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Easy to build UIIF remote sMtch UHF remote switches can be tricky to build and align. Not this one. It comes with a ready made transmitter so all you have to do is assemble and tune the receiver. By GREG SWAIN One of the main aims in designing this new UHF Remote Control Switch was to make it as easy to build as possible. Most people will use it to switch a car burglar alarm on and off but it can also be used with house alarms or for switching mains appliances. When installed in a car, it will have a range of about 10 metres, depending upon the degree of shielding by the car body and the provision of an antenna. 46 SILICON CHIP Apart from its obvious convenience, remote switching also offers improved security and eliminates exit and entry delays. There is no need for hidden switches inside the car and all sensors can now be wired to the instant trip inputs of the alarm. Malcing it easy Remote control switches have been described before but all required the constructor to build and align both the transmitter and the receiver. That's where the problems started. Many constructors find it difficult to set the transmitter frequency correctly and then match it with the receiver, particularly if they don't have access to a frequency meter. The way around this problem was obvious - use a ready made transmitter that could be supplied as part of the kit. This has several advantages: (1). It eliminates the fiddly task of building and tuning the transmitter; (2). It eliminates a source of possible uncertainty if problems are encountered; and (3). It ensures that the transmitter is tuned to the correct frequency (304MHz). To ensure that the transmitter is .---------------------------------+8.5V, 820!] 1M RF AMPLIFIER 150k Q3 PN3563 1M E .01I .,. L2 : 1.66T, 0.8mm ENAMELLED COPPER WIRE CW ON 5mm DIA FORMER SUPER REGENERATIVE DETECTOR AMPLIFIER 39k 2.2 BP* +4.25V ~------------+B.5V WIRE ACCORDING TO TRANSMITTER CODE 270k 10M t I R.ELAY 120n MIN 8.2k 16 ~ Dl-'-----"An LED1 A 14 RELAY DRIVER 10 390k +vs .,. 02 1N4002 +VSD---N-+-'""""' 12V VIEWED FROM BELOW 820!:l + -VS°"]_ .,. 390!:l POWER SUPPLY 10I 100k UHF CAR ALARM SWITCH .,. Fig.1: the incoming RF signal is amplified by Qt and detected by Q2. IC1a-IC1d amplify and square up the detected signal which is then fed to decoder stage IC2. When the correct code is received, pin 11 of IC2 goes high and triggers the output stage (latch IC3a and relay driver transistor Q4). always on frequency, the LC tuned circuit used in previous designs has been scrapped and replaced by a SAW (surface acoustic wave) filter which is precisely tuned to 304MHz. This eliminates the need for transmitter alignment and means that there is very little drift due to temperature variations. Another advantage of this transmitter is that it automatically switches off after about 10 seconds if the button is accidentally held down. As well as saving batteries, this feature is now also a DOTC (Department of Transport & Communications) requirement. The transmitter is supplied in a plastic keyring style case and is powered from a 12V lighter battery. A red LED flashes when the on/off button is pressed. Because the transmitter comes ready made, all you have to do is assemble and tune the receiver. There's just one adjustment to make and that's to a small trimmer capacitor on the receiver PCB. In practice, you simply activate the transmitter and adjust the trimmer until the receiver responds (ie, the relay triggers). Receiver circuit Fig.1 shows the circuit details for the receiver. It can be broadly divided into four sections: an RF amplifier and detector stage (Ql & QZ), an amplifier and comparator stage (ICla-lCld), decoder IC2, and a latch and relay driver circuit (IC3 & Q4). When the transmitter button is pressed, a 9-bit code word is broadcast as bursts of 304MHz oscillation. This signal is picked up by inductor L1 which forms a tuned circuit with the 22pF and 8.2pF DECEMBER1989 47 When mounting the parts on the PCB, keep all leads as short as possible, particularly around RF stages Q1 and Q2. The trimmer capacitor (VC1) must be installed with its flat side towards coil L3 (see Fig.2). We soldered the ICs directly to the PCB but you can use sockets if you wish. capacitors. The signal is then coupled to the base of Ql via a 3.3pF capacitor. Ql functions as an RF amplifier stage with bias supplied by the 8200 and 1800 resistors. The amplified signal appears across 12 in Ql 's collector circuit and is coupled to super-regenerative detector stage Q2. The circuitry associated with Q2 is actually both an RF oscillator and a quench oscillator. The RF oscillator comprises Q2, 13, TCl and the 5.6pF, 220pF, 33pF and 2.2pF capacitors. In practice, TCl is adjusted so that the RF oscillator runs at the transmitter frequency (ie, 304MHz). The quench oscillator includes 14, a 