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433MHz Wireless Data Range Extender There are many ‘remote control’ devices which rely on a 433MHz data link. You may have one and not even realise it – an alarm remote, a garage door/gate controller or even an outdoor weather station are just some examples. But is yours 100% reliable? Is the range a bit less than you’d like? Perhaps the remote unit is too far away from the receiver – or are there obstacles in the way? Here’s the answer: a small, solar-powered repeater placed between the transmitter and receiver with clear line-of-sight to both, giving you the reliability and extra range you need. by John Clarke T here are quite a few devices Using a commercially made whip which is also a better location for recepwhich transmit periodic bursts tion by the receiving unit (ie, placed in antenna for the transmitter and/or receiver can improve the range compared of data on the 433MHz UHF between the two devices). It stores the received data and then, to the simple wire antenna, as can a ‘LIPD’ band, including a number of our designs, such as the Driveway Monitor after a short delay, re-transmits the longer half-wave antenna (340mm for same signal in the same frequency 433MHz). (see July and August 2016 issues). But we must caution you that if This includes some commercial band. So this design is suitable for exdevices too, such as remote weather tending the wireless range by up to two your transmitter is close to the 25mW stations. Unfortunately, it isn’t always times, where line-of-sight transmission legal limit (10mW, UK), using a better is possible. But it’s also extremely ef- antenna (with higher gain) may be ilpossible to get reliable reception. Sometimes this is because there are fective at improving the signal integrity legal. That’s because 25mW (10mW, hills, trees or buildings between the where the two units have obstructions UK) is the effective radiated power between them, including buildings, limit, so it takes into account antenna transmitter and receiver locations. gain. Each increase in antenna gain Other times, it’s because of limited trees and terrain. of 3dB is equivalent to doubling the antenna sizes or the 25mW legal limit output power. (10mW in the UK – see panel oppo- Other things to try first So you cannot legally use a +3dBi ansite) placed on unlicensed devices Before building a Repeater, there are operating in this band (many 433MHz some simple ways to improve range tenna with devices that exceed 12.5mW transmitters are far weaker than this). that may give you what you need. The (5mW, UK) transmit power. Antenna orientation is important too. Even the weather can have an impact: first step is to try a better antenna. Typically, our projects use a short Having the transmitter and receiver a shrub or tree that has little to no effect in dry weather can play havoc with length of wire as the antenna, sized antenna both with the same orientato be one-quarter of the wavelength. tion, eg, both oriented vertically or UHF signals in the wet. While 433MHz signals aren’t at- This is around 170mm for a 433MHz both horizontally may improve range. If these changes prove to be imtenuated as much as higher frequen- transmitter or receiver. practical or not effective cies (eg, 2.4GHz, enough, then it would which is also used Features make sense for you to for data), if you’re  Extends the range of 433MHz transmitters build this Repeater. A suffering from mar Overcomes ‘line-of-sight’ limitations caused by trees and obstacles Repeater is placed in the ginal signal any Receives 433MHz signal and re-transmits at 433MHz after a short delay signal path between the way, it could be  Suitable for use with projects that transmit intermittent signal bursts transmitter and receiver. enough to stop data  Discrimination of genuine signal from noise In this case, the Repeater getting through. comprises a UHF receiver This Repeater can  Repeater chaining possible (with caution) and UHF transmitter, plus be placed in a loca Adjustable delay period a microcontroller, some tion where it can  Adjustable maximum data rate detection memory and a power supclearly and reliably  Solar power with LiFePO4 cell storage ply. Once the Repeater receive signals from  Up to 200m open-space range with optimised antenna receives a valid signal, it the transmitter, and 16 Practical Electronics | May | 2020 With a solar panel to keep the internal battery charged, you’ll never have to touch it once completed. Get up to double the range you had originally! is stored and then after a delay, retransmitted, to be received by the receiver. This effectively increases the range for the transmission as it can be placed Is this 433MHz repeater legal to use without a license? Australia and New Zealand In a word, yes. You can view the ‘LIPD’ class license for the 433MHz ‘ISM’ band, which applies to everyone in Australia, at: