raoul@eplunix.UUCP (Ignacio Nico Garcia) (06/24/88)
There is some question as to just how tough it is to build a phone, or a phone system on your own. If I had 50 research engineers and the resources of a multi-national company like Western Union ( I *think* they bought the first patent rights), it wouldn't take very long at all. There are even lots of published circuits today that do the job. What is amazing is that they built the first system with only resistors, capacitors, relays, and transformers. No diodes. No transistors. No IC's whatsoever. The damn things are so durable you can crack nuts with the old ones. In terms of building a system for one's own house, I worked out the circuits for a system at my student living group. I had designs that included intercom, PA, a phone line that was always on the ringing line, and all the bells and whistles I could think of. Then I thought of the reliability and maintainability of a home-built customized system and just upgraded our phone service instead. Essentially, home or custom designers can do weird things to their circuits and get away with it. The phone company, however, had to pay for all repairs and complaints with their own time and money, and could save lots of money by manufacturing lots of unkillable units instead of marginally cheaper but unreliable widgets. Look at the phones on the market now. The solidest ones are the old Ma Bell models that Ma Bell had the service contracts for. Who says monopolies are always bad things? nico
knt@cbnews.ATT.COM (kirk.n.trost) (04/05/89)
Does anyone have a simple circuit for pulse dialing a telephone without using a relay (i.e. non-mechanical)? I want to be able to control the pulsing via a TTL signal from a microprocessor. Thanks, Kirk ...
larry@kitty.UUCP (Larry Lippman) (04/06/89)
In article <5417@cbnews.ATT.COM>, knt@cbnews.ATT.COM (kirk.n.trost) writes: > Does anyone have a simple circuit for pulse dialing a telephone > without using a relay (i.e. non-mechanical)? > I want to be able to control the pulsing via a TTL signal from > a microprocessor. For all intents and purposes, a pulse-dialing circuit is ALSO the circuit which determines whether the associated telephone, modem or whatever is on-hook or off-hook. Your request is interesting because, offhand, I cannot recall ever seeing a solid-state dialing circuit except in a self-contained telephone. Every piece of externally-powered equipment that I can recall used a relay for this purpose, with the relay type being either reed, mercury-wetted reed or conventional "low-mass" contacts. Mind you, there is no reason why a solid-state circuit cannot be used; it's just that several design criteria need to be met in order for the circuit to be reliable. While these design criteria are non trivial, they are not overly difficult, either. Most designers seem to say the hell with it, and use a relay. I must admit that I, too, fall in this category and use a relay - even though I explicity understand the required design criteria and am capable of designing such a circuit with minimum effort. I am assuming that you wish to have a dial pulsing circuit that is COMBINED with the on-hook/off-hook control function. The design criteria for a solid-state circuit is the same for both functions, and it would seem contrary to your requirements to say, use a relay for the on-hook/off-hook control. So, to answer your question, the following design criteria need to be met for a solid-state dialing circuit (not necessarily in order of importance): 1. The portion of the circuit which actually connects to the telephone line must be optically isolated from the TTL control line and its power supply. In my opinion, there is just no way around this requirement that will protect BOTH the telephone line AND the dialing apparatus from electrical fault. 2. The switching device is a suitably-selected transistor. I would consider an NPN silicon transistor with an hfe of at least 100 at 150 mA; you want this transistor to saturate quickly and conduct with a minimum of base current. The Vceo MUST be at least 200 volts. The collector current should be rated at least 250 mA to eliminate having to use a heat sink. You are going to switch a current that varies between 25 mA on a very long CO loop to 125 mA if you are sitting right next to a PABX. While a typical current may be more of a mean between these two values, you need to consider the whole range for a reliable, conservative circuit design. The Vceo rating is particularly important. While the DC on-hook voltage of a telephone circuit is typically between 48 and 52 volts, there are four circumstances which can result in the switching of MUCH higher voltages. First, an AC voltage as high as 120 volts @ 20 to 30 Hz _superimposed_ upon 48 to 52 volts DC can be present during the ringing cycle. If the designed circuit is idle this transistor must withstand these voltages without getting upset and conducting through breakdown. Second, during dialing there may be inductive transients on the line as a result of the inductive reactance present in bridged mechanical ringers; these inductive transients can be as high as 120 volts. Third, inductively and capacitatively coupled transients may exist on the telephone line as a result of lightning; these transients can exceed 200 volts DC. Fourth, long central office loops may have loop extenders which can result in on-hook voltages as high as 100 volts DC instead the usual 48 to 52 volts DC. You can and perhaps should protect the transistor with a metal oxide varistor (MOV) or a zener diode having a voltage rating of say, 180 volts DC. The use of a MOV or zener is NOT a substitute for a Vceo of less than 200 volts. Under no circumstances do you want any false conduction of the transistor or its protective MOV or zener at a voltage less than 150 volts since this may cause false ring tripping. While I have suggested a silicon NPN power transistor, you could consider a darlington device, a power MOSFET or a VMOS power device. 3. The polarity of the telephone line voltage presented to the transistor above needs to be assured through the use of a full wave bridge rectifier circuit (i.e., telephone line in series with AC leads of bridge). 4. The effective voltage drop across the dialing circuit should be less than 5 volts. You are going to lose 2 volts across the bridge, so the transistor cannot have a collector-emitter drop of more than 3 volts with applied base current in the on state. 5. The collector-emitter of an optoisolator will switch the base current to turn on the switching transistor. BEWARE that in many circuit designs the optoisolator Vceo rating must be as high as the switching transistor itself. This optoisolator will be driven by the TTL signal through a suitable driver capable of providing 20 mA or so through the LED. This optoisolator provides the requisite electrical isolation between the TTL circuit and the telephone line. 6. Assuming an effective 60% break +/- 2% for the dial pulsing function at a dialing rate of 10 pps, the effective switching ON and OFF times of the dialing circuit should be well under 2 milliseconds. Your microprocessor or whatever will be required therefore to open the circuit for 60 ms followed by a 40 ms closure for one dial pulse; this gives a 60% break. 7. The dialing circuit must also pass the voice frequency signal on the telephone line when it is in an off-hook state. For a decent circuit design, the AC insertion loss between 300 and 3,000 Hz should not exceed 0.2 dB. The traditional way to assure this is to bridge a capacitor across the dialing circuit. HOWEVER, this requires some careful circuit design since you do not want to create an excessive transistor collector current by discharging a capacitor each time you go off-hook or dial pulse; a series resistor may be in order. Also, the presence of too much capacitance will result in effective dial pulse distortion. This area can be the tricky part... Following the above design criteria will result in a good, reliable and conservative circuit design. As I said in the beginning of this article, the design effort is non-trivial, but not that difficult, either. Most of these problems can be readily solved in one fell swoop using a relay, which is why most people say screw it and do it that way. :-) <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231, 716/773-1700 {att|hplabs|utzoo}!/ <> FAX: 716/741-9635, 716/773-2488 "Have you hugged your cat today?"
gs940971@longs.LANCE.ColoState.Edu (glen sunada f84) (04/18/89)
In article <5417@cbnews.ATT.COM>, knt@cbnews.ATT.COM (kirk.n.trost) writes: > Does anyone have a simple circuit for pulse dialing a telephone > without using a relay (i.e. non-mechanical)? > I want to be able to control the pulsing via a TTL signal from > a microprocessor. > > > Thanks, > > Kirk ...