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Control equipment via the telephone line with this Have you ever thought of controlling equipment via the humble telephone line? This project allows you to do just that. It can control up to nine separate appliances and can be connected either directly to the telephone line or acoustically coupled via your answering machine. By DARREN YATES & GREIG SHERIDAN Picture this: you 've just arrived at work and remembered that you have forgotten to turn the house alarm on. Do you drive all the way home again to turn it on or do you take a punt and hope your house doesn't get robbed? Or what about this? You go out The DTMF Decoder is designed to work with commercial DTMF encoders like the "Pocket Tone Dialler" from Dick Smith Electronics. 30 SILICON CHIP somewhere and decide that you're going to come home late , but you haven't put the front verandah light on. Obviously, you're not going to drive home just to turn a light on. But will you be able to find the steps, the door and the keyhole without tripping over the cat in the dark? Now if you're really clever, you can use a little mental telepathy and your problems are solved. For us more terrestrial beings, telephony is the next best thing! If we could use the telephone system like a remote control , then it would be a simple case of ringing home , pressing a couple of buttons and the job would be done. Most phones are now connected to the ISDN network, so you could quite easily ring up from England and turn on your house alarm in Australia! This may well be an extraordinary example but it can be achieved easily with our new DTMF (Dual-Tone Multiple Frequency) decoder board. You can use it to control up to nine appliances at once. In addition to the decoder board , you need a DTMF encoder like the one pictured in this article. This features a 12-key kepad and looks rather like a small pocket calculator except that it lacks a display. Each time you press a key, the device generates a dual-tone output and this can be heard via a small loudspeaker. In use, you carry the encoder with you while the decoder sits at home , connected to your telephone line and the appliances you wish to control. To switch an appliance on, you first dial your home number and the decoder automatically "answers". After that, it's simply a matter of pressing a couple of keys on the keypad; ie, the corresponding key plus the Enter (#) key. Want to turn three appliances on (or off) at the same time? No problem - just press the three corresponding keys in turn and then the # key. Want to turn an appliance off again? - just press its key and the # key again. Up to nine appliances can be controlled in this manner. To make the circuit as versatile as possible, the decoder board features four toggle outputs (ie, outputs which turn on or off at each press of the button) plus four momentary outputs (ie, outputs which turn on for only a brief period). It's up to you how you use them to control your appliances. DTMF decoders The DTMF standard has been around for a number of years now and is used in all new phone systems. However, encoding and decoding these frequenci es has not always been an easy task. The DTMF system works like this: All the parts for the DTMF Decoder are mounted on a PC board. It can either be plugged directly into the telephone line in parallel with your existing phone or acoustically coupled to the line via a telephone answering machine. if we take a 16-key keypad like the one shown in Fig.1, we can split it into four rows and four columns. Each key ha~ a particular position in this 4 x 4 matrix. For example, key "8" corresponds to row 3 and column 2. Now a particular frequency is set aside for each row and for each coiumn. For the four columns (1-4), the frequencies are 1209Hz, 1336Hz, 1477Hz and 1633Hz. For the four COL 1 897 COL 2 COL 3 COL 4 0000 0 0 0 0 ROW 1 STO OTMF 770 (H,) 852 [?J[IJQJ0 ROW 3 941 [:]000 ROW 4 1209 1338 1477 ROW 2 1833 STD OTMF (HJ) Fig.1: the row & column arrangement for a standard 4 x 4 keypad. Each key generates a unique combination of frequencies when pressed. rows, the frequencies are: 697Hz, 770Hz, 852Hz and 941Hz. When each key is