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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 one: by John Clarke T HE SECURE REMOTE CONTROLLED MAINS SWITCH (we’ll call it the Switch from now on) is ideal for switching motor loads such as pool pumps, water pumps and any number of applications where you find it convenient to switch power to an appliance remotely. The high security of this design means that it can be used for remote-controlled doors, gates and door strikes, maintaining the security of your home or premises. As is typical for security remote controls, the handheld unit is pocket-sized. Many commercially-made remotecontrolled mains switches are available, such as Jaycar Cat MS6148 and Altronics Cat A0345. Wi-Fi controlled mains switches are also available, like the Blaupunkt smart Wi-Fi plug BSP2EM. These rely on a mobile phone app for control. These are all fine for their intended purpose, but the relays they use to switch mains power are not suitable for appliances that include motors. While rated at 10A, the relay contacts will quickly be destroyed when used to power items such as a pool pump. Also, controllers relying on a mobile phone app could become obsolete 72 Silicon Chip should support for that app be dropped or become incompatible with newer phones. We covered this phenomenon in the February 2022 editorial. Secure codes The use of secure codes is not only necessary for security applications; it is also very useful to ensure that a neighbour or passer-by using a similar remote control does not inadvertently switch your appliance on or off while controlling their own equipment. Editor’s note: our motorised security shutters have rolled up or down more than once when we were nowhere near the remote control! So this is not just a theoretical risk, and it definitely has security implications. The security of this design also means that you can build more than one Switch without being concerned about interference between them. The unique transmission code ensures that the Switch receiver will not be activated by anything other than one of the paired handheld remote controls. Rolling codes for high security The remote control code sent by the handheld remote units can be Australia's electronics magazine considered an electronic lock similar to a physical key. This key is a specific code sent by the transmitter to the receiver; it is a long sequence of on and off signals sent in a particular order and over a set period. The code must be correct for the receiver to respond. With a fixed remote control code, an intending thief can receive and store the code sent by the remote control and re-transmit it in an attempt to operate the receiver. However, with a rolling code, the reused code will not trigger the receiver. That’s because the receiver requires a different code each time. Each code that’s transmitted differs markedly from one transmission to the next. The codes sent are based on an algorithm (calculation) that both the transmitter and receiver have in common, based on a unique numerical value that is stored within ICs in both the remote control and the receiver. The handheld remote will have a unique identifier different from any other handheld remote. The code possibilities of a rolling code system run into the trillions. This renders any attempt to break the code by sending out guessed codes totally unrealistic. The odds of picking a siliconchip.com.au MAINS SW TCH Transmitter Professional-looking key-fob enclosure 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 power switch uses secure wireless transmission so that nefarious people can’t intercept the commands and override your control. It can also switch high current loads and includes an adjustable timer. Up to 16 separate handheld remotes can control the same receiver. correct code at random for our rolling code transmitter, for example, is one in 2.8 trillion. Even then, the code needs to be sent at the correct data rate, with the correct start and stop bit codes and other transmission requirements, including data scrambling that changes for each transmission. Other features Our Switch comprises two parts: a professional key-fob-style transmitter and a separate receiver. The key-fob has three pushbutton switches and an acknowledge LED that briefly lights up each time one of the switches is pressed. Up to 16 different key-fob transmitters can be used with one receiver. The receiver has a 30A mains relay making it suitable for switching power to motors. The relay can be switched on or off, or switched on for a fixed time, using the remote control or a switch on the receiver. The on-period can be adjusted from 1/4 second to 4.5 hours in two ranges. Another feature is brownout detection; it automatically switches off should a brownout occur. This is when the mains voltage drops to a siliconchip.com.au lower than normal level, usually because of a supply fault. This lower voltage can cause motors to overheat and burn out. Motor burn-out occurs because the current through a motor’s induction windings increases when it is not spinning at its correct speed, which is likely when the supply voltage is