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By JOHN CLARKE & GREG SWAIN Crook in bed with a bad back, mate? Want someone to bring you a nice hot cup of tea? Don't shout - just press the button on a small keyring transmitter to attract attention. The traditional way for someone confined to bed to attract attention is to shout at the top of his (or her) lungs. But when one of our staff members (who had best remain nameless) tried that recently, there was no response. His pathetic whimperings were muffled by two sets of closed doors and by a TV set going full-bore in the family room. That started him thinking (and not before time). What was needed was a unit that would sound a bell or a chime in the family room in response to the press of a button on a small transmitter in the bedroom. Aha! Why not base the circuit on 56 SILICON CHIP the UHF Remote Alarm Switch described in the March issue for switching car burglar alarms on and off? And that's exactly what we've done. We simply stripped the Remote Alarm Switch receiver of any unnecessary circuitry and married it to a two-tone chime IC, the SAB0602. So, whenever you press the button on a small keyring transmitter, the receiver picks up the signal and sounds a pleasant two-tone chime. For want of a better name, we've called the finished project the Remote Alert. It works like a charm. And it eliminates enraged yells, fist banging on walls, and the aforesaid pathetic, self-pitying whimperings. Another ideal use for this project is as a wireless doorbell. In this version, the transmitter PCB can be housed in a small plastic project box, together with a 9V battery and a pushbutton switch. Depending on circumstances, the completed unit should have a range of 12 metres or more. By the way, you could have as many "bell pushes" as you wanted. Just build as many transmitters as necessary (one for each door?) and code them all exactly the same. To keep costs low and simplify construction, we've designed a new printed circuit board for the receiver. This PCB leaves out any unnecessary circuitry from before and includes the two-tone chime IC. No changes are necessary for the transmitter circuit so we have simply reused the original PCB. Transmitter Fig.1 shows the transmitter circuit. It consists of a digital encoder integrated circuit (IC1) and a UHF oscillator operating at 304MHz. ICl is a Motorola MC145026 9-bit trinary encoder. Trinary code is like binary code except that it uses three logic states instead of two (ie; high, low or open-circuit). Only one transmitter code word (one 9-bit word) is possible, as selected by connections to the IC's nine address inputs, Al fo A9. In keeping with the trinary states just mentioned, each of the nine address inputs can be connected to the + 12V rail, to ground or left open circuit. In our application, because of the particular decoder used in the receiver, the A9 input must either be connected to the + 12V rail or to ground. The 9-bit code word appears as a series of pulses on pin 15 of ICl. The frequency of these pulses is set by the two resistors and the capacitor connected to pins 11, 12 and 13. For our circuit, the frequency is about 2kHz. Pushing the Transmit button (St) causes the IC to deliver the coded word from pin 15. This is used to key the UHF oscillator Qt on and off at a rate of 2kHz. When pin 15 is high ( + 12V), Qt oscillates. Qt is a BFR91A which is a surface mounting transistor intended for use in UHF and microwave amplifiers. Inductor 11 and the 2-6pF capacitor form a tuned circuit load for the collector while the base is grounded (to AC signals) by a 470pF capacitor. Stray capacitance between the emitter and collector of Qt provides positive feedback which causes it to oscillate at 304MHz. To increase the oscillator's output, the emitter deger ,: rntion resistor is bypassed with J 1.5pF capacitor, which is critical in value. The value of this capacitor cannot be too large because this would reduce the positive feedback and stop oscillation. The transmitter is powered from a 12V lighter battery (VR22, EL12, GP23 or equivalent), but will work from voltages down to as low as 4.5V (a 9V battery is used in the doorbell transmitter version). The When you press the button on the small keyring transmitter, the receiver picks up the signal and sounds a pleasant two-tone chime. You can also build the transmitter into a larger case for use as a doorbell. + LOW 14 IC1 MC145026 13 12 TE11 ... 