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Build this telephone off-hook timer How many times have you tried to have that afternoon snooze or that romantic dinner for two only to be interrupted by the telephone? This circuit takes the phone off the hook for you and then automatically replaces it after a preset time period. Block diagram Power for the circuit is derived directly from the incoming phone lines via on/off switch Sla and bridge rectifier D1-D4. This bridge rectifier ensures that the correct supply polarity is applied to the circuit, regardless of the polarity of the phone lines. When power is first applied, IC1 (the timer circuit) begins counting. About 5 seconds after this, it turns on the "Start-Up" SCR (SCR1) and this closes the line to simulate the phone being taken off hook. When IC1 reaches its preset time period, the timer latches and its output goes high and turns on SCR2. SCR1 an d SCR2 form a standard commutating circuit. When SCR2 is turned on, it steals current from SCR1 via capacitor Cl . This immediately turns SCR1 off, w hich places the phone back on-hook. SCR2 then turns off again a short time later since the current through R2 is insufficient to maintain conduction. The block diagram of Fig.1 shows how the circuit works in principle. Circuit details By DARREN YATES Phone calls in the middle of the night or right in the middle of dinner are annoying, particularly if they're the "sorry... wrong number" types. Most people use the age old solution to this well-known problem; they simply take the phone off the hook. The problem here though is that it is equally simple to forget to put the phone back on the hook. It can be quite a few hours later before you realise that it's been off. The Telephone Offhook Timer can save you from missing those important calls. By turning the circuit on, it automatically takes the phone off the hook for either 30 minutes, one hour or two hours, and then replaces it back "on the hook" again. The project is based on one common CMOS IC and a couple of silicon controlled rectifiers or SCRs, and fits neatly into the smallest zippy box. It's quite possible that you will already have most of the components lying in your junkbox. The Oflhook Timer has three preset time periods: 30 minutes, 1 hour & 2 hours. It automatically puts the phone back on hook at the end of the timing period. 64 SILICON CHIP Refer now to Fig.2 which shows all the circuit details. It's a bit more complicated than our block diagram but all the basic elements are easily recognised. At the heart of the circuit is IC1 which is a 14-stage ripple-carry binary counter. This IC carries a built-in oscillator circuit which is set to operate at approximately 1.3Hz by the RC components on pins 9, 10 & 11. Switch Slb selects between the Q12, Q13 and Q14 outputs of ICl, corresponding to the 30-minute, 1-hour and 2-hour time periods r_yspectively. Thus, when power is first applied, IC1 begins counting up. WhenQ4goes high, the pulse is .fed to the gate of SCR1 and triggers it on. This takes the phone off-hook and also lights LED 1 which is the off-hook indicator. ZD2 is there to ensure a constant supply voltage of 4.3V across the LED and its associated 220Q current limit- Fig.1: block diagram of the Oflhook Timer. Shortly after switch on, SCR1 turns on (to take the phone off hook) & timer IC1 starts counting. At the end of the timing period, SCR2 turns on & turns SCR1 off to place the phone back on hook. SCR2 then turns off again a short time later. 0-0 PHONE LINE START UP SCR1 TIMER IC1 voltage rail for ICl. Normally, with the phone off hook, this rail will be about 3.5V. Construction All the parts except for the switch and LED are mounted on a small PC board coded SC12106921 (61 x 42mm). Fig.3 shows the parts layout on the board. The parts can be installed in virtually any order but it's best to mount the smaller components first. Leave the SCRs and electrolytic capacitors till last and make sure that you don't confuse the two zener diodes. We suggest that you check each resistor with your multimeter before mounting it on the board, so that you can be sure of its value (see Table 1 for TURN OFF SCR2 OSCILLATOR HOLD 01 ing resistor of when SCR 1 turns on. When the selected output from IC1 (either Q12, Q13 or Q14) subsequently goes high, SCRZ turns on and turns SCR1 off as described