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An easy-to-build UHF remote switch This UHF remote switch is based on a readymade receiver front-end, so it's really easy to build & get going. You can use it to switch your car or house alarm on & off, or to control lights & other appliances. By GREG SWAIN Although it's mainly intended to switch burglar alarms, this simple project can be used wherever you require a single channel remote control and could even form the heart of a garage door controller. It uses a small hand-held transmitter, has a range of about 100 metres in open air, and uses a receiver that measures just 138 x 42 x 30mm (W x D x H). There are two relays on the receiver board and these are activated each time you press the transmitter button. You can activate the main (or switch) 22 SILICON CHIP relay in one of two modes, depending on how you install a single wire link. If you select momentary mode, the relay turns on when you press the transmitter button and remains on only while the button is held down. It immediately turns off again when the button is released. Alternatively, in latched (or toggle) mode, the main relay changes state on each press of the transmitter button. Press the button once, and the relay turns on. Press it again, and the relay turns off. This mode would be used to switch most burglar alarms on and off, for example. Note: the alarm on/ off inputs should be wired across the normally closed (NO) contacts of the switch relay. The second (or indicator) relay is activated briefly each time the main relay switches on or off, regardless of the mode of operation. When the main relay switches on, the indicator relay closes its normally open (NO) contacts for about 0. 2 seconds. Conversely, when the main relay switches off, it closes its contacts for about 0.1 seconds. This second relay in intended to briefly activate a car's hazard lights, . to indicate whether a burglar alarm is · being turned on or off. The short pulse indicates that the alarm is on; the longer pulse indicates that the alarm is off. most other applications, the "indicator" relay would not be required and so it could be left off the board. In 01 1N4148 18 Cl .001 Al R4 A2 820 C4 6.8pF C3 2•7pF B 17 304MHz SAW FILTER CS 4.7pF 01 2SC3355 E A3 + 'T' ~ A4 12V : ..&.. c2 · .001+ ~ + AS AS IC1 AX5026 A7 A~K AS C E B VIEWED FROM BELOW 16 15 0 R1 111 UHF REMOTE CONTROL TRANSMITTER ~ 9 14 Fig.1: the transmitter is based on trinary encoder ICl. When St is pressed, it generates a series of pulses at its pin 17 output to switch transistor Qt on & off. This transistor is wired as a Hartley oscillator & operates at 304MHz due to its tuned collector load & the SAW filter in the feedback path. The relay contacts are all brought out to a screw terminal block adjacent to one edge of the PC board, along with the supply connections. The main (switch) relay has both NO and NC contacts, while the indicator relay has one set of NO contacts only. SAW resonator Unlike some UHF remote switches, a SAW resonator is used in the transmitter to ensure frequency stability. This SAW filter also makes the transmitter easy to align, since its oscillator will only spring into action and pulse a LED in series with the power supply when the single tuned circuit . is virtually dead on frequency. This clever technique eliminates trial and error adjustments and means that the transmitter can be quickly and accurately alignedto 304MHz (ie, the frequency of the SAW resonator). And although it doesn't directly set the transmitter frequency, the SAW filter will quickly pull the oscillator to 304MHz when it starts oscillating if there is some drift in the transmitter tuned circuit. At the other end of the RF link is a factory-built front-end module that's accurately aligned to the transmitter frequency. This module is fitted with a row of pins along one edge and mounts on the main receiver board just like any other component. It eliminates quite a lot of work, since you don't have to wind any coils or align a receiver front end in order to get the project going. To ensure a compact assembly, the module is entirely made up of surface-mount components. It accepts the signal from the antenna and outputs a digital pulse train which is then fed directly to a digital decoder IC. We'll look more closely at how this decoding circuitry works shortly. How it works -transmitter The transmitter is based on an AX5026 trinary encoder IC - see Fig.1. When pushbutton switch Sl is pressed, this IC generates a sequence of pulses at its output (pin 17). The rate at which these pulses are generated is set by the lMQ timing resistor between pins 15 and 16 (Rl), while the code sequence is set by the connections to the address lines Al-A 12. Each address input (A1-A12) can either be tied high or low or left open circuit (0/C), giving more than half a Main Features Range: ........ .. ...................... ... : ............ 100 metres. Main Relay: ................. ................... ..... Momentary or latched operation. Indicator Relay: .................................. . Pulses on for 0.2s when main (switch) relay turns on; pulses on for 0.1 s when main relay turns off. No. of Code Combinations: ................. 531 ,441. Receiver Current Consumption: ....... ... 1mA approx. (relays off). Receiver Dimensions: .. ....................... 138 x 42 x 30mm (W x D x H). DECEMBER 1992 23 + 0 Fig.2 (left): the receiver circuit is based on a pre-built front-end module. It processes the RF signals from the transmitter & feeds the resultant coded pulse signals to ICl, an AX-528 trinary decoder. This IC then drives the relay circuits via D1 & link LKl for momentary operation, or via flipflop IC2a & link LK2 for latched operation. I 0 ; i. ... a a:~ ,. -> c.., N- + I,--!•· "'C> 0$! 1---1•· oo o~ million possible codes - 531,441 to be exact. The 12-bit code pulses generated by ICl are used to switch transistor Ql. This transistor is connected as a Hartley oscillator operating at 304MHz, as set by parallel tuned circuit 11, C3, C4 & C5. The SAW resonator provides a narrow-band feedback path. Its lowest impedance is at its resonant frequency of 304MHz and thus the tuned collector load must be set to this frequency in order for Ql to oscillate. C3 is used to adjust the centre frequency of the tuned collector load. This point corresponds to maximum current consumption and is found by adjusting C3 to obtain peak brightness from the indicator LED (LED 1). Power for the transmitter is derived from a miniature 12V battery (GP23 or equivalent) and this is connected in series with the pushbutton switch (S1). When S1 is pressed, the current drawn by the circuit is only a few milliamps, the exact figure depending on the code word selected at address lines A1-A12 . I• ... ,-.N ON :::, 0 + I,--!•· O>c, o- a: w > w 0 0 w N:C ><O ....----➔ _,; . ., _... N- w a: _...< ► IC ....I 0 ><Z --'W a: "'0 "' 1- z 0 I• o<> 0 w 0 l- L....IINU\,,.-+-.IJ\-... + o lrl•· ~ .. 0 U)Q w o- a:~ a: ..,.., Oe> LL :c :::, ., ,. IC!;; ...cc-,. How it works - receiver . _... o"' - >< .. < C ., C ... C .., < ... .... C C .... ... C ... C . < ,. N < . . . . . , " ' " ' " ' . . . . . , , , . _ _ . . . . . . , , , . _ _ . . . . . . , , , . _ _ . . . . . . , , " - - ' . . . . . ,1" - - ' . . . . . , , , , . _ _ . . . . . . , , , , . _ _ . . . . . . , , , _ . . . . . , , , _ _ ,, _ _ , 24 SILICON CHIP _ Fig.2 shows the circuit details of the receiver. Its job is to pick-up the coded RF pulses from the transmitter and decode these signals to drive the relays. As already mentioned, the receiver is based on a complete "front end" module which is supplied ready made, tested and aligned to 304MHz. This module processes the received signal · via a bandpass filter, an RF preamplifier, a regenerative detector, an amplifier and a Schmitt trigger. Its input is connected to a short antenna, while its output delivers a digital pulse train that's applied to the input (pin 14) ofICl. ICl is an AX-528 Tristate decoder and is used to decode the 12-bit pulse signal that's generated by the transmitter. As with the AX-5026 encoder, this device has 12 address lines (AlA12) and these are connected to match the transmitter code. If the code sequence on pin 14 of ICl matches its address lines, and the code sequence rate matches its timing (as set by Rl) , the valid transmission output at pin 17 switches high. This then drives the remainder of the circuit via one of two possible paths, to provide either latching (toggle) or momentary operation for switch relay RLYl. For toggle operation, the output from ICl (pin 17) is applied to the clock input (pin 3) offlipflop IC2a via a filter circuit consisting of R2 , R3 & C5. This filter circuit