pressed, the corresponding row and column frequencies are added together and fed down the phone line. For example, if we press the "8" key, the resulting output will consist of two frequencies - 85 2Hz and 1336Hz - on top of each other. These days, all the necessary tones are generated by a single IC. In fact , we featured a DTMF encoder in Garry Cratt's Amateur Radio column in the September 1989 issue of SILICON CHIP. It produced the correct pairing of tones depending upon which key you pressed, and used a single Texas Instruments TCM5089N IC. There's no longer much incentive to build your own encoder, however not when you can now buy professional DTMF encoding units for around $25. These units include both the keyboard and a small loudspeaker, which you hold up to the microphone of your telephone receiver. We used an encoder from Dick Smith Electronics while developing this project and it worked extremely well. Decoding Generating the necessary dual tGJnes is one thing but decoding them from amongst the noise and other rubbish on the phone line is something else. To do this properly in the past meant using eight phase locked loop (PLL) tone decoders (one for each frequency), plus some noise reduction circuitry on the input - all of which meant the number of ICs required to do a decent job went through the roof! That problem was solved when Motorola came up with the MC145436 DTMF decoder IC. The internals of this IC, shown in Fig.2, contain everything we need to decode the tones into a 4-bit binary code. It includes mains frequency and dial tone rejection, as well as eight switched capacitor filters to decode each frequency. It also has the necessary decoders to produce a 4-bit binary number at the MAY1991 31 Fig.2: block diagram of the MC145436 DTMF receiver IC. It includes mains frequency and dial tone rejection circuitry in the front end plus eight switched capacitor filters to decode each frequency into a 4-bit binary code. Timing for the chip is supplied by an external, low-cost 3.579545MHz crystal. output (which corresponds to the key pressed). Note that although 16-key keypads can be used with the MC145436 , our project has been designed to use the more commonly available 12-key encoders. Block Diagram To get a better overall view of how the DTMF Decoder works, let's take a look at the block diagram in Fig.3. The DTMF signal is fed to the circuit either directly from the telephone line, using a suitable isolation transformer, or from an electret microphone placed near the loudspeaker of an answering machine. Switch Sl selects either of these inputs and feeds the signal to the MC145436 (IC3), where all the initial decoding is done. The output of this IC is, as previously mentioned, a 4-bit binary code which corresponds to the key that was pressed. This 4-bit code is fed into a 4-to-16 decoder, which produces a single high output for each key. Since most keypads have only 12 keys, and since we also need three of these keys for entering, clearing and resetting the decoder, we have nine possible output lines. These nine lines plus the master 32 SILICON CHIP reset line are then fed into a userselect matrix. This allows you to decide which keys drive which particular output. The output driver section consists of nine open-collector transistors. Four of these lines are toggle outputs; ie, pressing a key turns a particular output on and pressing it again turns the output off. The other five are momentary, ie, a particular output is on for as long as the enter key is held down. The output drivers can be used to drive relay coils or optocouplers for Triacs, for example - which ever you prefer. The keypad If you buy a standard keypad, it will have the digits 1-9 plus "*", 0 and " # " symbols. The "*" and "#" symbols are fixed but the 10 remaining keys are available for you to swap and change to different outputs. For example, you can make keys 1-4 the toggle outputs or you can make 2, 3, 5 and 8 the toggle outputs, but more about this later. Circuit diagram Let's take a look at the circuit diagram ofFig.4 and see how it all works. Starting at the input, the phone line is connected via relay contacts RL1 to a 600Q:600Q isolation transformer. This