Fullscreen HTML5Med Res (??MB)HTML4

“DO NOT DISTURB” TELEPHONE TIMER Al Bell invented one of the most useful – and one of the most frustrating – communications devices of all time. What do you do when you don’t want the ’phone to disturb you? T here are times when you simply don’t want to be disturbed by anything, let alone a telephone – especially by that incessant “ring ring… ring ring… ring ring…” Of course, the person at the other end of the line doesn’t know that you are, well, umm, let’s just say you’re otherwise engaged. But you can imagine such times, can’t you? What do you do? Take the phone off the hook? Sure, that works – until a couple of hours/days later you start to wonder why no-one’s ringing you. What you need is either something to remind you to put the phone handset back on the hook – or better still, do it for you so the process is automatic. And that is exactly what the SILICON CHIP “Do Not Disturb” Timer does. Even better, it does it without you by JOHN CLARKE lifting the handset in the first place: you simply set the time period you’re going to be “busy” – whatever that is – and press the start button. The phone then effectively goes off-hook (ie, anyone ringing will get an engaged signal) until the time period is com- plete. The phone is then restored to its “normal” (ie, on-hook and waiting for calls) state. As far as time periods go, you have everything from a sprint to a marathon – 7.5 minutes to a whopping two hours. And if the reason for your not wanting to be disturbed ends prematurely you can hit the “hang up” button at any time. You can even make the offhook time period indefinite by not selecting a time period. If that sort of sounds like you’re defeating the purpose for making the device in the first place, it’s a great little security feature if you don’t want anyone else to use the phone while you’re away from July 2001  43 PLEASE NOTE This Telephone Timer is NOT an Austel-approved devi ce. The penalty for using a no napproved device, if dete cted and subsequent prosecut ion took place, could be a he avy fine, up to $10,000. This almost-larger-than-life photograph shows the completed project. The leads emerging top and bottom plug into the phone and wall socket. It doesn’t matter which way around they go. phone but that is unimportant as far as we are concerned. The timer works by connecting a load across the telephone line to simulate an off-hook condition. This off-hook condition means that there is a nominal 20-25mA drawn from the line. The timer starts and maintains the off-hook condition until the end of the time period or until the “end” switch is pressed. At this time it removes the load across the line, fooling the exchange into believing that the handset has been put back on the phone, readying it for a call. The circuit it – especially if the little box is hid50VDC to around 3-6VDC. The voltage The full circuit for the Do-Notden! drops because of the load provided Disturb Timer is shown in Fig.1. It by the telephone. The voltage drop Speaking of little box, the timer is comprises a 4060 counter IC, a 7555 (or more accurately the significant housed in a small plastic case with a timer IC, a LED, two transistors and a increase in current) is detected at 6P6C telephone socket located at each few diodes, capacitors and resistors. the telephone exchange and so the end. It has a 4-position DIP switch and Diodes D1-D4 provide full wave rectwo tiny pushbutton switches poking tification for the telephone through the box top – one to lines, necessary since the start the timer and one to stop polarity of the line voltage it manually. A LED indicates is indeterminate. y lit faci er tim sh ni Fi d when the phone is off-hook. Power for IC1 & IC2 is an t ar St  us at st k Installation is simple: you oo derived via the rectified f-h of s ow LED indicator sh just unplug the telephone line  telephone line voltage via socket from the phone, plug it  a 220kΩ resistor feeding Powered from phone line ite into the timer, then plug in a zener diode ZD1. This fin de in Four time periods plus modular lead from the timer to  develops a nominal 5.6V the phone. across the supply pins of In case you’re feeling a sense IC1 & IC2, smoothed by telephone is recognised as being of deja-vu, yes, we have described a the 10µF capacitor. off-hook, in anticipation of receiving similar device before – back in July The trigger input to IC2 (pin 2) is either the tones or pulses required to 1992, in fact. But that used an LM3909 initially held high via the 470kΩ redial a number. timer and alas, those devices are no sistor connected to the positive supply If the phone is simply left off-hook more. Hence this new, improved rail, while the threshold input (pin without another number being dialled, model! 