isolates ICl from IC2 and the filtering action of C5 is useful iflong wires are attached to the clock input of IC2 (eg, if the optional manual override circuit is connected). ICZa, a 4013 D-type flipflop, has its Q-bar output conn ected to its data (D) input via R5 to provide toggle operation. Thus, each time pin 17 of !Cl goes high (ie, wh en a valid code is detected), a clock pulse is applied to IC3 and its outputs (pins 1 & 2) toggle. C4 and R5 prevent the flipflop from changing state at less than 1-second intervals. This time constant acts as a debounce circuit and eliminates inadvertent multiple toggling when the transmitter button pressed. Assuming that the circuit is wired in toggle mode, ICZa's Q-bar output (pin 2) drives transistor Ql via R6. When pin 2 of ICZa switches high, Ql turns on and pulls QZ's base low. QZ thus turns on and activates relay RLYl to operate a set of changeover contacts. The relay now remains on until the transmitter button is pressed again. When that happens , ICZa's output switches low and so Ql, QZ and RLYl all turn off. D4 protects QZ by quenching any back-EMF spikes that are generated when RLYl turns off. Momentary operation If momentary operation is selected, ICl 's output is fed directly to Ql via Dl and a lkQ resistor (R4). ICZa is not used for this mode. Now, when the transmitter button is pressed, pin 17 ofICl goes high and Ql, QZ and RLYl all turn on as before. However, when the transmitter button is released, pin 17 of !Cl goes low again and so Ql, QZ and RLYl all turn off. C3, in company with R6 & R7, provides a 1-second switch-off delay when the transmitter button is released. This protects the circuit against drop-outs due to short breaks in the transmission when the transmitter button is pressed (eg, due to contact b ounce). Thus, when momentary operation is selected, RLYl only remains on for as long as the transmitter button is held down. Conversely, when toggle operation is selected, it only changes state each time the transmitter button is pressed. Indicator relay The indicator relay, RLYZ, works differently to RLYl. It is actuated briefly each time Ql (and thus RLYl) changes state, regardless of the mode of operation. Each time Ql turns on, capacitor C7 charges via R12 , DZ, R13 and the base-emitter junction of Q3. As a result, Q3 turns on during this charging period and operates the indicator relay (RLYZ). After about 0.2 seconds, C7 is fully charged and so Q3 and RLYZ turn off again. Similarly, each time Ql turns off, QZ also turns off and capacitor CB charges viaR12 , D3, R13 and the baseemitter junction of Q3 . Q3 and RLYZ thus turn on while CB is charging and turn off again about 0.1 seconds later when the capacitor is fully charged. Resistor RlO discharges C7 when Ql turns off, while Rl 1 discharges CB when QZ turns on. Switch SZ and its accompanying 4.7kQ resistor (R15) provide an optional manual override for the circuit. When SZ is pressed, it provides a positive-going pulse to the clock input of ICZa and so ICZa toggles and switches the relays as described previously. Alternatively, if the circuit is wired for momentary operation, pressing SZ pulls Ql's base high (via R15, RZ, Dl, R4 & R6) for as long as the switch is held down. The receiver module can be powered from any+ 12V DC rail and draws approximately lmA. This rail directly powers the relay driv.er circuitry, since the relays can only w9rk down to about l0V. The front end of the receiver, including the module and the two !Cs, is powered from a +5V rail derived from 3-terminal regulator REGl. PARTS LIST Transmitter 1 transmitter case 1 PC board, 30 x 37mm 1 miniature PC-mount pushbutton switch 1 12V battery, GP23 or equiv. 1 304MHz SAW resonator Semiconductors 1 AX-5026 Tristate encoder (IC1) 1 2SC3355 NPN transistor (01) 1 1N4148 silicon diode (D1) 1 3mm red LED (LED1) Capacitors 2 .001 µF ceramic 1 6.8pF ceramic 1 4.7pF ceramic 1 2-?pF miniature trimmer Resistors (0.25W, 5%) 1 1MQ 1 6.8kQ 1 1kQ 1 150Q 1 82Q Receiver 1 PC board, 137 x 42mm 1 front-end module (aligned to 304MHz) 1 SPOT 12V relay 1 SPST 12V relay 1 7-way PC mount screw terminal block 1 pushbutton switch plus 4.?kQ resistor for manual override (optional, see text) Semiconductors 1 AX-528 Tristate decoder (IC1) 1 4013 dual O-type flipflop (IC2) 1 BC548 NPN transistor (01) 2 2N2907 transistors (02,0 3) 1 78L05 3-terminal