100k .0022 + 220k E TYPE MARKING 8 ... L1 : 32mm OF 0.71mm TINNED COPPER WIRE FOR SMALL VERSION. LARGER VERSION USES PCB TRACKS. toN LARGE VERSION ONLY UHF REMOTE TRANSMITTER SC03-1-288 Fig.1: the transmitter uses an MC145026 trinary encoder IC to key UHF oscillator Qt on and off. A1-A9 are connected to give the address code (see text). battery is bypassed by a 0.1µ:F capacitor located near IC1 and by a 0.047µF capacitor near the tuned circuit for Ql. When S1 is closed the current drawn by the circuit is a few milliamps, the exact figure depending on the code word selected at Al to A9. The current through LED 1 is abou ~ 7mA. When St is open, AUGUST 1988 57 4,70 r - - - -.....- ......--~-'Wl,,,.....--------~~----.----.....---+sv ANTENNA L4 L1 .,.. .,. 12VDC + PLUG-PACK 47 + 100 + 16VW+ 16 -_i 16VWL ,. . 1 INPUT A1 2 A2 3 220 A3 OUTPU 4 5 A4 11 IC2 MC145028 A5 4 39k 7 10 12 A9 .,. DECODING OPTIONS ~SPEAKER L1 : 15T OF 0.63mm ENAMELLED COPPER WIRE ON 3.2mm DIA. FORMER L2 : 1.5T OF 0. 71 mm TINNED COPPER WIRE ON 5mm DIA. F29 FERRITE CORE L4 : BT OF 0.63mm ENAMELLED COPPER WIRE ON 3.2mm DIA. FORMER 13 AB SC03-1-DBBB-1 8 1sv~ so .0068t .022+ REMOTE ALERT l 0 33 · 6 15 A6 14 IC3 SAB0602 ,. ,. 0.1+ Fig.2: the receiver circuit can be divided into four sections: an RF input amplifier and detector (Ql, Q2), a tuned 2kHz amplifier (ICl), trinary decoder IC2, and a chime generator circuit (IC3). Power comes from a 12V DC plugpack transformer. the current drain is less than 0.2µA. The transmitter can be built into one of two cases, one of which is small enough to be attached to a key ring. We'll talk about these later in the section on construction. Receiver The receiver circuit is shown in Fig.2 and is identical to the circuit used for the Remote Alarm Switch as far as the output (pin 11) of ICZ. It consists of four sections: an RF input amplifier and detector (Qt and QZ), a tuned ZkHz amplifier (IC1), trinary decoder IC2 , and the chime generator circuit (IC3). The transmitted signal is picked up by the antenna which is loaded 58 SILICON CHIP by inductor 11. The signal is then coupled via a .001µF capacitor to the base of Qt , which is an RF amplifier with a tuned collector load. Signal from the collector of Qt is fed via a voltage divider consisting of a 2.7pF and a 22pF capacitor to self-oscillating detector stage QZ . Whenever signal is received, this stage oscillates at 304MHz. When no signal is received, the circuit is quiescent. The detected signal from QZ appears across the .001µF capacitor connected to its base. This capacitor bypasses the 304MHz signal but not the ZkHz pulse modulation which is superimposed on the signal fed to the antenna. This ZkHz pulse signal is coupled via 14 and a 2.ZµF capacitor to ICla, an inverting op amp with a gain of about 470. L4 is there to keep RF energy out of this op amp stage. ICtb is a Schmitt trigger. It squares up the amplified signal from ICla before feeding it to ICZ, the trinary decoder. ICZ is an MC145028 decoder which is compatible with the MC145026 used in the transmitter circuit. It is set up to respond only to the unique code word sent by the transmitter. This is done by connecting the address inputs Al to A9 in exactly the same way as for IC1 . in the transmitter. When IC2 detects a correct code 1=' -~,:~®:~~ T ~/0., { ~ .0470 1Al-H1G_"- -~ ~--Lo_w A7• • •A6 A9 AS• •A4 A3• eA2 <<at>)t2·6pF ~ctlOOk Al• ~i220k + Fig.3: parts layout for transmitter PCB. Substitute a battery snap connector for the battery clips and leave out the LED and PC-mounting switch if you are building the doorbell version. Fig.4: to code the transmitter each A1-A8 input is connected to the high rail, the low rail, or left open circuit. A9 must he connected high or low; it cannot be left open circuit. The transmitter PCB fits neatly into the small keyring case which is available from Dick Smith Electronics. Power comes from a 12V lighter battery. Fig.5: mount the parts on the transmitter PCB as shown here. The RF transistors (Qt & Q2) are installed from the copper side of the PCB, with the part number visible from the component side. Winding details for the coils are shown on Figs.2 & 6. from the transmitter, the output at pin 11 goes high. This triggers the two-tone chime IC (SAB0602) which produces a rich, heavily modulated chime sound. Note that the chime IC only sounds once and will not retrigger if the transmitter button is held down (ie, if pin 11 of IC2 stays high). IC3 drives an 80 loudspeaker via a 330µF capacitor. The O.lµF capacitor and 33k0 resistor on pins 6, 7 & 8 set the chime tone frequency, while the 0.33µF capacitor on pin 3 provides a degree of high frequency filtering. Power for the circuit is derived from a 12V DC plugpack transformer . This feeds a 7808 3-terminal regulator and several filter capacitors to produce a regulated + 8V supply. This supply regulation is necessary to prevent