Miniature PCM Remote Control

Transcription

1 Miniature PCM Remote Control part 1: transmit at 433 MHz and 950 nm! Here s a design that should gladden the hearts of many model builders. A really small proportional remote control unit built using standard - low-cost components. It s ideal for loads of applications and so flexible that simply by changing on-board jumpers you are free to choose the transmission medium: radio, Infrared or two-wire! 12 Elektor Electronics 10/2001

2 Remote control main features: Lightweight Simple construction with no set-up or alignment 6 channels 4 proportional and 2 digital Switched outputs for 5 A load Four standard servo connectors Trim function on 4 channels Small, light and compact PCM transmission method Selectable Infrared or radio operation Current saving transmission method Type-approved and licence-exempt 433MHz Tx and Rx modules Voltage supervision in both the transmitter and receiver Built-in speed regulator in receiver Built-in soft start switch in receiver Speed regulator and softstart controller suitable for loads <15A Battery Low indicator Servo reverse (change the servo travel direction) for all 4 servos Operating voltage: transmitter 3.3 V to 4.8 V (3x AA Alkaline cells or 4x Nicad cells) Receiver 5 V from on-board BEC voltage regulator 4 potentiometers Microcontroller It used to be that constructional projects using RF transmitters and receivers would only be attempted by the more courageous electronics enthusiasts. Thankfully since the advent of complete integrated RF modules we can forget the tedious set-up and adjustment process and just treat the RF stage as another plug-in building block. The design described here is very flexible and can use these RF modules to implement a radio remote control. Microcontrollers are also employed in the transmitter and receiver to replace the discrete shift registers, clock generators and timing circuitry that you normally find in remote control designs. The result is a compact, reliable and low cost remote control unit. The low radiated power emitted by these licence-exempt radio modules limits the range of the units so this design is only really suitable for controlling indoor models. A speed controller is also built 2 pushbuttons 433 MHz LPD Tx module Infra-red sender into the receiver allowing direct connection to the models drive motor. Using the infrared control option a low-cost 4-channel proportional control system can be built that simply cannot be matched for price and performance by anything available commercially. The twowire control option is useful if the device to be controlled is in a fixed location, for example the receiver could drive two model servos controlling the pan and tilt of a remote security camera. This remote control design is a good demonstration of how microcontrollers can be used to implement features that not so long ago would require dedicated circuitry. The design need not stop here. If, for example you decide that a menu driven transmitter together with a keypad and LCD would better fulfil your needs then it is entirely possible to implement this. Many features of the microcontroller together with unused ROM are just waiting for an inspired programmer to make good use of them. The transmitter The block diagram shown in Figure 1 indicates that a microcontroller does most of the donkey-work. It digitises the analogue resistance values of the four input potentiometers, checks for push button presses, and generates the transmitted signal protocol. The signal can be transmitted at RF or by infrared. Even if you are only intending to use the transmitter at RF it is worth fitting the infrared components to make the finished unit more flexible. The additional cost incurred by fitting the IR diode and its drive transistor is relatively low. During initialisation the transmitters microcontroller will read the state of the input at port pin P0.7. If it is high (jumper JP2 not fitted) then RF control will occur. If it is low (jumper JP2 fitted) then signals will be sent using infrared. When the controller sends infrared signals the driver stage uses more peak current than with the RF option. The controller formats the output signal differently by modulating the Pulse Code Modulation (PCM) data at 36 khz and reducing the data rate in order to prolong battery life. The same modulation is used by remote controllers for TVs etc and gives good transmission security. The components used for the transmitter circuit shown in Figure 2 are low-cost and widely available from numerous suppliers but its worth taking a closer look at some of the more unusual items: Figure 1. Transmitter block diagram The Joystick assembly The remote control system should be small, economical, simple to build and suitable for 10/2001 Elektor Electronics 13