previously. At the same time, transistor Ql turns on and pulls pin 10 ofICl low to stop the oscillator and latch the counter at its present count. When SCR1 turns off, the phone is effectively placed back on-hook but that's not where the circuit operation ends. What happens now is that the 2.ZµF bipolar capacitor (Cl on Fig.I) quickly charges via ZD2 in parallel with LED 1 and its 220n current limiting resistor. As soon as the capacitor charges, SCRZ turns off because its associated 330kQ resistor is too high to allow sufficient "holding" current. Fig.2: the final circuit uses transistor Ql to stop the counter when the preset count is reached. The counter is then reset simply by turning the power off & on again. Note that the minimum timing period can be decreased for testing purposes to less than 3 minutes by connecting a 330kQ resistor across the 3.3MQ timing resistor. LED 1 also turns off during this time to indicate that the phone is back on hook. The circuit is reset by simply switching it off (using Sla). Supply rails WARNING! When the phone is taken off hook, the line voltage drops from a nominal 50VDC to about 6.5V DC due to the resulting 35mA current loop. This value is about the same as the loop current that flows when a telephone is physically taken off hook. The supply rail for the SCRs is derived directly from the output of the bridge rectifier and thus varies depending on whether SCR1 is on or off. This rail is also decoupled using a 10kQ resistor and 22µF capacitor and applied to ZD1 to derive a more stable In order to comply with the Telecommunications Act, we draw the reader's attention to the following: "This customer equipment has no AUSTEL permit and may be dangerous or damage a telecommunications network. Connection to a telecommunications network is an offence under section 253 of the Telecommunications Act 1991 and may attract a maximum fine of $12,000". 2~ 1 a PHONE ft . . . LINE ~ OFF 330k + 22 35VW! 2.2 BP 16 Q4 7 """""'; ELJc VIEWED FROM BELOW A 10k 0.1 IC1 4060 KAG B 33k 014 3.3M 013 3 2 012 1 RESET 12 ,.. .,.B Slb TELEPHONE OFFHOOK TIMER JULY 1992 65 Fig.3: install the parts on the PC board as shown in this wiring diagram. Be careful not to confuse zener diodes ZD1 &ZD2. Take care with the wiring to the switch, as it's easy to make a mistake here. The SCRs are installed with their metal faces towards the 2.2µF bipolar capacitor. the colour codes). Take care with the orientation of all polarised components. The two SCRs must be mounted with their metal faces towards the centre of the board. Push them down onto the board as far as they will comfortably go before soldering their leads. Once the board has been completed, connect the switch and the LED using hookup wire. Be sure to connect the LED the right way around - the anode TELEPHONE OFFHOOK TIMER Fig.4: this is the full-size pattern for the PC board. OFFHOOK 0 OFF-~ - 30 MINS 1 HR - 2HR - Fig.5: this artwork can be used as a drilling template for the front panel. lead is the longer of the two (see Fig.2 for pin connections). The adhesive label can now be attached to the lid of the case and used as a drilling template for the front panel holes. These include a mounting hole for the LED plus two smaller RESISTOR COLOUR CODES 0 0 0 0 No. Value 4-Band Code (1%) 5-Band Code (1%) 1 1 4.7MQ 3.3MQ 330kQ 33kQ 22kQ 10kQ 2200 yellow violet green brown orange orange green brown orange orange yellow brown orange orange orange brown red red orange brown brown black orange brown red red brown brown yellow violet black yellow brown orange orange black yellow brown orange orange black orange brown orange orange black red brown red red black red brown brown black black red brown red red black black brown 0 66 0 2 0 0 2 1 SILICON CHIP holes to accept the switch mounting screws. The square cutout for the switch slider can be made by drilling a series of small holes inside the marked rectangle and then filing the job to a smooth finish. A small hole must also be drilled in one end of the box to admit the telephone cable. Tie a knot in the cable so that it cannot be pulled back through the hole, then terminate the two leads to the indicated points on the PC board. The other end of the cable is connected to pins 2 & 6 of a standard telephone plug. Operation Now that you've finished the construction, you can plug it in parallel with one of