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Secure Remote Receiver 68m line-of-sight range Up to 16 remotes per receiver Mains-powered, quiescent power typically 0.8W Relay contact rating: 30A at 250V AC, meaning it can switch large mainspowered devices like pumps Relay on-timer ranges: 250ms to 60s or 60s to 4.5h (see Tables 3 & 4) Brownout protection: 192V AC switch off, 220V AC switch on DC supply current: 17mA with relay off, 100mA with relay on Part two: by John Clarke T HE SYSTEM COMPRISES ONE RECEIVER AND UP TO 16 KEY-FOB TRANSMITTERS. You can build multiple receivers if you have different equipment to control. There is no possibility of a transmitter triggering the wrong receiver due to the secure rolling-code system. The assembly of both modules is relatively straightforward due to the use of mostly standard parts. The transmitter PCB is small (30 x 45mm), and the onboard microcontroller is in an SMD package (SOIC-14). Still, anybody with reasonable soldering skills and proper equipment should be able to build it. Transmitter construction All the parts for the transmitter mount on a 30 x 45mm double-sided PCB that’s coded 10109212 – see Fig.3. Once assembled, this will be housed in a 65 x 25 x 17mm remote control case. This enclosure is designed for use with a 12V N battery, but we are using a button cell instead. So you will need to remove the curved plastic mouldings inside the front lid of the enclosure at the key ring end that locate the N-sized battery, using side cutters, to provide space for the cell holder to fit. 80  Silicon Chip Most of the parts except for the UHF module are mounted on the top of the PCB. The IC and 220W resistor are surface-­mount devices. IC1 will need to be programmed before soldering it in place. This IC can be obtained pre-programmed from Silicon Chip, or you can program it yourself if you have a suitable programmer. Start by soldering the 220W resistor in place. Tack solder one end of the resistor and remelt the solder to straighten it, if necessary, before soldering the opposite end. Then add a bit of fresh solder (or flux paste) to the first joint and heat it to reflow it so that it is nice and shiny. Next, fit IC1, making sure it is orientated correctly. Solder pin 1 to the PCB and check the alignment to ensure the IC pins all line up with the pads on the PCB before soldering the remaining pins. If any pins have a solder bridge, you can remove it with a bit of flux paste and some solder wick. Next, install the three switches, S1-S3. These are installed close to the PCB. Then fit LED1, ensuring its polarity is correct (the longer lead is the anode [A]) and positioned with the top of the LED lens 7.5mm above the top surface of the PCB. Australia's electronics magazine Mount the two 100nF ceramic capacitors next. The capacitor adjacent to S3 needs to lie over toward IC1. The UHF transmitter can now be installed on the underside of the PCB, with its pins bent so that it lies flat against the back of the PCB with 1mm of clearance. Check that it is correctly orientated before soldering its pins. Then mount the cell holder on the top of the PCB. The board assembly is completed by fitting the antenna. Make it from a 162mm length of 0.5mm diameter enamelled copper wire. Strip the insulation from one end by about 2mm using a sharp hobby knife, emery paper or sharp side cutters. Close-wind it on a 3mm mandrel (eg, a 3mm drill bit) and then stretch it out to 28mm overall length. Install the wire coil from the underside of the PCB with the stripped end into the antenna hole. Place the PCB assembly into the enclosure base before attaching the lid. The assembly is held together with the two self-tapping screws supplied with the key-fob enclosure. Then affix the front panel label that came with the enclosure to the lid. Note that the switches may not siliconchip.com.au MAINS SW TCH Transmitter Powered by a 3V CR2032 lithium cell, 200mAh+ recommended, giving more than two years of life with typical use Standby current: typically 60nA (526μAh/year) Active (transmitting) current: 10mA average over 160ms (900nAh / transmission) Registration current: 10mA average over 2.75s (15.5μAh per registration) Transmission rate: 976.5 bits/s (1.024ms per bit) Data encoding: Manchester code with a transmission time of 82ms Unique code generation: secure UHF rolling code control with 48-bit seed, 24-bit multiplier and 8-bit increment value This Remote Mains Switch uses a high-security rolling-code system, so it is suitable for triggering motors that open doors or gates. It’s also very robust, allowing it to switch motor-based appliances like pool pumps and water pumps. Last month, we described the circuitry; this article concentrates on its construction, testing, set-up, and use. initially all be operable; some adjustments might be required. In particular, switch S2 may not be able to be pressed due to the corner of the cell holder adjacent to S2 being a little too high to allow the bending of the enclosure lid lever for S2. In this case, file down that corner of the cell holder a little so the switch can be pressed (as seen below). Additionally, you might find that the switches are pressed in when the lid is attached. To avoid this, we will be supplying PCBs that are thinner than usual (1.0mm instead of 1.6mm). This thinner PCB should prevent the switches