transformer not only prevents high voltage DC from entering the circuit but also prevents us from superimposing any DC on the telephone lines. The relay contacts are normally open to simulate the telephone "on hook" condition (ie, no connection). When the phone rings, an AC voltage of about 50V appears across the phone line, and this normally activates the ringer or bell on an ordinary telephone. In our case, it activates a relay to couple the phone line to the isolating transformer. A .015µF capacitor AC couples the ring signal to a full-wave bridge rectifier formed by diodes D1-D4. The resulting DC voltage from the rectifier charges a l0µF capacitor via a l00kQ resistor. This is used to set a delay time so that the phone rings a number of times before the circuit "answers" the phone. The associated 1MQ resistor discharges the capacitor when the call has been answered. Zener diode ZD1 also plays a role in setting the time delay before the circuit "answers". Because it is connected in series with the base of transistor Ql, it ensures that Ql cannot turn on until there is around +12.6V across the capacitor (ie, 12V for the zener diode and 0.6V for the diode drop at the base of Ql). Once +12.6V has been reached, Ql turns on and pulls pin 2 of IC1 low. IC1 is a 555 timer connected in a monostable role. When pin 2 is pulled low, the 555 is set and its output at pin 3 switches high and turns on relay RLY1. This closes the relay contacts to give the "answered" or "offhook" condition. The lO0µF capacitor on pins 6 & 7 now charges via the 220kQ resistor until, after about 20 seconds, it reaches 2/3rds of the supply voltage (ie, +8V). During this time, the output at pin 3 is high, which means that you only have about 20 seconds to send the desired tone (or tones) down the line. When the v;oltage across the lO0µF timing capacitor reaches 2/3Vcc , pin 3 ofICl switches low again and the relay contacts open to give the on-hook condition. In other words, the circuit automatically hangs up after 20 seconds. Diodes D5 and D6 at the output of the 555 are there to protect it from voltage spikes or a latch-up condition TELEPHONE INPUT ISOLATION & ANSWERING OTMF DECODER 4 TO 16 DECODING MEMORY REGISTER (10 USER SELECTABLE OUTPUTS) SEm~ED . __ MASTER ___. RESET OUTPUT MATRIX MICROPHONE INPUT FROM ANSWERING MACHINE FIVE MOMENTARY OUTPUT DRIVERS which can occur when driving an inductive load such as a relay. Acoustic pickup The other way of coupling the DTMF signal to the circuit is via the FET-input mic insert (ie, by acoustically coupling it to the loudspeaker of your answering machine). If this method is used, your answering machine answers the phone and hangs up afterwards. IC2 is an LF351 op amp and is connected as a simple non-inverting AC amplifier with a gain of about 48. Its job is to amplify the signal from the microphone to a usable level. Switch Sl is used to select between the two inputs. The signal is then clipped by signal diodes D7 and DB, and then fed into the MC145436 (IC3). As already mentioned, this IC decodes dual frequency tones and generates a corresponding 4-bit binary number. A 3.579MHz colour TV crys- tal is used to generate all the necessary clock signals within the IC, but these crystals are very common and are quite cheap. Depending upon the input signal, the 4-bit code appears at pins 2, 1, 14 and 13 and is then fed directly to the inputs of IC4, a 4514 CMOS 4-to-16 output decoder. Valid signal indication Pin 12 of IC3 goes high whenever a valid input signal is detected. This is buffered by NAND gates IC10a & IC10b, which are then used to drive transistor Q2 and turn on LED1 (ie, the LED stays on for as long as a key is held down). This LED also stays on for as long as you hold down the "#" key, which enters your selected output into the memory register. The output of IC10b is also used to enable IC4 (the 4-to-16 decoder) via pins 1 & 23. While these inputs remain low (ie, while ever a valid tone Fig.3: the heart of the circuit is the DTMF decoder. This decodes the tones coming down the telephone line (either directly or via your answering machine) & outputs a unique 4-bit code for each tone received. These various tones are then decoded & used to activate the output driver stages (four toggle, five momentary). is received), the selected output of IC4 will remain high. When the particular key is released, all outputs of IC4 go low. Each of the 10 outputs from IC4, corresponding to keys 0-9, are then fed to the Set inputs of 10 RS latches comprising IC5, IC6 and IC7. These are 4043 CMOS quad RS latches and form the memory register. They are also Tri-State devices, which means that their outputs can have three states: high, low or high-impedance. Code entry To enter in a particular code to control one or more of the outputs, you simply press those keys in sequence and then press the"#" (Enter) key. As you press the output-select keys (ie, keys 0-9), they are stored in the memory register (IC4-7) . You can select as many of the outputs as you like at any one time. When you press the "#" key, pin 14 ,, To send the tones down the line, you simply dial the number, then hold the encoder against the mouthpiece & press the appropriate buttons & the ENTER key. .· I --.2--,, ',a·~, This close-up view shows the user selectable matrix in one corner of the PC board. You can either use the matrix we used (as shown on the wiring diagram) or you caffwire the matrix outputs to suit yourself. You don't have to install links for all the outputs either but don' leave out the master reset link. MAY1991 33 4 pcs. 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Box Q103, SYDNEY 2000 FAX: (02) 261 8905 SAME DAY MAILORDER DESPATCH* Post and Packing $5 - $25 ........ $3.00 $26 - $50 ...... $5.00 $51 - $100 ...... $6.00 $101 - $499 .... $8.00 $SOO + ............ FREE ai:!&l~l!,g of IC4 goes high. This enables the latches (IC5-7) via pin 5 of each IC. The selected latch outputs now go high and turn on their respective output driver stages as required. The high output generated by pin 14 of IC4 when you press the "#" key is also fed to a small delay network formed by a 0. lµF capacitor and lkQ· resistor at pin 13 of NAND gate IClOd. When the "#" key is subsequently released, pin 14 of IC4 goes low, the outputs of the latches are disabled, and pin 13 of IClOd is momentarily pulled low as the O. lµF capacitor charges via the lkQ resistor. This in turn switches the output of IClOd momentarily high and resets the latches (ie, outputs are low). This is how the momentary outputs work. They are on whenever the outputs of the memory register are high and the "#" key is held down . To produce the toggle outputs, we have used two 4013 dual D-type f!ipflops (IC8 & IC9) which are, naturally enough, connected in toggle mode. The outputs from the memory register are connected to the clock inputs of these four f!ipflops. This is done so that selecting that output with the keypad alternatively turns the output on and off. Each of these clock inputs and the common reset line have lOkQ resistors tying them to ground when the corresponding outputs of the RS latches (IC5-7) revert to high-impedance mode. This prevents the 4013s from clocking due to noise, as can occur without these resistors in place. As mentioned previously, the output drivers are BC33 7 transistors with open-collector outputs. Each has a LED indicator to show whether it is on or off, as well as a reverse-voltage protection diode to protect the transistor against large negative-going spikes when its relay turns off. Power supply Although the circuit is powered by a single 12V DC plugpack, the board splits this up into three supply rails: a non-regulated +12V rail which is fed to the output driver section (as the supply voltage here is not overly critical); a regulated +12V rail which is connected to the audio input circuitry; and finally, a regulated +5V rail which supplies the logic circuitry. Diode D18 prevents the circuit from being damaged if the power supply is connected in reverse, while the 7812 and 7805 regulator ICs produce the +12V and +5V rails, respectively. The three lOOµF capacitors provide filtering for the supply rails. Fig.4 (right): when the the phone rings, an auto-answer circuit based on Q1 & IC1 closes relay RL1 so that the tones can be fed via S1 to tone decoder stage IC3. This IC generates a 4-bit binary number in response to each