www.acma.gov.au/ Industry/Spectrum/Radiocommslicensing/Class-licences/lipd-classlicence-spectrum-acma The equivalent New Zealand document is at: https://gazette.govt. nz/notice/id/2017-go4089 Note that the New Zealand EIRP limit of −16dBm is the same as the Australian limit of 25mW. It is simply specified in different units. Neither of these documents place any restrictions on the use of the 433/434MHz LIPD band other than the maximum effective radiated power. There is nothing to limit how frequently you may transmit in that band, or how long the bursts can be. And there is no mention of repeaters whatsoever. Since our repeater uses commercially available 433MHz transmitters, which comply with the power limit, and since it only transmits after the original transmission has ceased, it is entirely legal to operate in Australia and New Zealand. However, we do not recommend that you use this repeater with any signals which transmit frequently. Typically, you would use it in conjunction with a device that sends a short burst (well under one second) no more frequently than, say, once every 30 seconds. If you used it with a device Practical Electronics | May | 2020 closer to both the transmitter and receiver than they are to each other, and possibly in a more advantageous location (eg, higher up) where there will be fewer obstacles in the way of both signal paths. Other types of repeaters exist, which operate slightly differently to this one. For example, many repeaters retransmit the received signal on a different frequency. That prevents conflicts between the transmitter and receiver and allows the repeater to operate with effectively no delay. But the final receiver must be able to receive on the new frequency, so that type of repeater is not ‘transparent’. transmitting rapidly, you could blanket the 433MHz band with transmissions in a 100-200m radius. The Class License states that: ‘If interference occurs, the onus is on the user of a LIPD to take measures to resolve that interference, for example by retuning or ceasing to operate the device.’ (Retuning these devices would be difficult, if not impossible, without specialised equipment). So keep that in mind, and use common sense when setting up your transmitting device and repeater(s). In particular, stick to an aerial that is quarter wavelength – no longer. It is the responsiblity of the constructor to behave responsibly. Be aware of rules, don’t break them. From the PE editor My thanks to a long-term reader and a PE columnist for providing advice for using this project in the UK. United Kingdom In the United Kingdom, LPD433 (https:// en.wikipedia.org/wiki/LPD433) equipment that meets the respective Ofcom Interface Requirement can be used for model control, analogue/ digitised voice, low-cost home automation and remote keyless entry systems. There is significant scope for interference however, both on frequency and on adjacent frequencies, as the band is widely used Ofcom, together with the RSGB (Radio Society of Great Britain) Emerging Technology Co-ordination Committee This Repeater retransmits in the same frequency band as the received signal. That means that the final receiver does not need to be modified in any way. But the Repeater has to wait for the end of the transmission before resending. Otherwise, the received and transmitted signals will interfere, and the receiver could even go into a loop, continually retransmitting the same data. Compatible projects Some of the projects we have previously published that can benefit from using this Repeater include:  UHF Remote Switch (January 2011)  Versatile 10-Channel Remote Control Receiver (June 2014)  Driveway Monitor / Infrared to 433MHz UHF Transceiver (July 2014). All the projects mentioned above used the standard 433MHz UHF transmitters and receivers sold by Jaycar and Altronics (as shown overleaf). have produced guidelines to help mitigate the side effects of interference to an extent. (http://bit.ly/pe-may20ofcom1). UK power limitations In IR 2030 – UK Interface Requirements 2030 (http://bit.ly/pe-may20ofcom2), Ofcom states (p.18) that applications must limit maximum power to 10mW, speech is not allowed and users must stick to a 10% duty cycle (ie, you must not hog the band, blocking other people’s transmissions, you must be off the air for nine-times longer than you are on the air). Do not make the common mistake of believing this is an unlicensed spectrum in the UK – there is no such thing. Bands like 433MHz are ‘license exempt’. This means if you stick to the license published by Ofcom you don’t need a specific licence, but there are still rules to follow. Non-compliance is a criminal offence and Ofcom can (and do) prosecute (usually when something is interfering with some other user). The RSGB outlines (http://bit.ly/pemay20-rsgb) the modes in which radio amateurs can use 433MHz frequencies. Bottom line Used sensibly this project can be operated legally in the UK if you use legal modules. Also, do stick to the quarter-wave short antenna wires. (NB, linking repeaters in a chain might be considered unsportsmanlike and possibly illegal.) 