3 +4V5 S3 A1 UT1 3 P1 20k P2 20k P3 20k P4 20k 18k R2 15 R6 D2 R1 470Ω C1 220µ 25V ON/OFF D1 10Ω IR-LED R5 2 Radiometrix TX2 UHF Transmitter P0.0 P P0.1 IC1 P P0.2 P P0.3 P LPC768 9 P0.4 P P0.5 P0.6 P0.7 P1.2 P1.1 P R4 1k T1 TIP110 ZTX603 BT1 4V5 R3 JP1 5 X1 X2 6 7 X1 S S1 S2 P5 P6 P7 P8 10k C2 15p 6MHz C3 15p infrared encoding Figure 2. Transmitter circuit diagram. use with standard model servos from Graupner, Multiplex, Futaba and Ripmax etc. One of the first hurdles in building this proportional remote control system was to track down an economical joystick assembly to use for control input. The US Company CTS produces a suitable assembly. Its miniature joystick is self-centring, has low mechanical play and can optionally have a switch fitted (Figure 3). The most important technical features of this joystick are: Figure 3. Miniature joystick assembly from CTS. 1 Million operating cycles Sturdy metal housing 2 Potentiometers Available potentiometer resistor values between 10 kω and 150 kω Selectable potentiometer resistor tolerance from 10% to 30%. Available with in-built pushbutton switches Switch contact resistance less than 0.1 Ω Switch rating V Push button life: 100,000 operations Unfortunately the joystick control paddle passes through a circular opening in its housing. This has the effect of limiting control when the joystick is moved to the extremes of both the X and Y axis simultaneously. The joystick housing will therefore require a little modification and most competent modellers will have no problem in squaring out the opening to allow maximum deflection of the paddle at all positions. The paddle itself can also be modified to improve the feel of the joystick. On the prototype we used some drilled-out plastic guides from ball-joints to extend the sticks. Standard linear or turn potentiometers can be substituted for the joystick assembly if the combined X- Y control is not necessary in your model. In series with the potentiometers are also trim pots that allow the servo position to be finely adjusted or trimmed out. This feature is useful for example when setting up a model car to ensure that hands off it steers straight ahead. Microcontroller The central control element in the transmitter is the 87LPC768 microcontroller from Philips. This device inputs analogue values from the joy- 14 Elektor Electronics 10/2001

4 Figure 4. The 433 MHz transmitter and receiver modules. The Infrared transmitter The infrared diode driver stage consists of Darlington transistor T1. The transistor is ideal as an LED power driver with a current gain of 2000 and a maximum collector current of 1 A. Two other alternative transistors are the BCX38 and TIPP110 but these have difsticks and pushbuttons and converts them into digital values using its analogue to digital (A/D) converters. A PCM signal is then produced and output to the IR diode or RF module. The microcontroller is based on the well-established Intel 80C51 architecture so for programming there are many low-cost development tools including shareware available. The 87LPC768 is described as a Low power, low price, low pin count Controller 1V23 Figure 5. Internal comparator of the controller. + supply voltage + R2 R microcontroller. A 6 MHz clock is necessary to cope with the processor intensive software routines and this produces a cycle time of 1 µs. The main controller features can be summarised as: 4 Kbytes ROM 128 Bytes RAM 32 Bytes Customer Code EEPROM Operating voltage V Two 16 Bit Timer/Counters 4 channel Pulse Width Modulator (PWM) using 10 Bits 4 channel A/D converter, 8 Bit resolution, conversion time MHz clock frequency integrated reset Selectable internal RC oscillator. 20 ma driver current from output port 18 I/O-Pins maximum, when internal Reset and RC Oscillator is selected 2 analogue comparators I 2 C interface Full duplex UART Serial In-circuit Programming The RF Module In recent times we have seen a big increase in the number of different applications using the 433 MHz band to send data. As well as supplying complete transmitter/receiver solutions some manufacturers produce integrated RF modules that can be used in many applications. The big advantage of these modules is that they do not require any set-up or tuning, they are supplied ready to go. The modules used in this circuit are produced by the company Radiometrix (Figure 4, together with the receiver module). The main features of the transmitter module are summarised in the following table: Transmitter TX2 Frequency 433 MHz, +9 dbm Modulation: FM Data rate 40 kbps max. Operating voltage 4.0 V to 6.0 Vdc Current consumption 6 ma The transmitter RF module has five pins enabling it to be soldered or plugged directly to the PCB. The module will begin transmitting as soon as power is applied provided that the modulation input signal is high. 10/2001 Elektor Electronics 15