6.8k0 resistor and a 390pF capacitor at the emitter of Q2. Its function is to ensure that RF oscillation does not occur in the absence of an input signal. Q3 is wired as a diode. It forms part of the bias network for Q2 and provides temperature compensation for this stage. When a coded input signal is received, Q2 oscillates at 304MHz and the detected signal appears 48 SILICON CHIP across the 6.BkO resistor. This signal is then applied to a low pass filter consisting of a 1.5k0 resistor and a .OlµF capacitor which removes the 304MHz RF signal but not the pulse modulation. The resulting pulse signal is ACcoupled via a 2.2µF capacitor to ICla which is a non-inverting op amp stage with a gain of about 27. From there, the amplified signal is fed to inverting op amp stage ICl b which has a gain of 18. IClc is wired as a Schmitt trigger. It squares up the amplified signal from ICl b and then feeds it to pin 9 of IC2 via voltage follower stage ICld. IC2 is an MC145028 trinary decoder and is used to decode the 9-bit pulse signal generated by an MC145026 encoder chip in the transmitter. It has nine tri-state address inputs (A1-A9) which are connected to correspond to the transmitter code. These address inputs can be tied high, low or left open circuit. In this project however, the A1-A8 address inputs can only be tied low or left open circuit, while A9 is permanently tied low. This simplifies programming but reduces the number of coding options from 13,122 to 256. This is quite adequate for most applications but there's really nothing to stop you from increasing the odds by tying some of the A1-A8 inputs high. We'll talk more about the coding later on. When IC2 detects a valid code from the transmitter, its output at pin 11 switches high. Just how the. circuit operates from this point on depends on where you install the 8.2k0 resistor in series with the base of relay driver transistor Q4. If the resistor is installed in position AA, then Q3 is driven by latch circuit IC3. IC3 is a D-type flipflop and is wired to change state whenever it receives a clock pulse from IC2. When power is first applied, pin 4 (reset) of IC3 is momentarily pulled high by the lµF capacitor. This sets Q (pin 1) low which means that Q4 is off. When the transmitter button is pressed, IC3 is clocked by the high on pin 11 of IC2. Thus, the Q output switches high and turns on Q4 and the relay. On the next press of the transmitter button, the Q output I ULE01 t, • • ~ •• 1. POLYESTER & CERAMIC CAPACITORS I RELAY • □ □ □ □ □ □ □ □ □ □ □ INC .-:--NO ◄ No. 3 2 1 1 1 1 1 1 1 1 2 Value 0 .1µ,F .01 µ,F .022µ,F 390pF 220pF 33pF 22pF 8.2pF 5.6pF 3 .3pF 2.2pF IEC 1 OOn 10n 22n 390p 220p 33p 22p 8p2 5p6 3p3 2p2 EIA 104 103 223 391 221 33 22 8.2 5.6 3.3 2.2 Fig.2: the 8.2k0 resistor near IC3 goes in position AA if you want the relay to latch and in position BB if you want the relay to turn only only while the transmitter button is pressed. RESISTORS □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ □ No. 1 2 1 1 2 3 1 2 1 1 2 1 1 2 1 1 1 Value 10MO 1MO 390k0 270k0 150k0 100k0 56k0 39k0 27k0 22k0 8.2k0 6.8k0 1.5k0 8200 3900 1800 1000 switches low again and the relay turns off. This means that RLYl is alternately latched on and off for each press of the transmitter button. LED 1 indicates when the relay is on. Now assume that the 8.2k0 resistor is installed in position BB. In this case, the latch circuit (IC3) is bypassed and Q3 is driven by the output of IC2. This means that Q4 is only on while ever the transmitter 4-Band Code brown black blue gold brown black green gold orange white yellow gold red violet yellow gold not applicable brown black yellow gold green blue orange gold not applicable red violet orange gold not applicable grey red red gold blue grey red gold brown green red gold grey red brown gold orange white brown gold brown grey brown gold brown black brown gold 5-Band Code brown black black green brown brown black black yellow brown orange white black orange brown red violet black orange brown brown green black orange brown brown black black orange brown green blue black red brown orange white black red brown red violet black red brown red red black red brown grey red black brown brown blue grey black brown brown brown green black brown brown grey red black black brown orange white black black brown brown grey black black brown brown black black black brown button is held down. As soon as the transmitter button is released, pin 11 of ICZ goes low again and thus Q4 and the relay turn off. Power for the circuit is derived from a 12V battery via a 78L05 3-terminal regulator. The two resistors connected to the GND terminal of the regulator jack up the output voltage to + 8.5V while the 10µ,F capacitor on the regulator output provides supply line filtering. Finally, a half supply rail ( + 4.25V) is derived from a voltage divider network consisting of