17 The 433MHz Data Repeater is based on commercial transmitter and receiver modules, as shown here. The Jaycar ZW3100 transmitter and ZW3102 receiver are shown on the left with the Altronics Z6900 transmitter and Z6905 receiver at right. They are for all intents and purposes identical; either will fit directly into our PCB. The Jaycar catalog codes are ZW3100 for the transmitter and ZW3102 for the receiver, while the Altronics catalogue codes are Z6900 for the transmitter and Z6905 for the receiver. This Repeater may be able to be used with some other commercial devices transmitting data in the 433MHz band; however, whether it will work depends on the details of those transmissions, so it’s hard to say that a particular device will or will not work until you try it. Keep in mind that you need to use the Repeater in situations where it doesn’t matter if the receiver could receive two identical packets in a short period. That’s because it may pick up both the direct transmission and the repeated transmission in some cases. In all the projects mentioned above, this should not matter, as the receivers are effectively ‘stateless’. That should be true of many other devices such as weather stations. But again, you will need to try it out to confirm that the receiver’s operation is not adversely affected by receiving multiple identical packets. Presentation Our Repeater is housed in an IP65 sealed box and that means it is suitable for use outdoors, in areas where it could be exposed to the weather. It is designed to be powered from a solar panel and uses a single-cell LiFePO4 rechargeable cell for power storage, so it can be used where there is no mains power available. This is ideal, as you can, for example, mount it up on a pole, where it will have a good ‘view’ of both the transmitting and receiving units, and it should also get plenty of sunlight to keep the battery charged. Circuit details The circuit diagram of the Repeater is shown in Fig.1. It’s based around microcontroller IC1, the previously mentioned 433MHz transmitter (TX1) and receiver (RX1), a 1024kbit/128kbyte static RAM (IC2), plus power supply parts such as the LiFePO4 charger (IC3) and 5V step-up regulator (REG2). Microcontroller IC1 monitors the signal from the UHF receiver (RX1), stores the received data in the SRAM (IC2) and then powers up the UHF transmitter (TX1) to send out the stored code that was previously received. IC1 also has two trimpots (VR1 and VR2) that are used to set the maximum data rate and the minimum retransmission delay; more on that later. Receiver RX1 is powered continuously from the 5V supply so that it can receive a signal at any time. When Screen1: the yellow trace at the top is the output from the UHF receiver, RX1. You can see the high-frequency noise before valid data is received. When there is a received signal, the random signal ceases and the transmitted code is produced instead. IC1 rejects the noise and only accepts the valid code, as shown in the cyan trace below. 18 there is no signal to be received, its data output pin delivers a high-frequency random (noise) signal. That is due to the automatic gain control (AGC) in the receiver increasing gain until it is receiving a signal, even if that signal is just amplified noise. When there is an actual 433MHz signal to receive, the AGC reduces the receiver’s gain to prevent internal clipping, ie, so it is not overloaded due to excessive gain. Since the AGC gain varies at a relatively slow rate, when the 433MHz signal transmission stops, the receiver output will be low for a few hundred milliseconds before the AGC action increases the gain sufficiently to produce noise again. The 433MHz transmitter and receiver use an elementary modulation system, known as ‘amplitude shift keying’, or ASK. When its input is high (one), the transmitter produces a 433MHz carrier. When its input is low (zero), the 433MHz carrier transmission stops. The data rate is usually fast enough that the receiver gain does not vary significantly during the burst, even though during the zero bits, there is no carrier. There are various schemes which exist to avoid having long periods of all 0s or all 1s, regardless of the data being transmitted, to help in cases like Screen2: the yellow trace at the top shows the original signal being received from the source, while the cyan trace at the bottom shows the signal being transmitted by the repeater. You can see how it does not start transmission until it has finished receiving an entire packet, and there is a short delay before retransmission, around 60ms in this case. Practical Electronics | May | 2020 433MHz Data Repeater Fig.1: the Repeater circuit – data transmissions are