low. During severe brownouts, the voltage can be so low that the motor will not turn at all, but current is still flowing in its windings. In that situation, the motor will quickly overheat and suffer permanent damage. The brownout detection protects the motor by switching off its power if the supply voltage falls below a preset value. Brownout detection is vital for mains-powered water pumps. Security and registration Each key-fob transmitter must be allocated an Identity number from 0 to 15, set by coding links on the PCB. Each transmitter is registered to the receiver by sending a synchronising code to the receiver when the receiver is in registration or learning mode. A facility is included to lock out a particular transmitter after registration. This is useful if a transmitter Australia's electronics magazine has been lost. If the lost transmitter is found, it can be easily re-registered. If the identity of the lost transmitter is not known, all transmitters can be locked out, and the ones that are still in use can be re-registered. The data is transmitted using UHF ASK as Manchester code. A zero bit is sent as a 512µs period of no transmission followed by a 512µs burst of 433.9MHz carrier. In contrast, a one bit is transmitted as a 512µs burst of 433.9MHz carrier followed by a 512µs period of no signal. Each transmission consists of four start bits, an 8-bit identifier, a 48-bit code and four stop bits, for a total of 64 bits. The start bits include a 16.4ms gap between the second start bit and the third start bit, while the code scramble value is altered on each transmission with 32 variations. Unique codes are generated using a 48-bit seed, 24-bit multiplier and 8-bit increment value initially set by a unique identifier within IC1 on the transmitter. The registration code is sent as two blocks. Block 1 sends four start bits, the 8-bit identifier, a 32-bit seed code and four stop bits. Block 2 sends four start bits, the 24-bit multiplier, the July 2022  73 8-bit increment and 8-bit scramble values and four stop bits. Again, the start bits include a 16.4ms gap between the second start bit and the third start bit. Circuitry The transmitter circuit is shown in Fig.1. It mainly comprises microcontroller IC1 and a 433.9MHz UHF transmitter. For IC1, the PIC16LF15323 was chosen for its very low standby current and the inclusion of a unique identifier called the Microchip Unique Identifier (MUI). We use the MUI to generate a rolling code sequence that is unique to the IC and thus the transmitter. IC1 is usually kept in sleep mode with its internal oscillator stopped and most of its internal circuitry switched off. In this state, it draws a typical standby current of 60nA from the 3V cell. You can verify this by connecting a 100kW resistor in series with the 3V supply with a switch across it. Apply power with the switch closed. After about 10 seconds, when the micro goes to sleep, open the switch and measure the voltage across the resistor. We measured 6mV, indicating a sleep current of 60nA. Switches S1 to S3 connect to the RA4, RC3 and RC1 digital inputs of IC1 while the Identity switches (1, 2, 4 & 8) connect to the RA0, RA1, RA2 and RC0 digital inputs, respectively. The Identity inputs are used to differentiate between different transmitters for a given receiver. If only one transmitter is used, it can be set for Identity 0, and none of the Identity pins need to be connected to circuit ground. At power-up, each Identity input is held high by pull-up resistors to the 3V rail that is inside IC1. The software then disables the pull-up resistor for any identity input that is kept low. That prevents that pull-up continuous sourcing current, which would otherwise be 25-200µA drawn from the cell per low input. The pull-ups for pushbutton switches S1-S3 are left on since they are only pressed momentarily. In contrast, at least one of the Identity inputs is always held low for Identity settings other than 0. IC1 is programmed to wake up from its sleep condition when any one of switches S1-S3 is pressed, and that corresponding input changes from high to a low. It then runs the program to send the rolling code for the function associated with the pressed switch. The rolling code and registration codes are sent via the 433.9MHz transmitter module. This module is powered via the paralleled RC5 and RC4 outputs of IC1, which go high to provide a nominal 3V to the Vcc input of the module. This way, it only draws current from the cell when in use. The code is applied to the data input of the module from the RA5 output of IC1. The antenna is a coiled length of wire. Fig.1: the transmitter circuit is quite simple, primarily comprised of a PIC16LF15323 microcontroller and a 433.9MHz UHF transmitter module. 