Telephone Remote Control Kit

The brains of the switcher is the Atmel AT89C2051 microcontroller, IC4. Incoming ring is detected via C4, R7 and the opto-coupler IC2 and connected to pin 6 of the microcontroller. This pin also has the Password Reset switch connected to it (more on this later). The incoming call is answered by connecting the circuit based around Q1 and Q2 (an electronic holding coil) to the line via IC1, a bi-directional opto-isolator. This circuit has a low DC resistance but a high AC impedance which is required by telephone circuits when the line is looped. The RC network consisting of R4, R5 and C3 is used to provide impedance matching to the telephone line. The metal oxide varistor protects the switcher from telephone line transients.

The configuration and values used here are optimized for the Australian telephone system but they should work in all other phone systems. They may be changed to suit other telephone systems if it is desired to get official approval of this device. DTMF detection and decoding is provided by IC3. This chip, an 8870, is a complete DTMF receiver which is able to detect and decode all 16 DTMF tone pairs into a 4-bit code. When a valid DTMF digit is detected the 4-bit code is placed on pins 11-14 and a data available output, pin 15, is set to a logic high.

The output relays, RL1-4, are controlled via the relay driver IC6. One output from the relay driver (pin 14) is used to output a 800Hz software generated tone into the telephone line via the impedance matching network. This tone is used to signal the user when commands have been completed or of any command errors.

User settings are stored in IC5, a 24C01 EEPROM. The microcontroller talks to this device using a 2-wire I2C bus via pins 18 and 19. IC7 provides two functions  a regulated 5V power supply output and detecting when the phone line is looped (in use). This chip is a Maxim MAX666 low power programmable voltage regulator with on-chip low battery detection, mainly intended for use in battery powered equipment. The regulator output voltage is set via the VSET input (pin 6). By tying this pin to ground the output is fixed at 5 volts. We have used the low battery input to monitor the state of the phone line. An idle (not in use) phone line normally has about 50VDC across its pair of wires. This voltage drops to less than 20V when the line is in use. The MAX666 Low Battery Input (LBI, pin 3) is one side of a simple voltage comparator which compares this input to an internal 1.30V reference voltage. When the voltage at the LBI input falls below the reference voltage then the Low Battery Output (LBO, pin 7) will go high. The phone line is connected to the comparator input via the diode bridge B2 and resistors R15 and R16. The diode bridge makes sure that the voltage polarity connected to LBI is the same regardless of which way around the phone wires are connected. The resistors form a voltage divider which is used to set the low battery threshold at LBI. For the values used the LBO output will go high when the phone line voltage drops below 25 volts, indicating that the line is in use (looped).

PRE-TESTING

Insert IC7 (MAX666) and connect a 9-12V power supply to the DC input jack. Use a multimeter to check the 5V supply. Easiest place to measure this is across pins 20 (+) and 10 (-) of the IC4 socket. If all is well then remove power and insert the rest of the ICs into their sockets. Check that the IC legs are actually inserted into the socket and not accidentally bent up underneath the IC body (quite a common fault).

Before you can use the kit for the first time the various user settings must be set to their default values. To do this, press AND hold the 'Password Reset' button while powering up the kit. Hold the button down for a further 2 seconds before releasing it. This procedure is also used you user forget the password.

The default value for each setting is:

Password ................ 0000

Tamper ................... 3

Rings to answer ...... 3

Auto hangup ........... 30 seconds

Lockout .................. Disabled

Each of these settings can be changed by the user to suit.

Connect the kit to the phone line using a suitable cable. Note that the kit uses the two inner pins on the RJ-11 plug. Some cables do not  check before using. Turn on the power. If using the kit for the first time follow the procedure described previously to set the user settings to their default values. The kit is now ready for incoming calls. Dial the phone number to which the kit is connected. After 3 rings it will answer with 4 short beeps. Type in the password (0000) followed by the '#' key. If the password is correct the kit will respond with 4 short beeps. It is now waiting for a command.

Entering a * at any time after the first character aborts

and starts again.

1. *<1-4># - operate one of the four relays 1,2 3 or 4.

2. #<1-4># - release one of the four relays 1,2,3 or 4.

3. *5# - set password

4. *6# - set tamper setting

5. *7# - set number of rings to answer

6. *8# - set auto hangup time

7. *9# - set lockout time

8. *0# - forced disconnect

9. #0# - release ALL relays

Commands 3-7 require additional data. After the command is entered the unit will issue a single, short beep as a prompt for the data. Enter the required data followed by # to finish. A description of the data to be entered is given in the section "USER SETTINGS".

The beeps are used to acknowledge commands, prompt for more data or indicate errors.

A "forced disconnect" command will cause 3 long beeps to be sent back to the user before the switcher hangs up.

These are stored in non-volatile memory (EEPROM) and are not affected by loss of power. None of these settings are defined initially - they have to be 'set' to their default values before you can use the kit (see "USING THE KIT FOR THE FIRST TIME").

From 4 to 8 digits long using any number from 0 to 9.

The number of attempts allowed at entering a valid password. A value of 0 disables this feature (unlimited). If the tamper setting is exceeded then the unit will automatically disconnect and go into lockout if enabled (see Lockout time).

Number of incoming rings required before the unit will automatically answer the call. A value of 0 disables this feature (does not automatically answer). Valid ring is defined as having an "off" time of more than 1 second between each burst of ring. So the North American single ring, and the European double ring are both seen as a single ring. If the caller hangs up BEFORE the number of rings to answer have been received then the unit will wait 10 seconds before 'resetting' itself to wait for the next call. When this feature is disabled the unit expects the the call to be answered 'externally', eg. by an answering machine. Everything else works as normal, including entering a valid password. The only difference is that it does not automatically answer the incoming call.

This is the length of time that the unit will wait for a VALID command. If this time is exceeded then the unit will automatically disconnect itself (hang up) from the call. A valid command includes entering a correct password at the start. A value of 0 disables this feature (no auto hangup).

This only comes into effect when the tamper setting has been exceeded on the PREVIOUS call. This is the length of time that the unit will wait AFTER disconnecting (hanging up) before it will answer the next call. A value of 00 disables this feature (no lockout). For example, suppose the tamper setting is 3 and the lockout time is set to 10. The unit receives a call and answers it. If a valid password is NOT entered in 3 attempts the unit will disconnect (hangup) and then wait for 10 minutes before it will answer any further calls. In other words, further calls a 'locked out' for 10 minutes. This security feature helps prevent multiple attempts at 'hacking' into the unit by introducing a delay time between calls being answered.

In this case you will need to follow the procedure in "USING THE KIT FOR THE FIRST TIME". This will reset the password to 0000. However it will also reset all the user settings to their default values. You will have to re-enter them as required.

1. Check that all components are in their correct place and the correct way around. Check for unsoldered joints and solder bridges or splashes.ﾠ

2. Is the 5V supply OK?

3. Any IC legs bent up under the IC body?

4. Is the phone line cable using the two inner pins on the RJ-11 plug?