picked up by UHF receiver RX1 and fed to microcontroller IC1’s RB0 input. They are then stored in SRAM IC2, and once the transmission is complete, read back out of the SRAM and sent on to UHF transmitter TX1. IC1 then waits for a programmable delay before listening for another transmission. Power from the rechargeable LiFePO4 cell is stepped up to 5V by REG2, and that cell is charged from a solar panel using charge-management chip IC3. this. One such scheme is Manchester encoding, where each bit is encoded as either a low (0) then a high (1), or a high (1) then a low (0), at a fixed rate. The UHF transmitter and receiver pair can transfer data at up to 5kbits/second using Manchester encoding. Distinguishing signal from noise The receiver’s AGC action poses challenges for our software, since it needs to be able to distinguish a series of zeros and ones that form part of a genuine data transmission from the zeros and ones that result from the Practical Electronics | May | 2020 amplified noise in the receiver, when there is no signal present. IC1 monitors the signal from the UHF receiver at its RB0 digital input (pin 6). Each time the voltage level changes, it decides whether it is just due to noise or a valid data signal. Valid data is determined by comparing the received data rate to the maximum rate setting. This is set using VR1, which also varies the voltage at test point TP1. With TP1 at 0V, the maximum data rate is 233bps, and with TP1 at 5V, the maximum data rate is 5kpbs. Intermediate voltages give intermediate maximum rate values. If the incoming data rate is higher than the rate setting of VR1, the data is assumed to be noise and is rejected as invalid (see Screen1). If the data rate is less than the maximum data rate setting, the data is considered valid and so it is stored in memory. As soon as the data rate exceeds the maximum rate setting, it is assumed that the transmission is complete and so the data which has been stored is then transmitted. This is done by reading the data out of the RAM and feeding it to digital output 19 The circuit is powered from a single 600mAH LiFePO4 cell. The quiescent current draw is around 9.4mA (when the transmitter, memory and LEDs are off). This means the cell will last for around 60 hours or 2.5 days when fully charged. Fig.2: this shows how the boost converter generates 5V to run the micro and UHF transmitter and receiver from the 3.2-4.2V cell. The control circuit pulses the base of internal transistor Q1, which pulls current from the cell through inductor L1, charging up its magnetic field. When Q1 switches off, that magnetic field collapses, D1 is forward-biased and CL charges up to 5V. This is regulated by feedback to the control circuit via the voltage divider formed by trimpot VR3 and a 10kΩ resistor. RA4 (pin 3) of IC1 at the same rate that it was received. At the same time, TX1, the UHF transmitter is powered up and transmits this stored data (see Screen2). IC2 is the memory that is used to store the data. It is a 1024kbit random access memory organised as 128k × 8-bit bytes. The memory is read and written via a Serial Peripheral Interface (SPI). When writing, data is sent to the SI input of IC2 (pin 5) from the SDO (pin 8) output of IC1, one byte at a time. When reading, data is sent from the SO output of IC2 (pin 2) to the SDI input (pin 7) of IC1; again, one byte at a time. In both cases, the data is clocked by a signal from the SCK (pin 10) of IC1, which is fed to the SCK input of IC2 (pin 6). The memory SPI interface is enabled by a low level at the chip select (CS) input (pin 1) of IC2, which is driven from the RB3 digital output of IC1 (pin 9). To write to the memory, the CS line is brought low and then a write instruction is sent from IC1 to IC2, followed by the memory address to write to. In our application, this is always the first location (address zero). This is a 24-bit address sent as three 8-bit bytes. The seven most-significant address bits are always zero since only 17 bits are required to address the 128k bytes in the RAM. Following this, data can be written, one byte at a time. By default, the address is automatically incremented after each byte of data is written, so bytes are written sequentially to the RAM. We store the received data as 16-bit values. The most-significant bit (bit 15) indicates the received level, low (0) or high (1). The remaining 15 bits are used to store the duration that the data stayed at that level. This period is stored in increments of 4µs, resulting in a 4µs minimum period and 131ms maximum. Reading data out of the memory is a similar process to writing, except that a different instruction is used and the 20 data is sent in the opposite direction, from IC2 to IC1. Power saving features Since we are powering the Repeater using solar panels and a small cell for storage, its power consumption must be minimised, especially when idle and waiting