tone received and feeds it to a memory register based on ICs 4-7. The 9 decoded outputs are then used to drive five momentary output stages (Q7-Q11) & four toggle output stages (IC8 & IC9). Although no prov1s10n has been made for a power switch on the board, it is quite an easy matter to connect a small SPST power switch in series with the supply line. Construction All components except for the input audio switch and the microphone are mounted on a single-sided PC board coded SC12106911 and measuring 170 x 140mm. Before you begin soldering, check the board for any shorts or breaks in the tracks, particularly where the tracks run between the pins of the ICs. If you find any problems, either use a sharp knife to cut away the excess or add a touch of solder where PARTS LIST 36 1 plastic instrument case (optional, see text) 1 PC board, code SC 1206911 , 170 x 140mm 1 600Q:600Q telephone isolation transformer (Harbuch AT-251 or similar) 1 SPOT relay 1 SPST switch (for power on-off) 1 SPOT toggle switch (S1) 1 FET-input microphone insert 1 telephone plug and cable (Telecom approved) 1 3.579MHz TV colour crystal (X1) 3 4043 quad RS latches (IC5-7) 2 4013 dual O-type flipflops (IC8IC9) 1 4011 quad 2-input NANO gate (IC10) 2 BC548 NPN transistors (01,02) 9 BC337 NPN transistors (03011) 1 7812 12VOC regulator 1 7805 5VOC regulator 16 1N4004 power diodes (01 -06, 09-018) 2 1N914 signal diodes (07,08) 1 12V 1W zener diode (ZO 1) Semiconductors 1 NE555 timer (IC1) 1 LF351 FET input op amp (IC2) 1 MC145436 OTMF decoder (IC3) 145144-to-16 decoder (IC4) Capacitors 1 100µF 35VW PC electrolytic 1 100µF 25VW PC electrolytic 2 100µF 16VW PC electrolytics 1 10µF 16VW PC electrolytic 2 1µF 50VW PC electrolytics SILICON CHIP 6 0.1 µF 63VW (5mm lead pitch) polyester 1 .022µF 63VW (5mm lead pitch) polyester 1 .015µF 63VW (5mm lead pitch) polyester 1 .01 µF 63VW (5mm lead pitch) polyester 1 .0047µF 250VAC polyester (WIMA) Resistors (5%, 0.25W) 2 1MQ 11 3.3kQ 1 470kQ 1 2.2kQ 1 220kQ 9 1kQ 4 10OkQ 1 680Q 7 10kQ 6 470Q 1 4.7kQ Miscellaneous Tinned copper wire, hookup wire, cable ties, etc. ~ >-" 18 ~ ~ I .,. .,. 61 f 10k .,. .,. 5 + v 1 ::1 l 1 +5V 1 -- ,...,,, TOGGLE 1 + : 03 .BCJJ] ,_._, 1 100 35VW ,. 12voc PLUG-PACK " -:- l 12 j 10k .,. 01 . OUT R 1 0 12 13 -t ~. 010 1N4004 ,. r 13 ~ B k \ OUT A S ~ "*" 1 K 23 I 22 __._.. 2: -• I TOGGLE 3 + ~E 12 14 18 17 I 9 0 10 8 I & 140 .,. J.. 10k I. -;:- IC9b 114 J 08 o3 4 : TOGGLE 4 'LOAD ,,. r "_ "11~ I .022 r ) I" J ) r:r-r,-m ~~ 043 4IC6 6 13 17 ~o 1 -gg LED5(~1') _ __ (+l ·- t lle~.~ iil:!Z I ~ I ,a 6 5 12 14 4 6 IB 18k 16 ~ l 1 I .12 114 _ ---- ~ •• 1C4 24 451 4 1s LED~LOAD 011 1N4004 .----11--,t---, I I ·-l- IN' ~ _3 : 2 ••.,.__ 12 ~F-"""'lr--' 8 Md~ 6 f ·-,. r l ELJc VIEWED FROM BELOW y DPTDCDUPLER Wt, ~ LOAD ' .01 10 9 o.1I .,. +5V DTMF TONE DECODER A~D DRl~ER B .,. ii Q }. ,.. MOMENTARY l : 5 TOGGLE 2 LED3 • RL ;;J -...r iiciai'' ~ N' 0 O __ lM + + 680n __ RL1 4 ...lNSl~ Wit~•· ± (I I Ml-5~ --, f +5V r: 2.2k !:- Q S1 4.7k j ~I ..- ;:. °'---o- AUDIDI L YM ·1 LINE -:- +12V UNREG O.lI AT251, 45035 ·~ ..,. 21 r~. . /: . - _. . . - ICBb ll CK S 9 D +12V REG iuwn.-=. _,ex, •-··--······ _ ..._..._.,_._-"'t +·O-_ _ _ UNREG +12V 1 I LF~51/"I" lt'--½--4'M-=t--l .,. I 5otw .I: .,. 100ki 0047 250VAC"'T"' Tllin----"11 .,. r) EHO MIC PHONE LINE 05 <at>½ ~ L I• ~ I,, 2O--=---1--4t----------------------' l .I: ,.i.r '-" 1 "'1 100 220k$ +12V REG • 2 7 5 64 3 1 2 0 B9M I Fig.5: if you intend plugging the unit directly into the phone line, IC2 & the microphone can be deleted. Alternatively, if you intend acoustically coupling the unit to your telephone answering machine, you can leave out the relay, the isolation transformer, diodes D1-D6, IC1, Ql & their associated parts. necessary. If you're not sure if you have shorted tracks or not, check the resistance between thern using your rnultirneter. Now take a look at the wiring overlay diagram of Fig.5. This shows you where each component is installed and rnust be followed exactly if you are to avoid problems. Begin the board assembly by installing the wire links. A number of these run parallel in close proximity to each other, so rnake thern as straight as you can so they don't short each