Fullscreen HTML5Med Res (??MB)HTML4

PICAXE APPLICATION SPECIAL PICAXE Telephone Intercom/Interface Here’s a very commonly requested circuit: something to link two telephones together so they actually work! It works with just about any modern (ie, touchtone-type) phone – and will even work with cordless phones or a combination of cordless/corded models. S omehow, just about every hobbyist has managed to score a phone or two for his/her junk box. None of them are ex-rental, of course – they have just somehow materialised. But wouldn’t it be nice if you could actually do something with them – like make them talk to each other? Here’s a simple circuit which does exactly that. It’s based on our new best friend, the PICAXE-08, which means it is dirt cheap and easy to build. When either phone is picked up, the other one will ring until it is answered or the other phone is hung up. And the really good news is that your calls don’t cost you a cent. Whether you build this as a toy for kids (that’s what the circuit was originally designed for), as an intercom between rooms or even buildings of your home, business, factory, whatever; serious or flippant . . . this will do it. Its range will depend to some degree on the cabling you use but could easily be a couple of hundred metres or so. And as we said before, you can even use cordless phones (or one of each) and make a fully wireless intercom. Incidentally, phones make really great intercoms in noisy areas because the earpiece is so close to the ear – and you can yell into the mouthpiece to get the message across! Design by David Lincoln so all we have to do is simulate the phone exchange, right? That’s pretty much how this circuit works. And we don’t need a big building full of electronics! Fig.1 shows a block diagram of what is required. A line interface does exactly what its name suggests: interfaces each phone to the line. That means we need two of them. We also need a transmission bridge, which connects the speech signals together. A controller determines when one phone is taken “off hook” (ie, the receiver is lifted) and so rings the other phone. Finally, a power supply provides all the voltages necessary to make the whole thing work. Fig.2 shows the circuit diagram, again broken up into its functional blocks. First of all, we will look at one of the line interfaces. These may look a little complicated at first but they are really quite simple. It works like this: when relay RLY1 is at rest (normally closed), power is supplied to the line via the two 330Ω Connecting two phones Unfortunately it’s not simply a matter of plugging one phone into t’other and expecting them to work. Phones rely on signals, voltages, etc from the telephone exchange – 18  Silicon Chip Fig.1: the Telephone Intercom in block diagram. You can compare these blocks to the circuits in Figs.2 & 3. www.siliconchip.com.au resistors. With a supply of around 30V, this means line current will be around 35mA (depending on the line resistance and type of phone). When RL1 pulls in, the normally open contacts close and “ring current” flows in the phone line, causing the phone to ring. What causes RL1 to pull in? The circuit detects an “off hook” condition by measuring the voltage across R2 (a 330Ω resistor). Zero volts across R2 means there is no current flowing, therefore the other phone is on-hook. When the phone is off-hook there will be up to 12V across R2 (again, depending on the line resistance and type of phone). The circuit around the base of Q1 performs the off-hook detection. R4, R5 and R6 form a voltage divider network across R2. Their combined resistance is high enough to have no effect on the line current. C1, in conjunction with R4 and R5, filters out any AC components which may be present in the line current. The lower half of the voltage divider, R5 and R6, turn Q1 on and off. When line current flows, the voltage at their junction is high enough to fully turn Q1 on. The voltage at Q1’s collector is compatible with TTL levels and is fed directly into the PICAXE, which is programmed to detect this as “off hook”. Q2 and its associated 10kΩ base resistor (R8) form a circuit which will operate RLY1 when there is a TTL-level signal at the “ring” terminal of the line interface. Obviously, this signal also comes from the PICAXE. D1 protects the transistor from the back-EMF generated when RLY1 releases. The speech signal appears directly across the speech terminals of the line interface and is connected to the other line interface via the transmission bridge (two 2µF capacitors). These capacitors block DC while allowing the speech signal (AC) to pass. Some of the speech signal will be lost due to R2 and R3. In a simple circuit such as this, with reasonably short distances between phones, there should be no problem, with enough speech signal left over to “drive” the other phone. The controller The PICAXE-08 microcontroller is programmed to read the the status of the off-hook signals from each phone and Fig.2: everything except the power supply. The two line interfaces are identical; the green labels are explained in the text. Components/connections marked * are only required while programming the PICAXE-08. www.siliconchip.com.au June 2003  19 Parts List - PICAXE Telephone Intercom/Interface Interface Unit Semiconductors 1 PICAXE-08, programmed 4 BC548 NPN transistors 4 1N4001 1A power diodes (IC1) (Q1,Q2) (D1, D2) Resistors (0.5W, 1%) 2 47kΩ 4 33kΩ 1 22kΩ 3 10kΩ 2 470Ω 1W 4 330Ω 1W Capacitors 2 2.2µF electrolytic (C1) 2 2µF non-polarised Miscellaneous 2 12V relays, SPDT contacts (RLY1, RLY2) * Components of second (identical) interface not numbered on circuit diagram then use logic to provide ring signals to the phones. We are not going to re-invent wheels by telling you how to program your PICAXE-08; that’s the purpose of Stan Swan’s “Fun With PICAXE” series which has been running in SILICON CHIP since February of this year. Suffice to say that the 10kΩ and 22kΩ resistors are only required while programming (in fact, they may well be part of your programming setup) and can be removed once programming is accomplished. We imagine that anyone building this project will program the PICAXE out of circuit. Software for the PICAXE is pretty straightforward – it is shown in a separate panel. The power supply The power supply circuit is shown separately in Fig.3. It supplies ring current, around 30V DC, 12V DC and 5V DC. Power Supply Semiconductors 1 BD139 NPN power transistor 1 7812 12V positive regulator 1 7805 5V positive regulator 2 1N4001 1A diodes (Q1) (REG1) (REG2) (D1, D2) Capacitors 1 470µF 35V electrolytic 1 1000µF 35V electrolytic 1 1000µF 50V electrolytic 1 100µF 50V electrolytic 4 100nF monolithic (C1) (C2) (C3) (C4) (C5-8) Resistors 1 1kΩ Miscellaneous 1 240V to 30V, 20-30VA transformer 1 12V AC 1A plugpack The output from a 12V AC plugpack is rectified by a voltage doubler circuit consisting of the two 1N4001 diodes, C1, C2 and C3. This produces around 32V (give or take, depending on the regulation of your plugpack) across C3, the main reservoir capacitor. This capacitor also provides a low impedance return path for the ring current. Half of the power supply output feeds a 12V regulator (REG1) then a 5V regulator (REG2) to give the +12V and +5V rails required by the line interfaces and controller. The 100nF capacitors around the regulators should be monolithic type; they bypass the supply lines to help prevent parasitic oscillations in the regulators. Q1 and its associated components form an active filter for the 30V supply. Any ripple across C3 would be heard as a very annoying hum in the phone earpieces and the filter reduces that hum to inaudible levels. The ring current is supplied by a 240V to 30V mains transformer connected backwards. This transformer needs to be rated at between 20 and 30VA. With a 12V input on the 30V winding, the output will be about 100V – enough to give you a nasty bite. So keep your fingers away from the ring current circuitry! The voltage will of course drop under load. Construction Fig.3: the power supply provides three DC outputs as well as the ring current. It is designed to operate from a 12V AC plugpack (12V DC will not work!). Transformer T1 is a small 240V:30V model used backwards. 20  Silicon Chip No PC board is provided for this project, the original being lashed together. Some readers may like to go the trouble of designing a PC board – it would make for a neater project. Regardless of which physical method is used, construcwww.siliconchip.com.au tion proceeds as would any project – smallest components first, polarised components, semiconductors then “hardware”. Don’t mount the PICAXE-08 yet – however, an IC socket is a good idea. Resistors R1 and R2 can get hot, so they should be mounted a few millimetres above any board to allow air circulation. It is NOT a good idea to use standard phone sockets or jacks for this project. Obviously it cannot be connected to the public phone network (not only will it not work, it’s illegal!) so to avoid the possibility of someone doing this by mistake, we suggest some other form of 2-pin plug and socket to connect this circuit to your phone lines. You will of course need a standard (modular-type) plug to connect to the majority of phones. Testing Ensure there are neither phones nor microcontroller plugged in. Apply power from the 12V AC plugpack (note that it must be AC, not DC) and measure the voltages out from your power supply. The main DC supply should be around 30-32V or so – the exact value is not critical and will vary a little depending on the mains voltage and the quality of your plugpack. The 12V and 5V supplies should be pretty-well spot on, as they are coming from regulators. Using an AC range on your multimeter, measure the ring voltage (the output from the transformer). It should be about 100V (and remember, it can bite a bit!). If all voltages are OK, disconnect power and wait until the capacitors have discharged. Plug in the PICAXE (assuming you have programmed it already!) and both phones. Reconnect power. When you lift one phone the other phone should ring. When it does, pick it up and verify that the ring stops. If the ring continues, it will be heard as a very loud buzzing noise in the earpiece (don’t put it against your ear as it could be quite painful!). If all is well and the ring has stopped, check that you can talk into one phone and be heard in the other. Duplicate the testing for the other phone. If all checks out, well done! Aw shucks! It doesn’t work! If the power supply voltages are not as they should be, check your wiring and component placement. There is very little else that could be wrong with the power supply. If a phone doesn’t ring, first check both line circuits. Use a multimeter to check the voltage between the phone terminals with the phone disconnected – it should be nearly the same as the DC output of the power supply. If that’s OK, measure the voltage across R2 with the phone on-hook and off-hook. It should be zero on-hook and around 6-12V off-hook. Repeat the voltage checks, on-hook and off-hook, at the collector of Q1. This time it should be about 5V and less than 0.5V respectively. To test the ring, disconnect power and wait for the capacitors to discharge. Remove the microcontroller, then reconnect power with both phones connected. Using a jumper lead, temporarily connect the ring terminal for each line to +5V. Verify that the relay operates and the phone rings. www.siliconchip.com.au Telephone Intercom - PICAX-08 Code main: let b0 = 0 loop: if pin4 = 1 and pin3 = 1 then atrest if pin4 = 1 and pin3 = 0 and b0 = 0 then ring1 if pin4 = 0 and pin3 = 1 and b0 = 0 then ring2 if pin4 = 0 and pin3 = 0 then clearing goto loop atrest: low 1 low 2 let b0 = 0 goto loop ring1: high 1 goto loop ring2: high 2 goto loop clearing: low 1 low 2 let b0 = 1 goto loop If all else fails, try substituting another phone or two (use phones that are known to be working). Note that older phones equipped with mechanical bells may not work properly with this circuit. Your phone lines Note our comments before about NOT connecting this system to the public phone system. Because it is a fully private system, you can use virtually any 2-wire cable between the phones. Phone wire is an obvious choice but you could use speaker wire, bell wire, even two strands of fence wire if they are on insulated posts! (Well, at least when the posts are dry!). And if you’re in the bush its probably a good idea to keep your phone wiring well away from any electric fence wiring ! The circuit should work with lines up to several hundred metres in length (depending on the type of wire used and most particularly the resistance). In fact, the line resistance will be the main factor in determining distance. Standard 0.5mm phone wire should be OK up to say a couple of hundred metres; longer runs may need thicker wire. And while there are regulations which don’t allow you to connect this to the mate’s place next door (ie, over the boundaries of your property), we could never condone your breaking those rules . . . SC June 2003  21