for data. This is done by switching off power to components when they are not required. The two trimpots, VR1 and VR2, are only connected to the 5V supply when their positions are being read. This is done only during the initial power-up process and when switch S1 is pressed. Any other time, the RA6 and RA7 pins that supply 5V to the trimpots are low (0V), to prevent current flow through the pots. This saves 1mA, which adds up to 24mAh per day. Similarly, the transmitter (TX1) is off until it is required to send a UHF signal. TX1 is powered directly from IC1’s RA2 and RA3 digital outputs (pins 1 and 2); these go high (to 5V) to power TX1. The power saving is considerable since TX1 draws some 10mA when powered and transmitting. This saves 240mAh/day. IC2 is on standby unless it is being used. So unless there is a valid data being received, it draws just 10µA instead of the 10mA required when it is active. Typically, the memory is only powered twice as long as the transmitter; the first half being the reception period and the second half being the transmission period. This also saves around 240mAh/day. Microcontroller IC1 typically draws 1.7mA and UHF receiver RX1 draws 2.9mA. The transmit and receive LEDs are powered when TX1 and RX1 are active respectively, and draw about 3mA. The LEDs can be disconnected using a jumper shunt (JP1) to save power if they are not needed. They are mainly provided for testing purposes. Charging circuitry The LiFePO4 cell is charged by IC3, which is powered from a 5V regulator (REG1) and this, in turn, is powered from a solar panel. Note that it connects to the circuit via a fuse (F1), which prevents damage if the cell is inserted incorrectly. If the cell is reversed, current will flow through diode D2 and blow the fuse. Diode D1 prevents damage if the solar panel is accidentally connected with the wrong polarity. IC3 is a miniature single-cell integrated Li-ion/LiPo charge management controller. It charges the cell at a constant current, up to a charge termination voltage of 4.2V. The charge current is set by the resistance at pin 5, and for our circuit, this is set to 100mA by the 10kΩ resistor. The charge LED (LED3) lights when the cell is charging. The 433MHz UHF transmitter (TX1) and receiver (RX1) can operate from 2.5-5V. Since the transmitter will have more output power and thus a better range when powered from 5V, rather than the 3.2-4.2V from the LiFePO4 cell, we use a step-up (boost) regulator to generate 5V to power these modules. However, the circuit can be built without this step-up regulator, if maximum range is not required. This saves time and money. The rest of the circuit will then be powered directly from the LiFePO4 cell. This would also extend the cell life as the step-up regulator is only around 70% efficient, and the lower supply voltage will also mean that less current is drawn by IC1, IC2, TX1 and RX1. Jumper link JP2 is used to select whether these components are powered from the 5V boosted supply, or directly from the cell. The voltage step-up is performed by TL499A switching regulator REG2. It comprises a switching control circuit, a transistor and a series diode. It requires inductor L1 to perform the boost function and a 470µF low-ESR output capacitor for energy storage and filtering. A simplified circuit showing the operation of the boost converter is shown in Fig.2. Initially, internal transistor Q1 is on and current flow begins to build through inductor L1 (at a rate limited by its inductance) until it reaches a particular value. This maximum current is set by the resistor connected to pin 4 of REG2. When Q1 switches off, L1’s magnetic field collapses and so current continues Practical Electronics | May | 2020 to flow to the load and output capacitor (CL) via diode D1. This current flow causes a voltage to appear across L1, which adds to the supply voltage (VIN), charging CL up to a higher voltage than the input supply. The process continues with Q1 switching on again, once L1’s magnetic field has mostly dissipated, and thus the field builds back up until Q1 switches off again. The output voltage is sampled via a voltage divider comprising trimpot VR3 and a 10kΩ resistor. This determines the proportion of the output voltage applied to pin 2 of REG2, which it compares against an internal 1.26V reference. The duty cycle of Q1 is controlled to maintain 1.26V at the pin 2 input. Therefore, by changing the resistance of VR3, we can vary the output voltage. The greater the attenuation of this resistive divider, the higher the output voltage must be to maintain 1.26V at pin 2. If VR3 is set to 29.68kΩ, the divider formed with the 10kΩ resistor reduces the output by a factor of 3.97. That means that the output will be 3.97 × 1.26V = 5V. Should the output voltage rise slightly above 5V, the TL499A will cease switching Q1 until the voltage falls slightly below the 5V level. Should the