6) is initially held low via the 100kΩ the telephone exchange will still recresistor to 0V. This sets the output at How does it work? ognise the telephone as being off-hook pin 3 low. or engaged and prevent incoming When you lift the handset on a NPN transistor Q1 is therefore off calls. After a certain time it will autotele-phone, the voltage across the and so is PNP transistor Q2. The matically send “engaged” tones to the tele-phone line drops from a nominal collector of Q1 is held at the rectified Features 44  Silicon Chip voltage from the phone line via the 2.2kΩ and 1kΩ resistors. This voltage also pulls the reset input of IC1 (pin 12) high via a 1MΩ resistor to reset this counter. Diode D5 prevents this input going above the IC supply rail. Pressing the start switch (S6) pulls the trigger input of IC2 (pin 2) low, setting the output (pin 3) high. Q1 is then switched on via the 10kΩ base resistor. In turn, transistor Q2 is switched on via the base current flow through the 1kΩ resistor and Q1. Transistor Q2 connects two 180Ω resistors in series across the phone line (via the D1-D4 bridge rectifier) to simulate the offhook loading. The voltage across the telephone line now drops to around 6V. Supply is maintained to IC1 and IC2 via diode D10 at the collector of Q2. The indicator LED1 will now be lit via the 1kΩ dropping resistor to indicate the off-hook condition. At the same time, the IC1 reset (pin 12) is released because of the low voltage at Q1’s collector. Components at pins 9, 10 & 11 of IC1 form an oscillator with the internal inverters operating at a nominal 0.88Hz rate. IC1 is a counter – each of its outputs, Q10, Q12, Q13 and Q14, goes high when the IC counts a certain number of pulses from the oscillator. In the case of Q10, it goes high when the count reaches 512 pulses. So if the oscillator is running at 0.88Hz, the output goes high after about 7.5 minutes. Similarly, Q12 counts 2048 pulses, or about 30 minutes, Q13 4096, or about 60 minutes and Q14 8192, or about 120 minutes. In case you were wondering what happened to Q11, 1024 pulses/15 minutes, the answer is that the chip does the count but there are insufficient pins on the IC to bring out all the counter outputs, hence we don’t get 15 minutes! If switch S1 is closed, this high will pull pin 6 of IC2 high to reset it. Pin 3 then goes low, switching off Q1, Q2 and the off-hook loading resistors. As far as the exchange is concerned, the telephone is now back on hook. Fig.1 (right): the circuit diagram shows the simplicity of the design: just one counter IC, one timer IC and a handful of other components. July 2001  45 Here’s the completed PC board out of its case, with the component overlay (Fig.2, right) printed at the same 1:1 scale. What you cannot see here is the switches and LED mounted high off the board to poke through the front panel. For longer times, IC1’s Q12, Q13 & Q14 outputs can also be selected to give timeout periods of thirty minutes, 1 hour or 2 hours respectively. These are chosen with switches S2S4 respectively. Note that if more than one switch is closed, it will be the lowest-time switch which will determine the timeout period, since its output will go high first. The others will be ignored. The “end” switch, S5, can be used to manually (and immediately) end the time period. It pulls IC2’s threshold input (pin 6) high regardless of the outputs of IC1, returning the circuit back to “on-hook”. High voltage transistors are specified to minimise the possibility of breakdown when the phone rings. The ring voltage can be 100Vp-p above the 50VDC line voltage. These transistors are rated at 250V, which will be adequate for preventing breakdown. The terminals for the incoming phone line are labelled as the tip and ring. These names are a throwback to the days when phones actually used 6.5mm phone plugs – the names are used these days to label which of the two wires is nominally “hot”. The two sockets are paralleled, allowing a phone connected to the socket to directly plug into the line, as if the timer were not there. Because the Timer takes the mini-scule amount of power it needs from the telephone line, no battery or other supply is required. This makes it extremely safe. Construction All components for the Timer mount on a PC board coded 12107011 and measuring 50 x 79mm. It all fits inside a plastic case which measures 83 x 54 x 31mm. A 50 x 77mm label is glued to the top of the case. Begin construction by checking the PC board for shorts and possible breaks in the copper tracks. The four corners of the PC board need to be cut to shape Fig.3: same-size artwork for the PC board. At right is the empty case, clearly showing the cutouts required for the phone sockets and the “surgery” done to the internal guides. 