from being pressed by the lid. But if you still experience this problem, you will need to trim the tops of the plastic pins on the lid of the enclosure that press on the switches with a file, sharp side cutters or a craft knife. Take care not to remove too much material, and test the switch operation after shaving off some of this plastic. Note that if you touch the junction of the two halves of the coin cell (the + and – contacts), that will cause a higher than expected leakage current due to oils from your skin being deposited on the insulating surface. This will discharge the cell quicker than expected. If you touch it like that, clean the cell with methylated spirits or isopropyl alcohol and avoid making contact across the cell halves your fingers. Receiver construction Many of the parts (but not all) fit on the PCB coded 10109211 that measures 159 x 109mm – see Fig.4. The off-board parts are the IEC mains input socket, GPO mains output socket, pushbutton switch S1, power switch S5 and the neon indicator lamp. Install the resistors first, taking care to place each in its correct position. The resistor colour codes are shown in the parts list, but you should also use a digital multimeter to check each resistor before mounting it in place. Fig.3: the top and bottom view of the PCB overlay and actual prototype PCB for the Transmitter half of the Secure Remote Controlled Mains Switch. siliconchip.com.au Australia's electronics magazine August 2022  81 Diodes D1-D5 are next. Make sure these are orientated correctly before soldering their leads. Then install a socket for IC1, ensuring its notched end matches the position shown in Fig.4. Do not fit IC1 yet – that step comes later, after the power supply has been checked. Regulators REG1 & REG2 are both mounted horizontally on the PCB. The first step is to bend their leads down through 90° so that they will go through their PCB holes. In each case, the regulator’s two outer leads are bent down 8mm from its body, while its centre lead is bent down 5mm from the body. Secure each regulator to the PCB using an M3 x 10mm machine screw and nut. Be careful not to get the regulators mixed up – the 7805 (REG1) is on the right-hand side. Tighten each assembly firmly before soldering and trimming the leads. Do not solder the regulator leads before tightening the mounting screws, as that could stress the soldered joints and fracture the board tracks. Next, install trimpots VR1 and VR2 (VR2’s screw adjuster toward the top of the PCB), transistor Q1 and the BCD switch. This must also be orientated as shown. The capacitors can then be mounted. The electrolytic capacitors are polarised and must be installed with the polarity shown (the longer lead is positive). You can install the two 100nF MKT polyester capacitors either way around. The two LEDs (LED1 and LED2) are mounted with the tops of the lenses 12mm above the surface of the PCB and the anodes (longer leads) to the holes marked “A”. CON1 and CON2 are 4-way and 3-way screw terminals. CON1 is made of two 2-way screw terminals dovetailed together by sliding them together along the side mouldings. Orientate CON1 with the wire entry toward RLY1. CON2 has connections made only to the two outside terminals. This is to increase the separation between the Active and Neutral connections. On our prototype, we removed the centre pin from the terminal. But if you find it difficult to remove, it can be left in place. The wire entry for this connector is on the left. Then fit the headers for jumpers JP1, JP2 and JP3. Now install the 433.9MHz receiver module, again ensuring it goes in the right way around. The pin designations are all clearly labelled on the back of the module, and you can also match the orientation of the module against the photographs. The antenna is made from a 170mm length of 1mm diameter enamelled copper wire. Form it into a spiral by winding it over a 6mm (or similar diameter) mandrel, such as the shank of a 6mm or 1/4-inch drill bit. As shown in Fig.5, it extends from the antenna PCB pad to another pad adjacent to REG1. Be sure to scrape away the enamel insulation from both ends of the antenna wire before soldering it into position. For safety reasons, the antenna must be fully enclosed in the plastic case. Under no circumstances should it be mounted externally, nor should any part of the antenna protrude from the enclosure. Otherwise, if a mains wire comes adrift inside the case, it could contact low-voltage circuitry and the antenna might also become live at 230V AC. The transformer mounts on the PCB and is held in place using two cable ties that are joined to provide a sufficient length wrap around the transformer body and PCB via holes provided on the board. The cable ties prevent the transformer from coming Fig.4: the overlay diagram for the receiver section of the Secure Remote Controlled Mains Switch. 