LINESIM Construction Notes
Construction & Housing
The unit may be constructed either as a portable boxed unit, or as an internal PC card. There is no functional difference between the two options.Advantages of construction methods:
Encased Unit
Can be used and transported independently of computer.
Does not require computer to be running when used with non-computer related devices.
Easier to show off to friends.
Does not need a separate power supply
Requires no casing
Occupies no space
Always ready for use with the computer
LINESIM Built on Prototype Card Ready for Insertion
Circuit Board
The component layout is left to the experience of the constructor.
For the encased version, I recommend construction on a single piece of DIP board. After testing, the circuit board can be mounted inside a suitable project box.
For the PC card version, refer to using a PC prototyping card. There is little difference in cost between the two methods, as the price of a prototyping card is offset by a boxed unit's need both for a suitable enclosure, and the components required to construct an independent power supply: transformer, rectifier, smoothing capacitor, 12V regulator and heatsink.
Test Gear
At minimum, you will need a multimeter. A logic probe will make debugging a lot easier. It is necessary to set the frequency of one oscillator. If a frequency counter or oscilloscope is not available, there is a work around mentioned in the testing section.
Power Supply Options
The unit must be powered by a 12v regulated and smoothed power supply with an output current of at least 200mA. If building an internal version, a suitable power supply is still required for testing purposes. A CB radio type power supply is ideal for this purpose. If building a boxed unit, you may wish to build the power supply internally. If your chosen case has no space for a transformer, you could use a suitably rated plug top a.c. adapter, with the main casing containing the remaining components of the power unit.
Encased Unit
Virtually any suitably sized project box is suitable as a casing. The only holes required are for the power lead and the wires to the socket doubler. The doubler is best mounted externally on the side of the case. Alternatively, by adapting the technique described here, you could flush mount the line sockets inside the case. Replace the phrase 'blanking plate' with 'casing'.Line Socket
Whichever style of unit is produced, two telephone sockets are required. These are most easily provided by a modified socket doubler. The following applies specifically to UK type sockets. If producing a unit for use in another country, you must adapt the information to suit. The modification consists of electrically separating the joined pins of the two sockets in the doubler, then wiring these pins to the correct points in the circuit. There are several designs of socket doubler on the market, some more suitable for conversion than others. The most suitable are those in which the sockets are linked using a small PCB. Doublers with sockets aligned as in A in the diagram, are more likely to be of this type.
Numbering of socket pins
For the purposes of this article, the pin numbering convention is as follows:
Looking into the socket with the pins uppermost, from the left, the pins are numbered:
If there are only four pins in each socket, the rightmost pin is 2.
Doubler Modifications
All Units
Remove the screw(s) and open the unit. Working on the socket's PCB, de-solder the wires coming from the plug. Break the plug from the body by bending it until it snaps away, then pull it clear along with its wires. Do not discard the plug. Removal of the plug will leave a hole in the rear of the casing through which cable or wires will exit. Examining the socket's PCB, there will be four or six tracks joining the pins of each socket. Cut all these tracks at a convenient point (see diagram).
Doubler to be used on a boxed unit only.
Modifications to Provide Surface Mounted Sockets
Prepare six 20cm lengths of thin flex and fashion them into three separate twisted pairs. Solder one pair to pins 2 & 5 of one socket and repeat with another pair on the other socket. Note: The third pair of wires is used to connect the bell capacitors CB1 & 2, and may be omitted in countries that do not use this system. Take the third pair and connect one wire to pin 3 of one of the sockets and the other wire to pin 3 of the other socket. To prevent confusion later, make a note of the connections of all wire pairs, then run them all through the hole in the rear half of the socket casing. Re-assemble the doubler ready for mounting to the side of the casing of the finished unit.
Flush Mounted Sockets
An alternative to using a modified socket doubler to provide surface mounted sockets, is simply to remove the sockets from the doubler and mount them directly onto the edge of your unit's circuit board, with aligned access holes cut into the side of the casing. Electrical connections are made to the mounting pins using the above illustrated pin numbering scheme.Doubler to be used on a PC card unit only.
To make the connecting flylead, take at least a one metre length of six core alarm / signalling cable. Solder a separate wire from the cable to pins 2, 3 & 5 of each socket in the doubler. Make a written note of to which pin and socket each of the six wires is connected, then run the cable through the hole in the rear of the doubler. Place a small plastic tie wrap around the outer insulation of the cable inside the doubler. This will act as a strain relief. Re-assemble the doubler. At the other end of the lead, connect each wire to one of the pins of a 9-way male 'D' connector. Make a note of the pin connections used, as you will need the information to wire correctly the female connector on the card.
Access Cable for Internal Version Using UK Sockets.