two lOOkO resistors. This + 4.25V rail is used to bias the non-inverting inputs of ICla, ICl b & IClc. Construction This project was developed by Dick Smith Electronics and is DECEMBER 1989 49 ;; L3 is the only coil that you have to wind yourself. It is made by winding 1 % turns of 0.8mm enamelled copper wire on a 5mm former. Scrape away the enamel coating from the leads before soldering them to the PCB. available as a complete kit (see panel). Construction is straightforward, with all parts mounted on two printed circuit boards. The main board is coded ZA-1518 and carries all parts except for the relay, the LED and a 3300 resistor. These remaining few components are all mounted on a separate small relay PCB. Fig.2 shows how the parts are installed on the main PCB. The order of assembly is unimportant but we suggest that you install the wire links first and then move on to the resistors and capacitors. After that, you can install the diodes, transistors and !Cs. Note that the 22kQ, 39kQ and 150kQ resistors in the base bias circuit of Q2 must be 1 % metal film types. Install the 8.2kQ resistor in position AA if you want the relay to latch on and in position BB if you want the relay to turn on only while the transmitter button is held down. Keep all leads as short as possible when installing the parts on the PCB. This particularly applies to those parts in the RF sections of the circuit (around Ql, Q2 & Q3). Cut the collector lead of Q3 flush with its body before installing it on the board. The coils can now be installed on the PCB. You don't have to worry about Ll since it forms part of the PCB pattern. 12 (4.7µH) and L4 (2.2µH) are supplied pre-wound and should be installed on the PCB using minimum lead length. 13 is made by close winding 1 2/3 turns of 0.8mm enamelled copper wire on a 5mm former (supplied with the kit). Remove the former after winding on the turns, then mount the coil by pushing it down onto the PCB as far as it will go [see photo). Scrape away the enamel coating from the leads with a sharp knife before soldering them to the board. Assembly of the main board can now be completed by installing the terminal block and the trimmer capacitor. Install the trimmer capacitor so that its flat side goes towards 13 (ie, the earthy terminal connects to the 5.6pF capacitor). Relay board The relay board has been designed to accept two different relays: Here's the location of the decoder address pins on the main receiver board. The prototype was coded by tying A1-A4 and A6 low, and leaving A5, A7 and A8 open circuit (see Fig.3). ◄ 50 SILICON CHIP Left: here's how it all fits in the case. The main board is mounted on the lid on 15mm threaded spacers while the relay board is mounted low at one end of the case on a single 6mm spacer. . .. l ~ •' . s:,: , r,. .~ A7 ' .• -~ ~· ., , A1 Fig.3: the transmitter and the receiver are coded by tying selected A1-A8 address pins low or by leaving them open circuit. The transmitter (left) is coded by cutting tracks while the receiver is coded by linking address pins to the earth track. An example code is shown here but you should choose your own code. either the DSE Cat. S-7120 relay rated at 2 amps or the S-7125 rated at 5 amps. The S-7120 will be supplied as standard with the kit but be sure to substitute the S-7125 if the load to be switched draws more than 2 amps, otherwise the relay's contacts will burn out. Fig.2 shows the assembly details for the relay board. Install the relay first, then mount the 3300 resistor and the 3mm LED. Note that the LED is mounted on the back of the board (see photo) and should be stood off the board by about 5mm so that it will protrude through the side of the case. Don't trim off the excess leads at this stage - you may need to adjust the height of the LED later on. Coding The transmitter and receiver must both be identically coded before they can be tested. If you don't use the same code for both, the project will not work. You program in the code you want by simply tying each Al-AB address pin low (ie, to ground) or by leaving it open circuit (0/C). For example, you could tie A1-A4 low, leave A5 0/C, tie A6 low and leave A7-AB 0/C. It's a good idea to write your selected code down on a piece of paper before you actually start making connections. As supplied, the transmitter has the Al-AB address pins of the encoder IC all tied low. Fig.3 shows the locations of these address pins on the copper side of the PCB. They correspond to pins 1-7 & 9 of the IC. Pin 10 (A9) is permanently tied low, as is pin 8 (which is a supply pin). To code the transmitter, first undo the self-tapping screw that holds the case together and remove the PCB (careful - don't lose the little plastic switch lever that sits in the lid). It's now simply a matter of cutting selected tracks between the address pins and the ground track to program in your selected code. The