voltage fall below 5V, the transistor will be driven with a higher duty cycle, to deliver more current to the output and bring it back up to 5V. Note that the 1.26V reference is only a nominal value and could be any voltage between 1.20V to 1.32V, depending on the particular IC. So VR3 makes it adjustable, to allow the output voltage to be set accurately. Chaining multiple repeaters As mentioned in the features panel, it is possible to have more than one Repeater, to extend the transmission range further – this is not recommended for UK operation unless you know what you are doing and how to avoid interefering with neighbouring signals. The Repeater closest to the source (original transmitter) will send the signal on to the second Repeater. When the second Repeater sends out its signal, the first Repeater must ignore it; otherwise the two Repeaters will endlessly retransmit the same packet. This is prevented by an adjustable delay between the end of each transmission and the unit accepting a new packet. This delay ranges from 50ms to 12.5s and is set using VR2. The voltage at TP2 indicates the delay setting, with each volt representing 2.5s. So for example, if VR2 is adjusted for 2V at TP2 then the delay is 2.5s × 2 = 5s. 0V gives a 50ms (minimum) delay. Construction The Repeater is built using a double-sided PCB coded 15004191, which measures 103.5 × 78mm and is available from the PE PCB Service. It fits in an IP65 sealed box measuring 115 × 90 × 55mm. Use the PCB overlay diagram, shown in Fig.3, as a guide during assembly. Start by soldering the battery charger, IC3. This is in a small five-pin SMD package. The correct orientation is evident since it has two pins on one side and three on the other. Tack solder one of the pins (ideally, at upper right) then check its orientation and solder the diagonally opposite pin. Then proceed to solder the remaining pins, and refresh the first joint with a bit of added solder or flux gel. If you accidentally bridge the three pins which are close together, add a little flux paste and then clean up the bridge with the application of some solder wick. The PCB has the option to use a DIP (through-hole) or SOIC (SMD) package for the memory chip (IC2). Only one type should be installed. If using the SOIC package, solder it next, using a similar procedure as described above. But first, make sure that its pin 1 dot or divot is at upper left, as shown in Fig.3. It should also have a bevelled edge on the pin 1 side. Practical Electronics | May | 2020 Parts list – 433MHz Wireless Data Repeater 1 double-sided PCB coded 15004191, 103.5 x 78mm, available from the PE PCB Service 1 IP65 enclosure, 115 x 90 x 55mm [Jaycar HB6216] 1 600mAh LiFePO4 cell (AA sized: 50mm diameter, 14mm long) [Jaycar SB2305] 1 12V 5W solar panel [Jaycar ZM9050] 1 panel label (see text) 1 15 x 8 x 6.5mm powdered iron toroid (L1) [Jaycar LO1242] 1 433MHz ASK transmitter (TX1) [Altronics Z6900, Jaycar ZW3100] 1 433MHz ASK receiver (RX1) [Altronics Z6905, Jaycar ZW3102] 1 PCB-mount tactile momentary SPST pushbutton switch (S1) [Altronics S1120, Jaycar SP0600] 1 2-way screw terminal with 5.08mm spacing (CON1) 1 2-pin header, 2.54mm spacing (JP1) 1 3-pin header, 2.54mm spacing (JP2) 2 shorting blocks/jumper shunts (JP1,JP2) 1 1A M205 fuse (F1) 2 PCB-mount M205 fuse clips (F1) 1 18-pin DIL IC socket (for IC1) 1-2 8-pin DIL IC sockets (optional; for IC2 and REG2) 1 PCB-mount AA cell holder 1 flag heatsink, 19 x 19 x 9.5mm [Altronics H0630, Jaycar HH8502] 1 IP65 cable gland to suit 3-6.5mm diameter cable 6 PC stakes (optional) 4 M3 x 5mm panhead machine screws 1 M3 x 6mm panhead machine screw 1 M3 hex nut 2 4.75mm long #0 panhead self-tapping screws 2 100mm cable ties 1 500mm length of 1mm diameter enamelled copper wire 2 175mm lengths of medium-duty hookup wire OR 2 175mm length of 1mm diameter enamelled copper wire (see text) Semiconductors 1 PIC16F88-I/P 8-bit microcontroller programmed with 1500419A.HEX (IC1) 1 23LCV1024-I/P 128kB SRAM in PDIP package (IC2) [Mouser, Digi-Key] OR 1 23LCV1024-I/SN 128kB SRAM in SOIC package (IC2) [Mouser, Digi-Key] 1 MCP73831T-2ACI/OT single cell Li-ion/LiFePO4 charger, SOT-23-5 (IC3) [Mouser, Digi-Key] 1 TL499A power supply controller (REG2) [Jaycar Cat ZV1644] 1 7805 5V regulator (REG1) 1 1N4004 1A diode (D1) 1 1N5404 3A diode (D2) 1 Green 3mm high-brightness LED (LED1) 1 Red 3mm high-brightness LED (LED2) 1 Yellow 3mm high-brightness LED (LED3) Capacitors 2 470µF 16V low-ESR electrolytic 1 100µF 16V electrolytic 1 10µF 16V electrolytic 1 470nF 63V MKT polyester 1 220nF 63V MKT polyester 2 100nF 63V MKT polyester 1 100nF multi-layer ceramic 1 10nF 63V MKT polyester (code 0.47, 474 or 470n) (code 0.22, 224 or 220n) (code 0.1, 104 or 100n) (code 0.01, 103 or 10n) Resistors (all 0.25W, 1% metal film) 3 10kΩ 4 1kΩ 1 330Ω 2 10kΩ miniature horizontal mount trimpots (VR1,VR2) 1 50kΩ miniature horizontal mount trimpot (VR3) 21 Fig.3: this PCB overlay diagram and the photo below show how to fit the components on the board. There are two possible locations for IC2, depending on if you’re using through-hole (DIP) or SMD (SOIC) versions. Be careful to orient the diodes, ICs, cell holder, transmitter and receiver correctly, as shown here. Some components can be left off if the solar battery charging function is not needed (see text). The SOIC package for IC2 is larger than that of IC3, so you should find it a little easier. Again, any accidental bridges can be cleaned up with flux paste and solder wick. Install the resistors next. A digital multimeter should be used to check the resistor values, as the colour codes can be hard to read. Fit the diodes next, making sure to insert them with the correct polarity, ie, with the striped ends facing as shown in the overlay diagram. D2 is considerably larger than D1. We recommend soldering an IC socket for IC1. The remaining ICs (including IC2, if using the DIP package version) can be fitted via an IC socket or soldered directly in place, which would give better long-term reliability. 22 Take care with orientation when installing the socket(s) and ICs. Additionally, make sure that IC2 and REG2 are not mixed up. Next, there are six optional PC stakes to install, which make wiring connections and test point monitoring easier. These are located at TP5V, GND, TP1, TP2 and one each for the antenna connection of RX1 and TX1. The capacitors should be mounted next, starting with the 100nF multilayer ceramic capacitor next to UHF receiver RX1, then following with the MKT polyester types, none of which are polarised. Follow these with the electrolytic types, which must be installed with the polarity shown; the longer lead goes into the pads marked with a ‘+’ sign, towards the top of the PCB. REG1 can be now fitted. It is mounted horizontally on a heatsink. Bend the leads so they fit the PCB holes while the mounting hole lines up with the hole on the PCB. Sandwich the heatsink between the regulator and PCB and do up the screw and nut before soldering the leads. Trimpots VR1 to VR3 are next. VR1 and VR2 are 10kΩ and would typically be marked with 103. VR3 is 50kΩ and may be marked as 503. Then install the LEDs, LED1 to LED3. In each case, the anode (longer lead) goes to the pad marked with an ‘A’ on the PCB. The bottom of the LEDs should be about 5mm above the PCB surface when soldered in place. You can then fit pushbutton switch S1. Install the 3-way and 2-way SIL headers now, for JP1 and JP2. Then fit the 2-way screw terminal, CON1, with the wire entry holes end toward the bottom PCB edge. L1 is wound using 17 turns of 1mm enamelled copper wire on a 25mmdiameter powdered iron toroidal core. These turns should be wound neatly around the perimeter, as shown in Fig.3. Remove the enamel from the ends of the wires using a hobby knife so you can tin them and then solder them to the PCB pads shown. The core is held in place with two cable ties that loop through PCB holes, as shown. The battery holder must be oriented as shown (red wire to +) and secured to the PCB using two self-tapping screws through the cell holder and into the slotted holes on the PCB. Cut the wires from the battery short and terminate them to the PCB. Insert the fuse clips for F1, making sure that the end stops in the clips are facing to the outside. Before soldering them, insert the fuse so that the clips are correctly aligned, for good contact with the fuse. Finally, the UHF transmitter and receiver can be mounted. These must also be oriented correctly. The pin markings are printed on the transmitter module. Orient the antenna pin connection on the transmitter and receiver so that they are adjacent to the antenna connections on the PCB. You have two options for the antennas: either use 170mm lengths of hookup wire coiled inside the box or, for better range (>40m), 170mm-long lengths of stiff enamelled copper wire protruding from the box. The extra 5mm in the lengths specified in the parts list is to give you enough wire to solder to the antenna terminals (for the hookup wire) or to bend over at the tip (for the enamelled copper wire). Practical Electronics | May | 2020 Having chosen which antenna wire you want to use, cut the appropriate lengths and solder them to the antenna PC stakes, or directly to the antenna pads if you are not using PC stakes. Note that you will need to scrape some insulation off the end of the enamelled copper wire (eg, with a hobby knife) so that you can tin and then solder it to the board. Mounting it in the box There is not much work required to mount the board in the box. We drilled a hole in the side for the cable gland required for the solar panel wiring. This hole is 25mm up from the outside base of the case opposite CON1. If you only require a UHF transmission range of less than 40m, the antenna wires can be bent around the