5 ferent pin-outs. Resistor R5 limits the diode peak current to approximately 240 ma. This figure was chosen as a trade off between battery life and transmitter range. R1 can be reduced to increase range as long as the peak forward current rating of the diode is not exceeded. The IR LED type TSUS5201 emits IR light at 950 nm with a beam intensity of 230 mw/sr at 1.5 A with a half angle of ±15. In principle any IR LED can be used but if you do intend to use a substitute choose one with a beam intensity >200 mw/sr at 1.5 A and with a wavelength of 950 nm. The IR LED operating current is given by the formula: 433 MHz LPD Rx module Infra-red receiver soft-start switch Microcontroller 4 servos 2 switches I LED = (V BAT V CESAT V F ) / R1 where: V BAT = operating voltage 4.5 V V CESAT 0.7 V V F = forward voltage drop of an infrared diode 1.6 V alternative: speed generator Figure 6. Receiver block diagram A crucial consideration affecting the reliability of the IR signals is the peak current that can be supplied by the battery. It is recommended to use either alkaline or Nicad cells to supply the high impulse current required by the IR driver stage. Cheap zinc-carbon cells may not be able to supply the necessary current. Servo reverse A 4 way DIP switch is conveniently included on the transmitter enabling the direction of travel of any of the M+ R8 M MBR745 M T1 SUP75N03 D2 T4 IRF4905L T5 R5 10k 27k R9 K3 SERVO 3 K2 SERVO 2 K1 SERVO 1 LM2940 IC3 D1 C7 C5 C6 C4 100Ω R4 27k R Ω R1 220µ 25V 470µ 25V 100n 100n C3 10µ 25V IC2 3 1 TEST K4 SERVO 4 1 P0.0 P P0.1 IC1 P P0.2 P P0.3 P LPC762 9 P0.4 P P0.5 P0.6 P0.7 P1.2 P1.1 P X1 X P2 T3 7 UR1 P1 T2 3 Radiometrix RX2 UHF Receiver A1 SFH5110 S1 ON/OFF BT1 2 SFH k R3 JP1 JP2 C1 X1 C2 R7 R6 15p 6MHz 15p speed control encoding infrared encoding 2x BUZ Figure 7. Receiver circuit diagram with two communication methods. 16 Elektor Electronics 10/2001

6 servos to be reversed. Voltage monitoring The minimum operating voltage of the microcontroller A/D converter is specified as 3.0 V. An internal comparator in the microcontroller is used to measure the supply voltage and detect when it gets close to this level. LED D1 is lit continually acting as a software watchdog but starts to blink when the supply voltage falls below 3.3 V indicating a low battery condition. The microcontroller has an in-built comparator with a reference voltage level of 1.23 V (Figure 5). The values of external resistor R2 and R3 are calculated using the following formula: Internal Reference = 1.23 V R3 is chosen as 10 kω R2 = 10 kω [(threshold voltage/1.23 V) 1] R2 = 10 kω [(3.3 V/1.23 V) 1) R2 = kω (nearest standard value: 18 kω) The Receiver The receiver block diagram shown in Figure 6 is divided into several functional blocks like the transmitter. Conrad Graupner /JR Futaba Low current consumption Interference suppression from continuous light sources (incandescent lamps or sunlight), or light sources pulsed at 36 khz or other frequencies (fluorescent lamps). The RF receiver for 433 MHz The receiver module type RX2 from Radiometrix is the partner to the transmitter module type TX2. The RX2 consumes approximately 13 ma and is a very small, lightweight unit. A significant advantage for modellers is that at 433 MHz the antenna length needs to be only 15.5 cm long. The main features of the receiver module are: Receiver frequency MHz Multiplex Figure 8. Pin-outs of different servo manufacturers. Robbe Microprop Simprop Receiver type: double superheterodyne Sensitivity 107 dbm Operating voltage 3 V to 6 V Current consumption 14 ma Digital data output As soon as power is applied to the RF receiver module it will be operational but without an in-range functioning transmitter module the receiver output will just be meaningless data. The Microcontroller Figure 7 shows the circuit diagram of the receiver. A central microcontroller is again used to do all the tricky stuff. It inputs digital control information from the transmitter signal and supplies control signals to four servos and two switched outputs. The infrared receiver Thanks to the growing popularity of infrared controlled equipment there are many integrated IR receiver chips on the market from different manufacturers all with broadly the same characteristics. The function of the IR receiver is to filter out any optical or electrical interference, demodulate the IR signal and amplify it. The receiver/demodulator IC s generally have three pins, two for connection to the power supply and one for the data output. The output pin can be connected directly to the input pin of a microcontroller. The IR receiver specified for this design is one of a family of devices that can operate in the frequency range from 30 khz to 56 khz. The main features of this device are: Integrated receiver diode and amplifier Electrically shielded Internal filter for PCM frequency. TTL and CMOS compatible. Active-low output. 10/2001 Elektor Electronics 17