46  Silicon Chip to clear the integral pillars in the case. The outline is shown on the copper side of the PC board. You will also need to drill holes for the integral mounting pins on the 6P6C sockets so that they clip in correctly to the PC board. The Altronics type of socket differs slightly to the one sold by Jaycar and so we have provided both hole positions for the mounting pins. The plastic case has integral side clips which will need to be removed with a sharp knife or chisel so that the PC board will slide into the case. Check that the PC board fits into the case without fouling. Insert the resistors in the appropriate place in the PC board using the accompanying resistor colour code table to select the right values. Alternatively, you can use a multimeter to measure the values directly. As the diodes are inserted, make sure the orientation is correct; likewise the transistors, LED, ICs and electrolyt- case and cut out the holes with a sharp knife. Testing Connect the telephone to one socket using a 6P2C (or 6P4C or 6P6C) extension lead and the telephone line into the other socket. You can test the unit by pressing the start switch and checking that the LED lights. You can also measure the voltage between pins 16 & 8 of IC1 and pins 4 and 1 of IC2. This voltage should be be around 5.6V as set by ZD1. Press the finish switch to check that Parts List – "Do Not Disturb" Timer 1 PC board coded 12107011, 50 x 79mm 1 panel label, 50 x 77mm 1 plastic case, 83 x 54 x 31mm 1 4-way DIP switch (S1-S4) 2 snap action PC board momentary closed switches (S5,S6) 2 4-way pin header 8 PC stakes 2 6P6C PC board mounting modular sockets 1 6P2C (or 6P4C or 6P6C) extension lead And here’s how it all goes together inside the case. There’s not much room to spare; in fact you’ll have to cut the corners off the PC board to allow it to fit around the corner pillars in the box. The case side guides also need to be removed. ic capacitors are also polarised. Note that Q1 and Q2 are oriented differently to each other. The electrolytic capacitor requires positioning with the positive lead where indicated. LED1 mounts with its top dome 19mm above the PC board, oriented with the cathode toward the edge of the PC board. The 6P6C sockets can be installed now, followed by the switches. Switches S5 & S6 must be oriented with the “flat” as shown. To be at the correct height, these are mounted on top of PC stakes which are cut down so that the top of the switch is 18mm above the PC board. Switches S1-S4 are mounted on a 2 x 4-way pin header so that its height is sufficient to protrude through the front panel. Place the PC board assembly in position over the case and mark out the cutout positions for the sockets. We cut the box with a fine toothed hacksaw and broke off the piece with pliers. The cutout then was filed to shape. Only cut the hole to the depth of the socket on each end of the box. Test the PC board for fit into the case and adjust any of the cutout sides accordingly. The lid will require a hole for the LED, the two switches and the DIP switch. Use the front panel label as a guide to positioning these. Also the flanges on the lid directly above the sockets will need to be filed flat so that the lid will sit flush on the case. Glue the front panel label to the Semiconductors 1 5mm high brightness red LED (LED1) 1 4060 binary counter (IC1) 1 7555 timer (IC2) 1 BF469 NPN high voltage transistor (Q1) 1 BF470 PNP high voltage transistor (Q2) 4 1N4004 1A 400V diodes (D1D4) 6 1N4148, 1N914 switching diodes (D5-D10) 1 5.6V 1W zener diode (ZD1) Capacitors 1 10µF 16VW PC electrolytic 1 0.47µF MKT polyester (code 474 or 470n) Resistors (0.25W 1%) 2 1MΩ 1 470kΩ 1 220kΩ 2 100kΩ 1 10kΩ 1 2.2kΩ 2 1kΩ 2 180Ω 0.5W July 2001  47 the LED goes off. Set the DIP switch to 7.5 minutes and press start again and check that the LED goes out after about 7-8 minutes. The time is only nominal and will vary depending on component tolerances and the particular 4060 IC. You can make the time periods shorter by changing the 0.47µF capacitor to a smaller value. Longer time periods can be achieved by changing the 0.47µF capacitor to a larger value. A bipolar electrolytic capacitor can be used in place of the MKT type but do not use a standard polarised electrolytic. A final check can be made by lifting the telephone handset and listening for the dial tone. This tone should last for about eight seconds, after which the tone will change to an engaged signal. Now try this with the handset back in place. Press the start switch on the timer and wait for, say, 10-15 seconds. Lift the telephone handset and check if there is engaged signal. If it is engaged you can be sure that the timer has caused the telephone to be off hook. SC S RESISTOR COLOUR CODE 5-Band Code (1%) 4-Band Code (1%) brown   No. Value brown black black yellow ck green brown bla wn bro Ω brown 1M nge ❑ 2 yellow violet black ora yellow brown let vio low yel kΩ wn ❑   1 470 red red black orange bro yellow brown brown nge ora ❑   1 220kΩ red red ck bla brown black black yellow brown wn bro red ❑   2 100kΩ brown ck brown black bla brown black orange brown ❑   1 10kΩ red red black brown brown wn bro red red red kΩ brown ❑   1 2.2 brown black black brown wn bro red ck bla wn bro wn ❑   2 1kΩ brown grey black black bro brown grey brown brown ❑   2 180Ω 48  Silicon Chip Fig.4: 1:1 artwork for the front panel. Use a photocopy as a template to drill the holes required. (This panel and the PC board artwork can also be downloaded from the SILICON CHIP website: www.siliconchip.com.au).