false triggering. Transmitter construction Construction of the transmitter mainly involves the assembly of a small printed circuit board. This board measures 46 x 33m and is coded SC0312882. Fig.3 shows where all the parts go. The keyring transmitter case r L2 FORMATION DIMENSIONS IN MILLIMETRES Fig.6: L2 is made by winding 0.71mm tinned copper wire. on a 5mm plastic former. The former is fitted with an F29 ferrite core for tuning. from DSE is supplied complete with PC-mounting battery clips. If you intend installing the PCB in the larger case for use as a doorbell, delete the battery clips and wire in a battery snap connector instead. You must install the IC with pin 1 towards the battery clip side of the PCB as shown. The RF transistor (Ql) is mounted on the copper side of the PCB (see Fig.1 for pinouts). The body of the transistor fits into a hole in the PCB with the type marking uppermost and the leads then soldered to the copper tracks (which should be pre-tinned). Note that some of the resistors are mounted flat on the PCB while others are mounted end on as shown in Fig.3. All the capacitors are mounted flush against the PCB except the .0022µF greencap which is bent to lie flat over the top of the IC (see photo). Pay particular attention when installing the switch (S1). It must be installed so that the flat side of the switch body is towards the battery terminal end of the PCB. The LED must be mounted so that the top of its body is about 13mm above the PCB surface. Delete both the LED and the PCmounting switch for the doorbell version in the zippy case. In this AUGUST 1988 59 r: 0 1-- a: 0 w 0 ...J <( 0 0 w 0 1-- 0 0 0 ~ w 0 L..: .:J o] 0 SC03 -1-288-2 The front panel and printed circuit board artworks are shown here for the convenience of those who prefer to make their own. Alternatively, you can buy ready made boards and panels from the usual suppliers (see page 96). version, the PCB is soldered to a panel-mounting pushbutton switch via two short lengths of tinned copper wire. It will be necessary to angle the board as shown in one of the photographs so that there will be enough .room to mount the battery. Receiver construction The UHF receiver and chime cir60 SILICON CHIP cuits are combined on a single PCB coded 03107881 and measuring 87 x 8 7mm. This is designed to clip into a plastic utility case measuring 159 x 96 x 51mm. Fig.5 shows the parts layout on the receiver PCB. Begin construction by installing all the low profile components such as the resistors, ICs and the wire link adjacent to IC2. When these have been install- ed, you can mount the capacitors and the 3-terminal regulator. The BFR91 transistors (Ql and Q2) are mounted on the underside of the PCB. As in the transmitter, the bodies of these RF transistors fit into holes, with the part number visible from the component side of the board. You will find it easier to solder the leads if you tin the copper tracks first. Transmitter 1 transmitter case (DSE Cat. H-2497, 31 x 58 x 17mm) 1 PCB, code SC0312882, 46 x 33mm 1 PC-mounting pushbutton switch, DSE Cat. S-1200 1 3mm LED 1 12V lighter battery (VR22, EL 12, GP23 or equivalent) Semiconductors 1 BFR91 NPN UHF transistor 1 MC145026,SC41342 trinary encoder Capacitors 1 1 1 1 1 1 0.1 µF miniature polyester .04 7 µF ceramic .0022µF metallised polyester 4 70pF ceramic 1.5pF ceramic 2-6pF ceramic trimmer Resistors (0.25W, 5%) 1 X 220k0, 1 X 100k0, 1 X 4 7k0, 1 x 1 OkO, 1 x 1 .5k0, 1 x 1 kO PARTS LIST Receiver 1 plastic utility case, 158 x 96 x 54mm 1 PCB, code SC03107881, 87 x 87mm 1 mini 80 loudspeaker 1 1 2V DC plugpack supply Semiconductors 1 MC145028,SC41344 trinary decoder 1 SAB0602 dual-tone chime 1 LF353, TL072 dual FET input op amp 1 7808 8V 3-terminal regulator 2 BFR91 NPN UHF transistors Capacitors 1 1 1 2 1 1 3 1 1 220µF 16VW PC electrolytic 1 OOµF 16VW PC electrolytic 4 7 µF 1 6VW PC electrolytic 1 OµF 1 6VW PC electrolytic 2.2µF 16VW PC electrolytic 0 .33µF metallised polyester 0. 1µF metallised polyester .022µF metallised polyester .01 µF ceramic 1 4 1 1 1 1 .0068µF metallised polyester .001 µF ceramic 22pF ceramic 3.3pF ceramic 2. 7pF ceramic 2.2pF ceramic Inductors and wires L 1 190mm 0.62mm enamelled copper wire L2 65mm 0.71mm tinned copper wire, 5mm former DSE Cat. L 1010, F29 ferrite screw core L3 3.3µH RF choke L4 100mm 0.62mm enamelled copper wire 1 300mm-length of 1mm solid core insulated wire (for antenna) Resistors (0.25W, 5%) 2 X 4.7MQ, 1 X 470k0, 1 X 180k0, 1 x 150k0, 1 x 1 OOkO, 2 X 47k0, 1 X 39k0, 1 X 33k0, 1 X 22k0, 1 x 18k0, 5 x 10k0, 1 x 1 kO, 1 X 4 700, 1 X 2700, 1 X 4.70 11 is made by winding a 190mm length of 0.63mm enamelled copper wire around a 3.2mm (1/8-inch) drill bit. Wind on 15 turns and strip the insulation from the ends with a sharp knife before soldering the coil to the PCB. 14 is wound in exactly the same fashion as 11 but requires only 8 turns. 