74 Silicon Chip Australia's electronics magazine The transmit indicator, LED1, is driven via the RC2 output of IC2 through a 220W current-limiting resistor and is modulated at the code transmission rate of about 1kHz. After sending the code, IC1 powers down the UHF transmitter and returns to sleep mode. During transmission, the current draw from the cell briefly rises to about 10mA. If you keep holding one of the buttons down after the transmission is complete, the current will drop to about 220µA until the button is released. This is due to the current flow in the switch pull-up resistor. Considering the quiescent current and intermittent bursts of higher current when transmitting, cell life should be more than two years with typical use. Receiver circuit The receiver (see Fig.2) uses a PIC16F1459-I/P microcontroller (IC1) and UHF receiver module with an onboard coiled wire antenna input to provide a very good reception range. When no signal is present, the receiver’s output signal is random noise since the module’s automatic gain control (AGC) is at its maximum. Upon reception of a 433.9MHz signal, the receiver gain is reduced for best reception without overload, and the coded signal from the data output of the module is delivered to the RC7 digital input of IC1. The Acknowledge LED (LED2) indicates whenever a valid signal is received. The RC5 digital output of IC1 drives NPN transistor Q1, which switches the relay coil. When RC5 goes high, it delivers current to transistor Q1’s base and Q1 powers RLY1. Diode D5 clamps the back-EMF that causes a voltage spike at the collector of Q1 as the relay switches off. The relay contacts are rated at 30A and 250V AC. The unit can be set up to power the relay for a fixed period or just switch it on or off continuously. There are two ways to toggle the relay on and off. The operation of switch S1 on the receiver depends on jumper JP3. When JP3 is open, the relay switches on with one press and off on the next. When JP3 is bridged and S1 is pressed, the relay is switched on for a fixed time and switches off at the end of this period, or when S1 is pressed again – see Table 1. siliconchip.com.au Fig.2: UHF transmissions are fed to microcontroller IC1 on the receiver, which decodes them. If they are valid, it controls the mains relay by changing the level at digital output RC5, which drives NPN transistor Q1 to power the relay coil. Table 1 – JP3 settings 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 July 2022  75 433.9MHz receiver module, while the +12V rail powers the relay. The outputs of REG1 and REG2 are filtered and stabilised using 100µF capacitors. A 100nF capacitor further decouples the 5V supply for IC1. Brownout detection The receiver fits into an IP65 sealed ABS plastic case, so it could be installed in a pool house or similar, to control a pool pump, among other possible uses. Being splashproof could also come in handy if it’s controlling a gate or garage door. It should be installed out of the elements, as the sockets and switches are not sealed. The remote control has three buttons, and usually, S1 on the remote switches the relay on with the timer to switch it off, S2 switches it on continuously (or for a much longer time if JP2 is inserted), and S3 switches it off. See Table 2 for more details. The timer period is set using trimpot VR1. The trimpot wiper can be adjusted from 0V through to 5V. This voltage is monitored at the AN6 analog input of IC1, which converts the voltage into a period from 0.25 seconds to 60 seconds. IC1’s digital input RA4 has an internal pull-up current from IC1. If JP1 is inserted, this pin is pulled low instead. In that case, the timing period ranges from one minute to 4 hours and 30 minutes – see Table 3. You can monitor the timer setting voltage at test point TP1. Table 4 shows the typical periods for selected trimpot positions. Identity The Identity selection is made using a BCD rotary switch (S4) with 16 positions, labelled 0-9 and A-F (for 10-15). This switch is only applicable to the lockout selections; it plays no part in the key-fob transmitter registration. S4’s four contacts connect to the RB7, RB6, RB5 and RB4 digital inputs 76 Silicon Chip of IC1. When the BCD switch is set at 0, all four inputs are high. Position 1 on the switch has the ‘1’ output at RB7 pulled low, while Position 15 (or F) sets all switch outputs at 0V. S3 provides the lockout or deregistering function for a transmitter. Pressing S3 will prevent the transmitter from operating the receiver identified by the number selected with the BCD switch. The Learn switch (S2) tells the program within IC1 to be ready to accept the synchronising signal from a handheld remote. While waiting for a signal from the remote unit, the Learn LED (LED1) stays lit. The Learn LED extinguishes once the synchronising signal has been received. Power supply Power for the receiver comes from the mains via transformer T1. The transformer’s 12V secondary voltage is full-wave rectified using diodes D1-D4 and filtered with a 470µF electrolytic capacitor at 3-terminal regulator at REG1’s input plus another 100µF capacitor at REG2’s input. The result is a pulsating 17V DC rail applied to REG1 & REG2, which in turn provide regulated +12V DC and +5V DC rails, respectively. The +5V rail is used to power IC1 and the Australia's electronics magazine IC1’s AN8 analog input is used for brownout detection. This input samples the 17V DC rectified rail via a voltage divider consisting of a 22kW resistor and trimpot VR2. VR2’s wiper voltage is filtered using a 10µF capacitor (to smooth out 100Hz ripple and transients) and applied to the AN8 input via a 1kW resistor. During the set-up procedure, VR2 is adjusted so that the voltage at AN8 is a DC voltage that is 1/100th that of the mains AC voltage. For example, the voltage is set to +2.35V if the mains voltage is 235V AC. If a brownout occurs and the mains voltage drops below about 192V AC, the voltage applied to AN8 will fall below 1.92V DC. This is detected by microcontroller IC1, which then switches the relay off to disconnect power from the mains output. The relay can only be switched on again manually when the mains voltage returns to normal. One small problem with monitoring the 17V rail is that while it does vary with mains voltage, it also varies with load. RLY1 has a coil resistance of 120W, so there is an extra 100mA drawn from the 17V rail when the relay is on. As a result, this rail drops when the relay is powered. Therefore, VR2 is adjusted while the relay is on, so the brownout voltage detection threshold is accurately set. When the relay is off, the voltage is expected to rise by about 3V as the relay load on the supply is removed. However, as the relay is latched off by a brownout and must be manually switched on again, that doesn’t matter. Next month We still have quite a bit of ground to cover as, besides assembling the two PCBs, we also need to describe how to fit them into their respective cases. Then we’ll go over the testing procedure, set-up, remote registration and de-registration and some more advice for using the Secure Remote Mains Switch. All of that will be in the second and final article in this series, next month. SC siliconchip.com.au Parts List – Secure Remote Mains Switch Receiver 1 double-sided plated-through PCB coded 10109211, 159 x 109mm, 1.6mm thick 1 IP65 ABS enclosure, 171 x 121 x 55mm [Jaycar HB6248, Altronics H0478] 1 433.9MHz UHF ASK receiver (RX1) [Jaycar ZW3102, Altronics Z6905A or equivalent] 1 3VA PCB-mounting 12V mains transformer (T1) [Altronics M7012A] 1 12V DC coil, 250V AC 30A contact SPST relay (RLY1) [Jaycar SY4040 or equivalent] 1 momentary push-to-close 250V AC panel-mount mains switch (S1) [Jaycar SP0716, Altronics S1080] 2 SPST PCB-mount tactile micro switches (S2, S3) [Jaycar SP0600, Altronics S1120] 1 4-bit DIL BCD PCB-mount rotary switch (S4) [Jaycar SR1220, Altronics S3000A] 1 SPST mains rocker switch (S5) [Jaycar SK0984, Altronics S3210] 1 10A mains panel socket with side wire entry [Jaycar PS4094, Altronics P8241] 1 panel-mount IEC mains socket with integral fuse holder [Jaycar PP4004, Altronics P8324] 1 M205 10A fast-blow fuse (F1) 1 10A IEC mains cord 1 panel-mount 230/240V AC neon lamp 2 2-way screw terminals, 5.08mm pitch (CON1) 1 3-way screw terminal, 5.08mm pitch (CON2) 3 2-way pin headers, 2.54mm pitch (JP1-JP3) 3 jumper shunts (JP1-JP3) 1 20-pin DIL IC socket (for IC1) Hardware 2 M4 x 6mm panhead machine screws and nuts (for relay mounting) 2 M3 x 10mm panhead Nylon machine screws (for IEC connector mounting) 6 M3 x 6mm panhead machine screws 4 M3 nuts 2 150mm cable ties (to hold down transformer) Wiring 1 20mm length of 3mm diameter red heatshrink tubing 1 400mm length of 10A light blue mains-rated wire ● 1 400mm length of 10A brown mains-rated wire ● 1 200mm length of 10A green/yellow mains-rated wire ● 1 400mm length of 7.5A brown mains-rated wire 1 170mm length of 1mm diameter enamelled copper wire 1 50mm length of 10mm diameter red heatshrink tubing 1 100mm length of 5mm diameter red heatshrink tubing 1 25mm length of 5mm diameter blue heatshrink tubing 1 25mm length of 5mm diameter green heatshrink tubing 12 100mm cable ties ● can be stripped from a length of 3-core 10A mains flex siliconchip.com.au Semiconductors 1 PIC16F1459-I/P microcontroller, DIP-20, programmed with 1010921R.HEX 1 7805 5V 1A regulator, TO-220 (REG1) 1 7812 12V 1A regulator, TO-220 (REG2) 1 BC337 500mA NPN transistor, TO-92 (Q1) 5 1N4004 400V 1A diodes, DO-41 (D1-D5) 2 3mm high-brightness red LEDs (LED1, LED2) Capacitors 1 470μF 25V PC electrolytic 1 100μF 25V PC electrolytic 2 100μF 16V PC electrolytic 2 10μF 16V PC electrolytic 2 100nF MKT polyester (code 104 or 100n) Resistors (all 1/4W, 1% metal film) 1 22kW 5 10kW 1 1kW 2 560W 1 330W 1 10kW miniature single turn top-adjust trimpot (code 103) (VR1) 1 10kW top-adjust multi-turn trimpot (code 103) (VR2) Transmitter (up to 16 per receiver) 1 double-sided plated-through PCB coded 10109212, 30 x 45mm, 1.0mm thick 1 RF Solutions ENCL_KIT3 3-switch key-fob enclosure [RS Components 4510674, Mouser 223-ENCL-KIT3] 1 Renata HU-2032-LF PCB-mount cell holder (BAT1) [element14 1319749, Mouser 614-HU2032-LF] 1 CR2032 3V lithium cell (BAT1) 1 433.9MHz UHF ASK transmitter (TX1) [Jaycar ZW3100, Altronics Z6900 or equivalent] 3 SPST two-pin momentary PCB-mount tactile switches (S1-S3) [Jaycar SP0611, Altronics S1127] 1 PIC16LF15323-I/SL microcontroller, SOIC-14, programmed with 1010921A.HEX (IC1) 1 3mm high-brightness red LED (LED1) 2 100nF 50V through-hole ceramic 1 220W 1% SMD resistor, M3216/1206 size 1 162mm length of 0.5mm diameter enamelled copper wire Resistor Colour Codes Australia's electronics magazine July 2022  77