other out. Don't worry about the keypad encoding links at this stage; CAPACITOR CODES 0 0 0 0 0 0 38 Value IEC Code EIA Code 0.1µF .022µF .015µF .01µF .0047µF 100n 22n 15n 10n 4n7 104 223 153 103 472 SILICON CHIP we'll corne to those later. Once you have finished installing the wire links, solder in the diodes and resistors. Sarne of the resistors are mounted upright and should be given neat rightangle folds so that they fit neatly into the board. Make sure you don't confuse the two signal diodes (D7 & DB) with the larger power diodes and check their polarity carefully against the wiring diagram. The zener diode (ZD1) is installed with a loop in one of its leads to protect it frorn thermal stress. Next, ·install the 5rnrn fixed-pitch polyester capacitors and the electrolytic capacitors. Make sure you get the polarity of the electroytics correct. Note that the .0047µF capacitor across the telephone line rnust be a 250VAC type. Now install the transistors, LEDs and ICs. Again, the overlay diagram will show you their correct orientation while the rnain circuit (Fig.4) shows the device pinouts. Note that all the ICs face in the sarne direction except for IC4 (4514) which faces the opposite way. The two regulator ICs are installed with their metal tabs towards the transformer. The 3.579MHz crystal can be soldered in next. It doesn't matter which way around it goes in. Check that the base of the crystal sits flush against the PC board. Finally, solder in the relay and the line isolation transformer. Depending upon which brands you get for these two components, you rnay have to file or drill out extra holes to rnake thern fit the board. In most cases though, the components should drop straight in and if you do have to rnake changes, they should be fairly minor. External wiring There's not rnuch to do here - just wire up the audio input selector switch (Sl) and the microphone, and install a couple of power supply leads. Note that light-duty shielded cable should be used for the microphone leads, as shown in Fig.5. Although we left our prototype in "bare-bones" state, we suggest that you install your unit inside a standard plastic instrument case. If this is done, the selector switch can be installed on the front panel, along with a polarised socket to accept the microphone leads. The optional on/off switch can be installed on the rear panel, along with a DC power socket for the plugpack supply. The power supply can be a 12V DC .1 amp plugpack. However, if you intend using only one or two of the output drivers with relays, then you could use a 300mA version. Setting up the keypad Before using the unit, you have to install the necessary links in the userselect matrix in the bottom righthand corner of the board. This matrix defines which key on the keypad operates a particular output. If you look at the grid on the board, the top row represents the keypad outputs from 0-9 but note that they are not in order. From left to right, they are: 7,5,6,4,3,1,2,0,8,9. The bottom 10 rows (from the bottom up) are as follows: MZ, Ml, R, T4, TZ, Tl, T3 , M3, M4 and M5, where M is a momentary output, T is a toggle output and R is the master reset for the toggle outputs. As an example, if you look at the overlay wiring diagram, the "9" output is connected to the 4th toggle output (T4), the "8" output is connected to TZ , the "O" output to the master reset (R), and so on. You can follow our scheme if you wish, or you can change the links to suit your own requirements. You don't have to install all the links to the momentary and toggle output rows either. For example, if you only wish to switch two appliances, then two links (plus the master reset link) are all that will be required. The completed PC board can be installed in a plastic instrument case, with the selector switch & microphone socket installed on the front panel. The telephone cable & plugpack supply leads can exit through the rear panel, via grommetted holes. An optional on/off switch can also be fitted to the rear panel. Don't leave out the master reset link; it must go in, although it can be controlled by any key you wish (except of course the"#" key). Once you have completed the board, check it thoroughly