Al-AB address pins on the receiver PCB can now be connected to match the transmitter code see Fig.3. Unlike the transmitter, Al-AB on the receiver are all initially 0/C. They can be tied low by bridging them to the adjacent ground track using a wire link. As mentioned previously, there Where to buy the kit A complete kit of parts for this project is available from Dick Smith Electronics stores or by mail order from PO Box 321, North Ryde, NSW 2113. Phone (02) 888 21 05. The kit comes complete and includes the transmitter, a pre-punched front panel, and a front panel label. The price is $79.95 plus $4.50 p&p. Quote Cat. K3257 when ordering. Note: copyright of the PCB artworks associated with this project are retained by Dick Smith Electronics. This close-up view shows the transmitter address pins. Initially, the A1-A8 address pins are all tied low, so it's simply a matter of cutting selected tracks. The transmitter time-out feature can be disabled to allow receiver adjustments by shorting the 4 7µF electrolytic capacitor. are some 256 possible codes to choose from but this can be increased dramatically if you elect to tie one or two of the address pins high. This can be done by installing small insulated links on the back of the board. Be careful you don't install a short between the positive and ground rails of the battery, otherwise nothing will work. Tuning Having completed the coding, you now have to adjust the tuned circuit in the receiver so that it matches DECEMBER1989 51 PARTS LIST 1 Auto Keeper transmitter (from DSE) 1 PCB, code ZA-1518, 130 x 70mm 1 relay PCB, 53 x 26mm 1 plastic case, 160 x 96 x 50mm 1 5mm coil former 1 6-way PCB-mounting terminal block 4 1 5mm tapped spacers plus screws & washers 1 6mm spacer 1 1 9mm x 1/8-inch screw plus nut 1 12V relay, DSE Cat. S-7020 (see text) Semiconductors 3 PN3563 NPN transistors (01 ,02,03) 1 BC337 NPN transistor (04) the transmitter frequency of 304 MHz. The tuning procedure is delightfully simple: • connect your multimeter between pin 7 of ICl and ground, and set it to a low AC voltage range; • connect a 12V DC supply to the main board (both the relay and the LED should remain off when power is applied); • activate the transmitter and adjust VCl for a peak reading on the meter. You will find that the peak is quite sharp so adjust VCl very slowly. (Note: you must use an insulated tool for this job). Alternatively, you can forget about the voltmeter and simply slowly adjust VCl until the relay turns on (don't forget to link the relay board to the main board first). After that, you can get someone else to activate the transmitter at progressively greater distances while you peak VCl for maximum range. By the way, the transmitter timeout feature is a nuisance when making receiver adjustments. It can be easily disabled by shorting out the 47µF electrolytic capacitor near one end of the IC. Final assembly Construction can now be completed by installing the receiver 52 SILICON CHIP 1 LM324 quad op amp (IC1) 1 MC145028 or SC41344 trinary decoder (IC2) 1 ~013 dual D flipflop (IC3) 2 1 N4002 or 1 N4004 diodes (D1 ,D2) 1 78L05 3-terminal regulator 1 3mm red LED (LED 1) Capacitors 2 1 OµF 25VW PC electrolytics 3 2.2µF 25VW bipolar electrolytics 1 1µF 25VW PC electrolytic 2 0. 1µF ceramic 1 0. 1µF polyester 2 .01 µF ceramic 1 .022µF greencap 1 390pF ceramic 1 220pF ceramic 1 33pF ceramic 1 22pF ceramic 1 1 1 2 1 8.2pF ceramic 5.6pF ceramic 3.3pF ceramic 2 .2pF ceramic 4-20pF trimmer capacitor Inductors L2 4.?µH L3 1 % turns 0.8mm tinned copper wire, 5mm dia. L4 2.2µH Resistors (0.25W, 5%) 1 2 1 1 2 3 1 2 1 10MO 1MO 390k0 270k0 150k0 1% 100k0 56k0 39k0 1% 27k0 1 2 1 1 2 1 1 1 22k0 1 % 8.2k0 6.8k0 1.5k0 8200 3900 1800 1000 Lt is part of the pattern on the main PCB. If need be, the range can be increased by soldering a 250mm-long antenna to the centre of the coil. PCBs in the plastic case. The main board is mounted on the lid of the case on 15mm threaded spacers while the relay board is mounted on one end of the case and secured using a single 6mm spacer, machine screw and nut. The lid of the case is supplied pre-punched to take the mounting screws and there is also an access hole for the tuning capacitor so that you can make adjustments with the lid in place. A front-panel label is supplied with the kit and this should be carefully affixed to the lid before the spacers are attached. Note that the relay board is mounted as low down in the case as possible. This is to provide clearance for the main board. You will have to drill holes in the side of the case for the mounting screw and LED, and to provide an exit for the external leads. Finally, if you want greater range (out to about 25 metres in open space or 10 metres in a car), connect a 250mm antenna to the centre tap of coil Ll (see Fig.2). The PCB comes ready-drilled to accept this lead, so making the connection is easy. ~