inside perimeter of the box. For maximum transmission range (up to 200m), the stiff receiver antenna wire should pass through a small hole in the upper edge of the box, and the receiver wire similarly should pass through a small hole in the lower edge of the box. Once it’s through, bend the tips over to form small 3mm loops. That prevents you poking your eye out on the otherwise sharp end. 1mm wire is used so that the wire is stiff enough to stay straight. The wire exit holes should then be sealed with a neutral cure silicone sealant. The Repeater PCB is held inside the case by M3 screws that go into the integral threaded bushes in the base of the box. The Neoprene seal for the lid needs to be placed inside the surround channel and then cut to size. The start and finish gap in this seal should be along the lower long edge of the lid. Labelling it To produce a front-panel label, you have several options. For a rugged label, mirror the design and print it onto clear overhead projector film (using film suitable for your type of printer). This way, the ink will be on the back of the film when the label is affixed. Attach with clear silicone sealant. Solar panel or mains power We used a 12V 5W solar panel to power the unit. A 6V panel would be more efficient, since we are reducing the voltage down to 5V. However, 6V panels aren’t easy to find. The panel power rating only needs to be 1W. If you want to run the unit from mains power, a 9V plugpack could be connected to CON1 instead. Make sure the plugpack is out of the weather, with only the low voltage wires going to the Repeater. In this case, IC3 and the LiFePO4 cell are not required, although you could leave them in so that the unit will run even during power outages (assuming the transmitting and receiving units are also battery-powered). If you’re leaving off IC3, you could also omit F1, D2, LED3 as well as IC4 and its associated parts. The 5V output from REG1 could then be directly used to power the circuit by connecting a wire link from the regulator output to the 5V terminal at JP2. Setting up It is essential that the shunt is not placed on JP2 until VR3 is adjusted for 5V at the output of IC4. To do this, insert the LiFePO4 cell into the holder and measure the voltage between the GND and TP5V PC stakes. Adjust VR2 for a reading of 5V. Reproduced by arrangement with SILICON CHIP magazine 2020. www.siliconchip.com.au Installation The Repeater should be mounted in a location that will give good reception of the original UHF signal. The LED indicators (LED1, LED2) will let you know if the signal is received and retransmitted if a shunt is installed on JP1. VR1 must be adjusted so that the receive LED does not flash at all, or at least not too often, when no signal is being received. But if it’s adjusted too far, the Repeater will not work, so you need to check that it is still retransmitting valid data. To achieve this, initially set VR1 fully clockwise and press S1 so that the VR1 setting is updated. More of the random signal noise will now be detected and the receive LED will flash now and then, followed by the transmit LED. Adjust VR1 anticlockwise a few degrees and press S1 to again update the setting. Check that the Repeater retransmits correctly. If the Repeater operates correctly, try further anticlockwise adjustment. The final adjustment will be a compromise between reliable Repeater operation and noise rejection from the UHF receiver. Adjusting VR1 too far anticlockwise will prevent successful Repeater operation. VR2 should be set fully anticlockwise if you are using a single Repeater. If you are using multiple Repeaters, set VR2 on all Repeaters fully clockwise, giving a 12.5s delay. If your transmitter can send signals more often than this, you will need to experiment with the maximum clockwise rotation of VR2 that will still cause all valid packets to be relayed. Remember that the settings for the VR1 and VR2 trimpots are only read by IC1 when first powered up and when S1 is pressed. LED1 and LED2 light when S1 is pressed, to acknowledge that the settings have been updated. Once you’ve finished adjusting VR1 and VR2, you will need to check whether the ultimate receiver is correctly decoding the retransmitted code from the Repeater(s). If not, you may need to move them. You can then permanently mount the Repeater(s). This is done using the mounting holes provided in the box corners. These holes are accessible when the box lid is removed. Alternatively, you could use a bracket and attached it to the box using the box mounting holes. Avoid drilling extra holes in the box as this could compromise its water-tight seal. Repeater mounted in its waterproof case. The blue and yellow wires are the 170mm-long transmitting / receiving antennas – they can be left ‘floating’ in the case, but ensure there are no bare ends to short to components or the PCB. Practical Electronics | May | 2020 23