82  Silicon Chip Australia's electronics magazine siliconchip.com.au Rolling Code Systems – frequently asked questions One question that’s often asked about rolling code systems is what happens if one of the switches on the transmitter is pressed when the transmitter is out of range of the receiver. Will the receiver still work when the transmitter is later brought within range, and the button pressed again? This question is asked because the code the receiver was expecting has already been sent (but not received), and the transmitter has rolled over to a new code. How does the system get around this problem? The answer is that the receiver will process a signal that is the correct length and data rate, but it will not trigger the relay unless it receives the correct code. So if the signal format is valid, but the code is incorrect, the receiver then calculates the next code that it would expect and checks this against the received code. If it matches, the receiver will trigger the relay; that means it missed one button press. If the code is still incorrect, the receiver calculates the next expected code, and it will do this up to 10 times, to handle cases where there have been multiple transmitter button presses out of range. If none of these are correct, the receiver then changes its operation to allow for a possible valid signal more than 10 codes ahead. The receiver waits for two valid separate transmission codes before restoring correct operation. On the first receipt of a valid transmission, it looks ahead up to 200 codes, so it is more likely the required valid code will be found, but it doesn’t trigger the relay. The Learn LED lights during this look-ahead operation. If a valid code is found, the receiver waits for the next code sent by the transmitter. This following code must also be correct before the receiver will operate the relay. If only one or neither code is correct, the receiver will not take action. If it’s more than 200 codes ahead, the transmitter will need to be re-registered to operate the receiver. You can test this process by switching the receiver off and pressing one of the remote control switches 10 times or more. Then switch on the receiver and press one of the switches again. siliconchip.com.au The Learn LED will light, indicating that the look-ahead feature beyond the initial 10 times is activated. The selected function on the remote should operate on the next press of the switch, and the Learn LED extinguishes. While two successive transmission codes could be intercepted, recorded and re-sent in an attempt to activate the receiver, these codes will not be accepted by the receiver. That’s because they have presumably already been received and processed, and the receiver has already rolled past those codes. It will look forwards but not backwards. Another transmitter with a different identity will still operate the receiver (provided it has been synchronised in the first place). That’s because the receiver tracks each transmitter’s rolling codes separately. Calculating the code Another question that’s often asked is how the receiver knows which code to expect from the transmitter since it changes each time. The answer is that the transmitter and the receiver both use the same series of calculations to determine the next code. They also both use the same variables in the calculation; unique values that no other transmitter uses. Without going into too much detail about how the actual rolling code works, here are the basics. We use a linear congruential generator (LCG) in conjunction with a 31-bit pseudo-­ random number generator (PRNG). The LCG uses an initial seed value, an addition value and a multiplication factor to produce a nominally random result. For example, if consecutive codes have the number 3 added and then multiplied by 49, with the same starting number, both the transmitter and receiver will go through the same sequence. But the actual numbers used are very large, making it difficult to predict the next code by peeking at a few values in the sequence. The code is 48 bits long, with 281,474,976,710,656 possible values (that’s over 281 quintillion or 2.8 x 1014). One problem with the LCG is that it can produce recurring values; depending on the factors, it can produce the Australia's electronics magazine same number more than once within a few hundred rolling code calculations. To prevent this, we include a second layer of randomisation. So once we have the value from the LCG calculation, this value is used in the PRNG to generate a pseudo-random number for the rolling code. The PRNG randomisation runs between one and 256 times before providing the ‘random’ number for the rolling code value. The number generated is then used as the seed in the LCG for generating the next number in the sequence. Using the PRNG makes it difficult to predict the rolling code sequence even if the multiplier or addition value for the LCG is known. We throw further complications by also using code scrambling. The calculated code is not sent in the same sequence each time. There are 32 possible scrambling variations that are applied to the code, so predicting the next code becomes very difficult. What if the transmitter rolling code is identical for two consecutive codes, and the first of these identical codes is intercepted and re-transmitted to open the lock? Our system has safeguards to prevent the same code from appearing twice in succession. There is a check for the same code repeating itself for consecutive codes. If the code is the same, the duplicate is effectively skipped, preventing this possibility. Multiple transmitters Wouldn’t the receiver lose its synchronisation if several transmitters were used? No, because each transmitter operates independently. Each of the 16 possible transmitters used with a given receiver has its