Last update: 16th Sept 2001

LINESIM Schematic Diagram

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Now you know what it does, here's how it does it. 

Telephone Line Simulator with Dialtone, Busy Tone & Ring Signal

Overview of Component Function

The 555 multivibrator IC1 is set to generate a frequency of 102.4 kHz, which clocks cascaded binary counters IC2 and 3. Several of the counter's outputs provide the frequencies and time periods required. IC4 is comprised of four R/S latches, each of which in either their set or reset states, store the four events: Dialling period, Line Busy, Call Ringing & Call Answered. Op-amps IC5A & B each detect on / off hook conditions at each line socket. The outputs of IC5A and B are each connected to the inputs of a pair of monostables, one of which is triggered by a positive pulse, the other by a negative pulse. The output pulses of the monostables, signal the occurrence of off-to-on hook and on-to-off hook transitions at either socket. IC5C detects call answering by monitoring the ringing current. Resistors R3 and R4 each supply current to one of the two line sockets. Coupling capacitor C2 provides the talk path between the two sockets, while allowing their DC voltage levels to be monitored individually.

Block Diagram

LineSim Block Diagram

Functional Description

For the purposes of this description, the term 'telephone' refers to any device designed to use a telephone line.

When either connected device goes off hook, the output of its associated voltage detector does the following:

  • Clears the 'Both ON' reset signal applying to counters IC2 & IC3, thereby allowing them to run.
  • Generates a positive pulse via IC8/C, which sets the dial period latch, whose output switches the dialtone through to the simulated line.
  • Selects the appropriate relay for later use.

Initialising A Call

The modem software in use, may be set to dial an arbitrary number, although this is not necessary. Only TONE dialling should be used. It is OK for dialling to overrun the dialtone period. The unit does not detect dialling, as this is unnecessary.

Ring Signal

To signal the end of the dial period, an output from counter IC3 pin 2 resets the dial latch IC4/B. If no busy condition exists, IC 7/A resets the Ring latch whose output energises the selected ringing relay and enables the Ring Frequency, Ring Break and Voltage Multiplier signals. Ringing continues until either the call is answered by a device at the other socket, or the caller hangs up.

Answer Detection

If the call is answered, op-amp IC 5/C senses across R22 the increased ringing current, sending its output high. This output sets the Ring latch, disabling the ring signal generator and de-energising the associated relay, thereby restoring the line connection. The Answered latch (IC4D) is reset, disabling all frequency generation for the duration of the call.

Busy Condition

Whenever both telephones are off-hook simultaneously, the Busy latch is set, preventing any subsequent ringing until both telephones are hung up. In this state, when the dial period ends, the 400 Hz tone is allowed to persist, interrupted by the Busy-Break signal from IC3/7. This produces a busy tone recognisable by modems and other automatic dialling equipment.

When a call is ended by one of the telephones hanging up, a positive pulse is sent to IC4/D pin 14, setting the Answered latch, which re-enables the astable IC1. In this state, the busy latch remains set; therefore the other telephone receives a dialtone followed by a busy tone. This arrangement prevents a new call being initialised until both telephones are placed on-hook together.

Ringing Tone

A simulated ringing tone is provided to the caller, derived from the dialtone and pulsed at the ringing frequency. Diodes D2 and D3 attenuate the ringing tone relative to the levels of the dial and busy tones. This is done to prevent modems from interpreting the ringing tone as a 'disconnected' or 'unavailable' tone, which would lead to premature disconnection.

Ring Signal Generator.

The voltage required to simulate a ringing signal, is generated by a Cockroft-Walton capacitor-based voltage multiplying circuit. The circuit is pulsed from the lsb output of the first binary counter, via an npn/pnp push pull buffer circuit. Driving the buffer from the counter's output, ensures a 1/1 mark-space ratio, essential for this type of multiplier. The ring signal is typically around 45 volts, sufficient to meet the 40 volt minimum for automatic answering equipment, without exceeding the 50 volt threshold considered safe for electronic projects.

Operational Limitations

  • This unit is not designed to simulate the characteristics of long transmission lines.
  • This unit is not intended for use as a long-distance intercom system.
  • When used with this unit, illuminated telephones may not light up as brightly as usual.
  • When first connected to this unit, electronic telephones may take a few seconds to begin working.