7 The 87LPC762 controller used in the receiver is a slightly reduced version of the 87LPC768 used in the transmitter. It does not have the four channel pulse width modulators or the A/D converters and the size of the program ROM is only 2 kbytes. Apart from this the internal circuit is the same. A 6 MHz clock frequency is used once again. Servo control Model servos are pulse width controlled. A pulse width of 1.5 ms will cause the servo actuating arm to travel to its centre point and varying the pulse width between about 0.8 ms and 2.3 ms will move the servo arm from one end of its travel to the other. The actual maximum and minimum values of the pulse width vary slightly from one manufacturer to another but the difference is not critical. The pulse is repeated approximately every 20 ms. The pulse width, pw, is calculated in the receiver microcontroller using the formula: pw [µs] = A/D value Where the A/D value is an eight bit value sent in the message from the transmitter (see the second part of this article in the coming month). The output pulses are generated using a built-in timer in the microcontroller. The four output pulses are produced one after another in order to reduce the peak current that would otherwise occur if the pulses were sent simultaneously to all four servos. The microcontroller port pins connected to the servos are configured as push-pull outputs. In the past, servos from different manufacturers had incompatible connectors. More recently equipment bought in the UK has standardised connectors conforming to the Futaba, Graupner/JR and Ripmax layout. The receiver PCB layout takes this into account so that the servo connector pins are suitable for this pin layout. If you are using older servos Figure 8 gives information on the pin-outs. Soft start/speed regulator The operating lifetime of motors and gearboxes can be extended by gently bringing them up to speed rather than switching them from zero to full power. Two methods of controlling the model motor are implemented in this design. Firstly if the soft start option is selected (jumper at port pin P0.5 not fitted) the left joystick switch is connected to the transmitter input at port pin P1.0 and corresponds to port pin P0.1 at the receiver. This output is connected to MOSFET T1. When the switch at the joystick is activated the motor speed will be ramped up to a maximum in about one second. This is accomplished by the controller switching the MOSFET with a PWM signal where the on/off ratio of the waveform gets longer until the MOSFET is switched fully on. If the speed regulator option is selected (jumper at port pin P0.5 fitted) then the up-down axis of the left joystick will become the motor speed controller. Pulling the stick downward from its neutral position will cause the motor speed to increase. The gate control voltage is 5 V so a logic level MOSFET is used. To ensure maximum power in the drive motor the MOSFET R DSON should be <0.001 Ω. The SUP75N03 meets this criterion and can switch 15 A continuous current. The gate inputs of all the MOS- FETS are tied to ground by 100 kω resistors to ensure that the transistors do not become conducting during microcontroller initialisation when the port output pins are tri-stated. Schottky-diode D2 protects the MOSFET from large voltage spikes produced by the motor. FET T4 is used to stop the motor suddenly when it is switched off. This brake function is necessary for power assisted model gliders to make the special propeller blades fold back at the end of the climb thereby reducing drag. Switched outputs The output port pins P1.0 and P1.1 have an on/off toggle action when push buttons S1 and S2 are pressed at the transmitter. Each press of a push button will cause the corresponding MOSFET T3 or T2 to change state. The BUZ11 MOSFETs specified here have an R DSON of 0.04 Ω and will have no problems handling 5 A continuous current. If the MOSFETs are used to switch inductive loads like relays then it is important to add protection diodes across the load to prevent destruction of the MOSFETs. These two MOSFETs can simply be omitted if you do not want this function and need to save weight. BEC The receiver includes a BEC or Battery Elimination Circuit. This allows the receiver and servos to be powered from the same battery pack that provides power to the electric motor. The voltage of this battery pack will be typically 7.2 V or greater. A lowdrop regulator type LM2940 (IC3) is used to supply the 5 V necessary for the receiver and servos. In some applications the BEC is not required in which case IC3 can be omitted and a +5 V supply is connected to point P1 on the PCB. The power dissipated in IC3 can be calculated from: P = (V batt - V BEC ) I BEC V batt will be the voltage of the battery pack while V BEC is 5 V and I BEC is the current to the receiver and servos. If this power dissipation is greater than that recommended for IC3 then it is necessary to fit a heat sink to IC3. A heat sink is certainly recommended if you are using high performance servos with this receiver. Lastly, for the BEC to function correctly the battery voltage must be greater than 6 V. The main features of the LM2940 are: 1 V maximum drop across the regulator 1 A maximum output 25 C Reverse voltage protection 26 V maximum input voltage Voltage monitor Just like the transmitter, the receiver also monitors its supply voltage. When it dips to below 4.5 V LED D1 will begin flashing. In normal operation the LED will be lit permanently to indicate correct operation of the software. The potential divider chain formed by R2 and R3 is calculated identically as it was in the transmitter but this time with a threshold voltage of 4.5 V. When a low voltage condition occurs it is stored in volatile memory. This ensures that even if the battery recovers slightly after working at full load it still gives a correct indication that the battery requires recharging. ( ) In the second part of this article we will look at the assembly of the units, and take a closer look at the transmitter and receiver software. 18 Elektor Electronics 10/2001