own different identity from one to 16. The codes sent by each transmitter are different due to the unique identifier within each transmitter IC that sets the rolling code calculation. Also, the code includes the transmitter identity value that differs between each transmitter. The receiver stores up to 16 different rolling code and calculation parameters, one for each identity, so each transmitter is treated independently. Therefore, even if one transmitter is not used for months while other transmitters are used frequently, its rolling codes will remain synchronised with the receiver. August 2022  83 adrift if the assembled unit is dropped. Without them, the transformer is only held by small pins that are secured in the plastic of the transformer body. The next step is to install the relay with its coil terminals toward CON1. Secure the relay to the board using M4 machine screws and nuts. Final assembly The Secure Remote Controlled Mains Switch is housed in an ABS enclosure measuring 171 x 121 x 55mm. You will have to drill and shape holes in one end of the case for the mains switch and IEC connector. The lid also needs holes drilled for the GPO socket, the neon indicator and pushbutton switch S1. A template for these cut-outs is shown in Fig.6. This can also be downloaded from siliconchip.com.au/ Shop/11/6418 and printed out. The large cut-outs (for the mains GPO and IEC connector) can be made by drilling a series of small holes around the inside perimeter, knocking out the central piece and filing the job to a smooth finish. The switch hole must not be oversized so that it stays clipped in when inserted into the cut-out. So take care with shaping it. Once the drilling and cutting is finished, install the PCB and power switch in the case. The PCB is secured using the integral brass inserts and four M3 x 6mm screws. The IEC connector must be secured using Nylon M3 x 10mm screws, although you can use metal nuts. The Nylon screws avoid the possibility of ‘live’ screws should a mains wire inside the enclosure come adrift and contact them. Before attaching the mains GPO, switch S1 and the neon indicator, you can print out the front panel label shown in Fig.7. Again, this is available for download from our website. Print it onto photo paper and cut out the holes for the switch, neon and GPO with a sharp craft or hobby knife. The panel will be held in place by the switch and the GPO. If the label is prone to drooping, use a small amount of clear tape to affix the corners or dabs of clear neutral-cure silicone sealant or glue. The wiring marked in Fig.5 must be run using 10A mains-rated cable. That includes the wires for switch S1. Note that brown wire is used for Active while the light blue wire is Fig.5: the wiring diagram for the receiver section of the Secure Remote Controlled Mains Switch. Note how the antenna is wound on the right-hand edge of the PCB. You can do this by winding it over a 6mm drill bit. 84  Silicon Chip Australia's electronics magazine siliconchip.com.au for the Neutral leads. The green/yellow-striped wire is for Earth wiring only, and the Earth lead from the IEC connector must go straight to the GPO. For the wiring not marked as 10A (for switch S1 and the relay coil), you can use lighter-duty 7.5A rated mains wire. Be sure to insulate all the connections with heatshrink tubing for safety, and cable tie the wires to prevent any broken wires from coming adrift. Secure the Active and Neutral leads to the GPO using cable ties passing through the holes in its moulding. Use neutral-cure silicone (eg, Roof & Gutter silicone) to cover the Active bus piece that connects the active pin to the fuse at the rear of the IEC connector. Take great care when making the connections to the mains socket (GPO). In particular, be sure to run the leads to their correct terminals (the GPO has the A, N and E terminals clearly labelled) and do the screws up nice and tight so that the leads are held securely. Similarly, make sure that the leads to CON2 are firmly secured. Testing Before applying power, check your wiring carefully and ensure that all mains connections are covered in heatshrink tubing. Then install the 10A fuse inside the fuse holder. Leave IC1 out of its socket for the time being. The Remote Mains Switch will be operated with the lid open when testing and making adjustments. During Fig.6: the lid needs to be drilled for the GPO socket, neon indicator and switch S1, while one side of the ABS enclosure needs to be drilled and shaped for the mains switch and IEC connector. siliconchip.com.au Australia's electronics magazine August 2022  85 Assembling the receiver is not difficult, but make sure you use mains-rated wire in the correct colours and add insulation and cable ties, as shown here and in the wiring diagram. this procedure, you must not touch any of the 230V AC wiring. This includes the transformer primary leads plus all wiring to the mains socket, neon lamp, switch S1, the IEC connector, the relay and CON2. Although all connections are insulated, it’s wise to be careful. In particular, note that the relay’s contact connections, the fuse holder’s terminals and switch (S2) could potentially all be at 230V AC. That applies whenever the device is plugged into the mains, even with switch S2 and the relay off. If your premises does not have a safety switch (Earth leakage detector, residual current detector or RCD) installed, consider using a portable safety switch for this part of the test. Rotate the timer trimpot (VR1) fully clockwise and apply power. Use your DMM probes to check for 5V DC (4.95.1V is acceptable) between pins 1 & 20 of IC1’s socket. If this is correct, switch off, disconnect the mains plug from the wall socket and install IC1. Take care to ensure that IC1 goes in the right way around – refer to Fig.4. 