will do) and your circuit ground connection. It is also advisable to position 2 'back to back' diodes across the secondary, to combat the possibility of high voltage ring signals from damaging your DTMF decoder chip. There are a number of similar interface circuits around the Net, so have a good look. Epanorama is a good place to begin your search. Austin Hellier Downunder

Folks, I noticed that there's been some 86 members look at the Link design, but so far, no replies. Can someone, anyone email in and tell me what you think? I'm always open to suggestions as to better ways of doing things. I'm also wondering if anyone's actually built one up? even if it's just on your SK-10 breadboard panels. Any takers? Austin Hellier Downunder

Some extra notes for those attempting to build the Link intercom: 1. Best to build it up on your SK-10 panels to ensure that all parts are working, before you go soldering them to a board. 2. Adjust R6 by making a call, then taking the called party's handset off hook - adjust R6 until the ring tone in your earpiece has halted. 3. Transformer must be 1K/8R type - 500R types won't do. 4. Some 556 timer chips don't fully switch their outputs off. 5. All phone handsets must be the same - all rotary dial or all pushbutton dial. 6. Use 9 volt buzzers as 12 volt ones aren't reliable due to the current drain pulling the supply voltage below thier operating voltage. regards Austin Hellier Project Link: http://www.electronics-lab.com/projects/telephone/013/index.html