12 is wound on a 5mm plastic former, the base of which is pushfitted into a hole in the PCB. The winding details are shown in Fig.6. Don't forget to screw in the F29 ferrite core. 13 is a standard 3.3µH inductor that can be purchased ready wound (eg, DSE Cat. 1-1765). The antenna is simply a 300mm length of single-core hookup wire soldered to the antenna input pad on the board (see Fig.5). Testing and alignment Both the transmitter and receiver must be coded before they can be tested. Initially, to allow testing, we recommend that only the A9 input on both the transmitter and receiver be coded. This input must be connected to either the high or The receiver PCB clips into slots in the side of the standard plastic case. For best range, you should make the antenna about 300mm long. low rails (ie, it must not be left open circuit). The Al-A8 inputs can all be left open circuit for the time being. The transmitter frequency can now be set to 304MHz by using a frequency meter. Temporarily connect pin 15 of IC1 to the positive AUGUST 1988 61 111r . . . ·. · . · ·~- - -[ir For the doorbell version, the transmitter PCB is soldered to the pushbutton switch on the case lid via two short lengths of tinned copper wire. Angle the PCB as shown to allow room for the battery. (ie, for maximum reading on the DMM). You may need to move the transmitter away from the receiver to obtain the setting for maximum sensitivity. If everything is working correctly, the chimes should now sound whenever the transmitter button is pressed. In fact, you may wish to disconnect one of the speaker leads during the adjustment procedure to prevent the chimes from continually sounding. When the correct setting has been found, drip some molten candle wax into the screw core of 12. This will prevent it from moving and thereby detuning the receiver. rail. This will set the oscillator in operation. Now hold the transmitter near the input of the frequency meter and adjust the trimmer capacitor for a reading of 304MHz. In some cases it may be necessary to connect a coil of wire between the input and ground of the frequency meter to obtain a satisfactory reading. Once the frequency has been set, remove the temporary connection to pin 15. Now connect the receiver to a 12V power supply and check that the output of the regulator is at + BV. If everything is OK, switch off and connect a multimeter set to read DC volts between pin 7 of IC1 and ground. Apply power and wait 10 seconds for the 2.2µF capacitor on the base of Q2 to charge. Adjust the slug in 12 for maximum signal when the transmitter switch is pressed Coding Because this project is not being used in a security application, there's really no need to worry too much about working out an ·elaborate security code. The main eA1 eA2 eA3 eA4 A&• A7• AB• A9• With this doorbell, you don't have to run connecting wires between the bell push and the chimes unit. Because the current drain is so low, the battery should last for its normal shelf-life. I eA5 LOW Fig.7: connect the Al-A9 receiver inputs to exactly match the transmitter code. Al·A8 can be high, low or open circuit; A9 must be tied high or low. 62 SILICON CHIP thing to watch out for here is that both the transmitter and receiver codes are made identical. For example, you may wish to code only the Al, A2, A3, A4 and A9 inputs (A9 must be tied high or low as mentioned above). The remaining A5-A8 inputs could all be left open circuit. Coding is achieved simply by connecting each A1-A8 input to the high rail or to the low rail, or by leaving it open circuit. For example, you could bridge Al to the high rail, A2 to the low rail, leave A3 open circuit, bridge A4 high and so on. Figs.4 and 7 show the Al to A9 code inputs on the copper side of the PCB for the transmitter and receiver respectively. It's a good idea to write your selected code down on a piece of paper before actually making the necessary connections. Once coding has been completed, the receiver PCB an be clipped into the plastic case and the wiring to the speaker and plugpack transformer installed as shown in Fig.5. You will have to drill holes in the side of the case to allow access for the supply leads and for the antenna wire. The loudspeaker is affixed to the lid of the case using an epoxy adhesive (eg, Araldite). Before mounting the speaker, attach the adhesive label to the lid and drill a 4mm hole at each marked location to let the sound out. Deburr the holes using an oversize drill. ~