8 Miniature PCM Remote Control (2) part 2: the software protocol In this second and final part of the remote control design we put it all together and take a closer look at the workings of the transmitter and receiver software. As with any communication system it is necessary to decide on a suitable message format to convey the information. This system does not need to comply with any existing communication standard so we are free to design our own. The main objectives are to enable a high data rate together with good reliability and error detection. The specification of the transmitters and receivers must also be considered to ensure, for example, we do not exceed the maximum clock rate or channel bandwidth. With these constraints in mind a message protocol is used (Figure 1) that is suitable for transmission using either IR or RF. 0.5 ms 1 ms 0.5 ms 2 ms 2 ms start bit A-D value switch channel address parity stop bit Figure 1. Message protocol. Start bit The start bit allows the receiver software to synchronise on a new message. The length of the high and low phase of the start bit ensures that it cannot be confused with any part of the rest of the message. channel number is also sent in front of each value. Switch bits The status of the two momentary action pushbuttons at the transmitter is sent using these two bits. A toggle action is implemented in software so that each time the switch is pressed the status of the corre- Channel address (2 bits) The channel address is used to indicate which channel the A/D value is intended for so that moving a joystick on the transmitter will cause the correct servo to move at the receiver. Two address bits allow four channels to be encoded. For example: Left joystick up/down value: Address 00 Left joystick left/right value: Address 01 A/D value (8 bits) The angular positions of the joysticks are read from the potentiometer values mounted in the joystick. These values are converted into 8 bit digital values in the A/D converter and transmitted in the message. The corresponding a b Figure 2.Control of the RF module (a) and the IR LED (b) Elektor Electronics 11/2001

9 Operating Range RF Module Radiometrix quote the free-field range of the modules used here as 300 m. This should be more than adequate for a radio-controlled car. Infrared The range of the infrared version is somewhat less than the RF variant. The IR link needs a line-of-sight path from the IR transmitter diode to the receiver either directly or indirectly by bouncing off a reflective surface. The IR receiving diode should therefore be mounted on the roof of the model or somewhere where it will be more likely to see the IR signal from any direction. The transmitting IR LED beam propagation pattern is cone shaped with an angle of ±15. If the receiver is not subject to direct sunlight then you can expect a range in the order of 5 m. sponding bit will change state and stay at this level until it is pressed again. Parity bit The parity bit allows the receiver to detect single bit errors in the transmitted message. The transmitter software counts all the 1 s in the transmitted message and if the count is odd then the parity bit will be set (1). If the count is even the parity bit is reset (0). The receiver simply counts all the 1 s in the received message (including the parity bit) if the result is odd it knows that the message was corrupted: Message = , Parity = 0 Message = , Parity = 1 Stop bit The stop bit marks the end of the message. It conveys no other information. A complete message is not sent every cycle. To achieve smooth, high Transmitter to receiver control action. speed servo positioning and reduce transmitter power consumption, information is only transmitted if there is a change in the position of the joysticks or if a switch is pressed. The transmitter software reads in the joystick and switch values, compares them to their previous values and prioritises any changes before a message is sent. If you tried sending all the data in every message with the data rate that is available in this system, the servos would have a very jerky response. The software also averages out the joystick position. It reads an A/D converter output four times, adds the values together and then divides the result by four. This process smoothes out changes and helps to reduce supply current. If you have an oscilloscope handy you will be able to see that the transmitter sends hardly anything when the transmitter controls are left untouched and its only when a switch is pressed or a joystick Transmitter Receiver output Left joystick up/down Servo at Pin P1.7 Speed controller at Pin P0.1 (jumper coding!) Left joystick left/right Servo at Pin P0.4 Right joystick up/down Servo at Pin P1.6 Right joystick left/right Servo at Pin P1.4 Switch 1 on Left joystick (Pin P1.1) Switch output on Pin P1.0 Switch 2 on Right joystick (Pin P1.0) Switch output on Pin P1.1 Soft start on Pin P0.1 (coding!) moved that a message is sent. This feature prolongs battery life, especially when the IR transmitter option is used. The receiver always stores and uses the last value sent until it is overwritten by a new value. The digital message stream is connected to the RF modules modulation input pin where it frequency modulates the 433 MHz carrier. If the IR option is selected the data must be modulated at 36 khz (Figure 2). The Transmitter and receiver programs The program flow for both the transmitter and receiver software is very simple and can be represented as a list of tasks: Transmitter 1. Initialisation of ports and internal controller hardware. 2. Watchdog reset. 3. Check the battery voltage level and control the LED correspondingly. 4. Input values from the four joysticks. 5. Check for pushbutton presses. 6. If any changes then send a message: For infrared: Modulate with a 36 khz carrier. For RF: Modulate the transmitter frequency with the message bit pattern. 7. If in infrared mode then wait 10 ms. 8. Return to 2. Receiver 1. Initialisation of ports and internal controller hardware. 2. Watchdog reset. 3. Check the battery voltage level. 4. Sample the receiver module output and check the signal tolerances. 5. Check the message parity. 6. Service the switch output signals, the soft-start switch and the speed regulator. 7. Calculate the servo pulse width from the 8-bit value. 8. Load the timer with the servo pulse width value. 9. Return to 2. A time of 200 µs is used to check the pulse length tolerance. The software for this project is available on floppy disk only. Due to contractual agreements with the author, the programs are not available as free downloads from the Publishers website. Construction Your chances of success in building a fully functioning control system are greatly 11/2001 Elektor Electronics 29