regulator (REG1) 1 15V 1W zener diode (ZD 1) 3 1N914 diodes (01,02,03) 2 1N4004 silicon diodes (04,05) Capacitors 1 100µF 16VW PC electrolytic 1 22µF 16VW PC electrolytic 3 10µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 2 0.1 µF monolithic 2 .0033µF ceramic Resistors (0.25W, 5%) 2 1MQ 1 330kQ 2 180kQ 2 100kQ 1 47kQ 2 10kQ 2 4. ?kQ 1 1kQ 1 10Q DEC EMBER 1992 25 Fig.3: keep all leads as short as possible when installing the parts on the transmitter board & take care with the orientation of the encoder IC. The receiver board can be wired for either momentary or latched operation of relay RLYl by selecting the location of a single link. The indicator relay (RLY2) at right is optional & can be left off the board if not required for your particular applicatjon. Zener diode ZD1 protects the regulator circuit against voltage spikes on the supply line. These spikes typically occur in automotive supply lines and are usually generated the ignition system. R14 and capacitors C6 & C10 provide decoupling from the supply line, while C9 filters the output from the regulator. The receiver has a sensitivity of 2µV for a valid data detect and this input level normally gives a 400mV p-p signal at the test point. This can rise to several volts peak-to-peak with normal input levels. The noise level at the test point (under no signal conditions) is approximately 110mV p-p. Construction Fig.3 shows the assembly details for the transmitter. All the parts, including the battery terminals and the switch (S1), are mounted on a small PC board which fits inside a plastic transmitter case. The most important thing to remember with the transmitter assembly ·is that all component leads should be kept as short as possible. Apart from that, it's simply a matter of installing the parts on the board exactly as shown in Fig.3. Be sure to orient IC1 correctly and note that the flat side of the trimmer capacitor (VC1) is adjacent to one end of the board. The SAW resonator and switch should both be mounted flat against the board, while the transistor should only stand about 1mm proud of the board. The LED should be mounted with its top about 7mm proud of the board, so that it later protrudes about halfway through a matching hole in the lid. Be careful with the orientation of the LED - its anode lead is the longer of the two. Check the board carefully when the assembly is completed - it only takes one wrong component value to upset the circuit operation. This done, slip the board into the bottom half of the case, install the battery and test the circuit by pressing the switch button. Don't worry if the LED doesn't flash at this stage - that probably won't occur because Ql will not be oscillating. To adjust the oscillator stage; press the switch and tune C3 using a plastic tool until the LED does start to flash. When this happens, the oscillator is working and you can then adjust C3 for maximum transmitter output (ie, maximum LED brightness). The lid of the case can now be snapped into position and secured using the small screw supplied. Receiver assembly Fig.4 shows the parts layout on the transmitter board. The first step is to decide whether you want momentary or latched operation for RLY1. Install either link LK1 for momentary operation or link LK2 for latched operation. RESISTOR COLOUR CODE 0 0 0 0 0 0 0 0 0 0 0 0 0 26 No. Value 4-Band Code (5%) 5-Band Code (1%) 3 1 2 2 1 2 1 2 2 1 1 1 1MQ 330kQ 180kQ 100kQ 47kO 10kO 6.8Kn 4.7kO 1kQ 1500 82Q 10n brown black green gold orange orange yellow gold brown grey yellow gold brown black yellow gold yellow violet orange gold brown black orange gold blue grey red gold yellow violet red gold brown black red gold brown green brown gold grey red black gold brown black black gold brown black black yellow brown orange orange black orange brown brown grey black orange brown brown black black orange brown yellow violet black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown brown black black brown brown brown green black black brown grey red black gold brown brown black black gold brown SILICON CHIP TEST . ii RELAY1 CONTACTS Fig.4: the front-end module is installed on the receiver board with its component side facing the adjacent .0033µF capacitor. Install either LK1 for momentary operation of Relay 1 or LK2 for latched operation on Relay 1. The rest of the parts can be installed in