for parts placement and solder splashes. When you're happy that the board is OK, connect the power supply with your multimeter in series with one of the leads and switched to amps. Now switch on and check that the quiescent current is around 30mA or so. If the current shoots up to greater than about 60mA, switch off quickly and check your wiring. Assuming everything is OK, you can now poke around the board with a voltmeter and look at some of the voltages to check that all is well. At the Vee pins of all the CMOS ICs , including the MC145436 , you should get a reading that's close to +5V. The LF351 and NE555 IC should have close to +12V on their Vee pins. If you don't get these voltages, turn the power off and then check your wiring again, particularly the orientation of the ICs. As a final check, us e the following test procedure to mak.e sure you get the right output. Using the board So how do you operate it? Let's say you've just turned the unit on. To start with , flick switch Sl over to the RESISTOR COLOUR CODES D D D D D D D D D D D D No Value 4-Band (5%) 5-Band Code 1 1MQ 470kQ 220kQ 100kQ 10kQ 4.7kQ 3.3kQ 2.2kQ 1kQ 680Q 470Q brown black green gold yellow violet yellow gold red red yellow gold brown black yellow gold brown black orange gold yellow violet red gold orange orange red gold red red red gold brown black red gold blue grey brown gold yellow violet brown gold brown black black yellow brown yellow violet black orange brown red red black orange brown brown black black orange brown brown black black red brown yellow violet black brown brown orange orange black brown brown red red black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown 1 4 7 11 1 9 1 6 MAY 1991 39 ::----, ..... ·-· ■■ D ■ D ■ DD - 0 40 SILICON CHIP a■ 0 Fig.6: before mounting any of the parts, check your finished board against this full-size artwork to ensure there are no shorted or open circuit tracks. microphone input, which is the easiest way for checking the output code. Now hold your keyboard encoder so that its speaker is next to the microphone and enter a "*" code to clear the memory register (note: you should do this each time power is applied to the board). Two things should happen. First, you should hear the tone coming out of the speaker of the encoder; and second, the VALID CODE indicator (LED 1) on the decoder board should be alight while ever you hold the key down. If you now press the enter or "#" key, LED 1 should light but all the other LEDs should stay off. This clears the memory register. The various outputs can now be tested. First, press keys 1-9 on the keypad in succession, so that LED 1 lights on each occasion. This lets you ··- ■■ DDD ■ -­ know that the circuit has "heard" your request. Now press the "#" key and check that the output LEDs all light up. When you release the"#" key, only four of the output LEDs should stay on. These should all correspond to the selected toggle outputs. The five LEDs that turn off should all .correspond to the momentary outputs. Next, we'll test the memory clear or "*" key. If you again press all of the keys from 1-9 and then press the"*", nothing should happen when you press the "#" key. That's because the "*" key clears the memory register. This means that if you make a mistake while entering the outputs you wish to select, you just press the "*" key and start over again. In our example, we used key "O" as our MASTER RESET key. If you press this and then press the"#" key, those four remaining LEDs should go out. If you have used another key for the master reset, press that key and the "#" key and you should get the same result. If you strike problems, .check the links in your output matrix to make sure that you have correctly matched the outputs to the keyboard code. What are the uses? The uses for the DTMF Decoder board are really limited only by your imagination. You can use it to turn on or off all manner of electrical items via the appropriate relays or optocoupled Triacs. Typical examples include house alarms, lights, water sprinker systems, radios, hifi systems, heaters and air-conditioners. If you are an amateur radio operator, you will already be aware of the uses of DTMF control systems. SC