86  Silicon Chip Power the circuit back up and, with your DMM set to read DC volts, adjust multi-turn trimpot VR2 so that the voltage between TP2 and TP GND is around 3V. This ensures that the relay can switch on so that you can proceed with calibration. Next, set the DMM to a high AC voltage range suitable for measuring mains voltage and carefully check the voltage between the Active and Neutral sides of the CON2 screw terminal Australia's electronics magazine block. Press switch S1 to turn on the relay, set your DMM to read low DC volts again and adjust multi-turn trimpot VR2 until the DC voltage between TP2 and TP GND is 1% of the mains voltage reading you got earlier. For example, if you measured 250V AC, adjust VR2 for a reading of 2.50V DC at TP2. Alternatively, if the mains voltage was 230V AC, set VR2 for a reading of 2.30V at TP2. This sets the brownout cut-out level to 192V AC. siliconchip.com.au The Acknowledge LED will light continuously during a brownout. The relay can only be switched on again via a (registered) remote transmitter or the switch on the receiver once the mains voltage has recovered after a brownout. Now that you’ve calibrated the unit, you can set jumper options JP1-JP3 and adjust the timer with VR1 (see Tables 1-4). Fig.7: you can either copy the front panel label from here, or download it from siliconchip.com.au/ Shop/11/6418 Registering a transmitter When registering a transmitter and for regular use, it is essential to have the transmitter and receiver apart by at least 1.5 metres. If the transmitter is closer than this, it could overload the UHF receiver and corrupt the signal, leading to incorrect registration or intermittent remote control operation. To register the transmitter with the receiver, press Learn switch S2 on the receiver. The Learn LED (LED1) will light. On the transmitter, remove the cell from its holder and reinsert it while pressing and holding switch S1. This will set the transmitter to Synchronisation mode (with the acknowledge LED lit) and send the registering code when S1 on the transmitter is released and then pressed again. The rolling code is continuously updated during the synchronisation time between when S1 is released and it is pressed again. This randomises the rolling code generation sequence to an undetermined point, due to the rapid rate that the code is recalculated – on average, around 500 times per second. The rolling code sequence is then well into its generating sequence. This makes it hard to guess the code based on possible MUI values, even if the initial seed value for the code generation is known. The acknowledge LED on the receiver will flash twice, and the Learn LED will extinguish once registration is complete. Test the transmitter and check that the receiver responds by switching the relay on and off. It will take a couple of attempts before the transmitter and receiver start working together. De-registering a lost transmitter Any transmitter that has been registered can be prevented from operating the receiver, for example, if a transmitter is lost and you don’t want it to be used by an unauthorised person. Deregister the lost transmitter by selecting the transmitter identity using BCD switch S4. The switch is labelled 0 to F; the labels A-F correspond to identities 10-15. Then press and hold the Clear switch (S3) for more than one second. The Clear LED will light initially, then extinguish after S3 is Table 1 – JP3 settings released and the transmitter is deregistered. If you are unsure of the identity of the lost transmitter, you can deregister all the registered transmitters, one at a time, then re-register the required transmitters again. Jumper options There are three jumper positions on the receiver board, and we’ve reproduced Tables 1 – 4 from last month, so you can recall what they do. JP1 selects the timer length multiplier (see Table 3). The range is 250ms to 60s with JP1 out (the x1 range) or 60s to 4.5 hours with JP1 in (the x255 range). Table 4 shows typical timeouts versus TP1 voltages as trimpot VR1 is adjusted. JP2 affects the function of the buttons on the remote control, as shown in Table 2. JP3 affects the function of switch S1 on the receiver, as shown SC in Table 1. Table 3 – JP1 timer settings JP3 in/out Receiver switch S1 function JP1 in/out Timer period Out Off if already on, otherwise on with a timer, range per JP1 Out 0.25-60s (1x) In Toggle on/off In 1m-4.5h (255x) Table 2 – transmitter switch functions Table 4 – Nominal period versus TP1 voltage Switch Function with JP2 out Function with JP2 in TP1 Time with JP1 out Time with JP1 in S1 Relay on with a timer, range per JP1 Relay on with a timer, 0.25-60s 0V 0.25s 1m S2 Relay on continuously Relay on with a timer, 1m-4.5h 1.25V 15s 1h 7.5m 2.5V 30s 2h 15m S3 Relay off Relay off 3.75V 45s 3h 22.5m 5V 60s 4h 30m siliconchip.com.au Australia's electronics magazine August 2022  87