any order, although you will find the assembly easier if you leave the larger components till last. These include the relays, terminal block and the front-end module. Make sure that all polarised parts are correctly oriented and that the correct part is used at each location. The front-end module comes with a row of 12 pins along one side and is simply mounted on one end of the PC board. Install the module so that its component side faces capacitor Cl. The trimmer in the module is factory preadjusted (and sealed) for 304MHz operation. It shouldn't ever be necessary to retune the receiver but it can be done by rotating the trimmer (at the top of the board) for maximum voltage at the test point. This voltage can be monitored by connecting a DMM set to AC volts between the test point and ground. However, unless you have a good reason to adjust the tuning, we suggest that you leave the trimmer alone. It's unlikely that you will do any better than the factory adjustment. To obtain a decent range, either a¼wave. or a ½-wave antenna must be connected to the input. This antenna consists of a length of insulated hookup wire and can be either 250mm or 500mm long. The latter will give slightly greater range if this is important. Testing When the assembly is completed, connect the receiver to a 12V DC power supply (a 9V DC plugpack should do) and press the transmitter button. If the project is working cor- rectly, you will immediately hear the relays operating. Check that each relay is operating correctly by connecting a DMM (set to ohms) across its outputs. RLYl should provide momentary or latched operation, depending on whether LKl or LKZ is fitted, while RLYZ's contacts should close briefly each time the transmitter button is pressed. Now check the line-of-sight range of the project. Provided the battery is fresh, it should operate reliably up to about 100 metres in open air, although this can be considerably reduced if the receiver is located indoors, depending on the building material. You can expect a range of 20-30 metres if the receiver is placed inside a car, depending on the location of the antenna. Changing the code Once the project is working correctly, you can code the A1-A12 address lines in both the transmitter and Where To Buy The Kit A complete kit of parts for this project is available from Oatley Electronics, PO Box 89, Oatley, NSW 2223, Australia. Phone (02) 579 4985. The price is $35 for the receiver (includes the frontend module) plus $34 for two transmitters (transmitters available separately for $20 each). Add $4 for packing & postage. Note:·copyright of the PC boards associated with this project is retained by Oatley Electronics. the receiver. You can make this code as elaborate as you like, depending on the security required, but make sure that the transmitter matches the receiver otherwise the unit won't work. Initially, all the A1-A12 address lines will be open circuit but you can tie selected address pins high or low by connecting them to adjacent copper tracks. In both cases, a +5V rail runs adjacent to the inside edge of the address pins, while a ground track runs around the outside edge of the address pins. For example, you might decide to tie Al, A2 and A8 high, tie A3 and A6 low, and leave the rest open circuit. Short wire links can be used to make the connections but note that you will have to scrape away the solder mask from the adjacent rail at each connection point so that the track can be soldered. Troubleshooting If it doesn't work, the first step is to check the supply pins of the two ICs in the receiver. You should find +5V on pin 18 ofICl and on pin 14 ofICZ. If the supply rail is OK, set you DMM to a low AC range, connect it to the test point, and check that the reading increases when you press the transmitter button. If it doesn't, then the receiver module is faulty (unlikely) or the transmitter is suspect. If the reading does increase, set your DMM to DC volts and check that pin 17 of ICl swings high when the transmitter button is pressed. Check the A1-A12 address lines and the timing resistor between pins 15 & 16 if this does not occur. If the reading does go high but neither relay operates, check transistor Ql and its associated base bias resistors (R6 & R7). If only one relay fails to operate, check its associated driver transistor (QZ or Q3). SC DECEMBER 1992 27