your existing phones via an adaptor socket. Now flick the switch to the 30 minute setting. Nothing will happen immediately. That is normal but after five seconds, when Q4's output goes high, the LED will come on. At the same time, you may hear a click from your phone. The phone is now "off hook" and no incoming calls will be received. If you now pick up the receiver, you should hear the engaged signal, confirming that the phone is indeed off hook. You will also notice that the LED dims when you lift the receiver. This is quite normal and it will continue to time out correctly when you replace the receiver. If you turn the unit on to the "2-hr" range, and then later decide you wanted only one hour's delay, you can switch the unit back to the "1-hr" setting provided you do so before one hour has passed since first switching it on. Similarly, if you first select the "1-hr" range and then decide on only 30 minutes, you must do it before 30 minutes has passed. Note also that if you turn the unit off and back on again quickly, you may not get the correct time you selected. If you wish to reset the timer, turn it off for at least five seconds and then switch it on to the desired range. Troubleshooting If it doesn't work, the first step is to check the wiring and the parts placement. In particular, check the wiring to switch Sl. If these checks reveal nothing, connect the unit to a 12-lSV DC power supply in series with a 5600 resistor, switch on and check that the output You can now afford a sate II ite TV system PARTS LIST 1 PC board, code SC12106921, 61 x 42mm 1 plastic jiffy box, 83 x 54 x 28mm 1 telephone plug and cable 1 double pole 4-position slide switch 1 5mm red LED For many years you have probably looked at satellite TV systems and thought "one day". You can now purchase the following K-band system for only: Semiconductors 1 4060 14-stage binary counter (IC1) 1 BC547 PNP transistor (01) 2 C106D SCRs (SCR1 ,SCR2) 1 5.1 V 1W zener diode (ZD1) 1 4.3V 1W zener diode (ZD2) 4 1N4004 silicon diodes (D1 -D4) *antenna, A 1.8-metre prime focus dish complete with all the Capacitors 1 22µF 35VW PC electrolytic 1 2.2µF 16VW bipolar electrolytic 1 0.1µF 63VW MKT polyester *or better). One super low-noise LNB (1.4dB *magnetic One Ku-band feedhorn and a signal polariser. Resistors (0.25W, 1%) 1 4. ?MQ 2 22kQ 1 3.3MQ 2 10kQ 1 330kQ 1 2200 1 33kQ * 30 Miscellaneous Insulated hookup wire, screws, solder etc Your receiver is pre-programmed to the popular AUSSAT transponders via the internal EEPROM memory. This unit is also suitable for C-band applications. of the bridge rectifier is about 5V. If you get 0V, check the diodes in the bridge rectifier and the wiring to Sla. Assuming tbat all is OK, plug the unit into the phone line and check that pin 16 ofICl is at about 4.5V with the LED off and about 3.5V with it on. If pin 16 is close to 0V but the output of the bridge rectifier is OK, check ZDl for incorrect orientation. If the supply rails are correct, LED 1 should turn on about 5 seconds after power is applied. If the LED fails to light, check the voltage across SCRl (ie, between anode and cathode). If the voltage is close to 0V, check the polarity of ZDZ and the LED. If the voltage is above 30V, check SCRl and its gate circuit components. Finally, if the circuit fails to switch off at the end of the preset period, check the wiring to Sl and the circuit around SCR2. You can speed up the circuit operation for checking by connecting a 330kQ resistor across the 3.3MQ timing resistor. SC $995.00 Here's what you get: mounting hardware. metres of low-loss coaxial cable with a single pair control line. *receiver lnfrared remote control satellite with selectable IF audio & bandwidth, polarity & digital readout. Call, fax or write to: AV-COMM PTY LTD PO BOX 386, NORTHBRIDGE NSW2063. Phone (02) 949 7417 Fax (02) 949 7095 All items are available separately. Ask about our C-band LNBs, NTSCto-PAL converters, video time date generators & Pay TV hardware. I I II ----------Name ... .. .. .......... .................... ... ....... I I II I Address..... .... ............. ..................... I I ........................... P/code ................ II Phone.............................. ... ............. I I I II YES GARRY, please send me more information on K-band satellite systems. .I ___________ .I ACN002174478 01 /92 JULY 1992 67