10 HOEK4 HOEK2 ELEKTOR (C) GENERALINTEREST increased if you use the professionally made PCBs available from Elektor Electronics. Transmitter HOEK1 UT1 S3 A1 D2 D1 R5 T The transmitter component placement is shown in Figure 3. The A/D converters need a stable supply voltage so the extensive earth plane helps to isolate the A/D converters from the large current pulses drawn by the IR LED. Fitting the components to the PCB begins by first soldering the five wire links in position. The microcontroller should be mounted on an IC socket. Ensure that the trim pots P1 to P4 will fit the unit. To achieve maximum range from the RF transmitter module it should be sited as far away as possible from any metal so when mounting it on the PCB try not to shorten the modules connection leads. A 1 cm length of PCB track forms part of the aerial so that 14.5 cm length of insulated wire connected to pin A1 should be sufficient to act as the transmitter aerial. The infrared LED is mounted and bent at 90 so that the IR light is beamed out of the front of the unit. P1 + BT1 - R6 S2 P2 S4 P5 P6 C2 X1 C3 R4 IC1 R1 R3 R2 JP1 S1 C1 P8 P4 P7 P3 (C) ELEKTOR HOEK3 COMPONENTS LIST Transmitter Resistors: R1 = 470Ω R2,R3 = 18kΩ R3 = 10kΩ R4 = 1kΩ R5 = 10Ω R6 = Ω P1-P4 = potentiometer, 20kΩ, linear P5-P8 = joystick (CTS # 252A104A60TB with internal switch) Figure 3. Layout and component placement of the transmitter PCB. Capacitors: C1 = 220µF 16V C2,C3 = 15pF Semiconductors: D1 = TSUS5201 or LD271 D2 = LED, red, 3mm T1 = ZTX603 (Zetex) or TIP110 IC1 = 87LPC768FN, programmed, order code Miscellaneous: X1 = 6MHz quartz crystal S3 = on/off switch, 1 make contact S4 = 4-way DIP switch JP1 = 2-way pinheader with jumper UT1 = 433MHz transmitter module type TX2 from Radiometrix (Farnell) Battery holder for 3 AA cells PCB, order code Elektor Electronics 11/2001

11 Receiver The receiver PCB layout is shown in Figure 4. To keep the receiver as compact as possible, all the components are mounted a little more snugly than in the transmitter. The six wire links are first fitted to the PCB, followed by the 20-pin socket for IC1. Once all the components have been fitted (except for IC1, IC2 and the radio module), power can be applied to the receiver card and with a DVM or scope you can check the voltages around the PCB. If everything is in order then the rest of the components can be fitted. Make sure that the metal screen on the RF module does not cause a short circuit with components on the board. Finally, a 15.5 cm length of insulated wire is C1 C A1 R1 K1 K2 K3 K4 IC (C) ELEKTOR UR1 D1 C5 R2 TEST R4 R3 R9 R5 T5 JP1JP2 R7 R8 D2 R6 C7 X1 C6 IC3 T3 T2 C4 P1 P2 C3 IC2 T1 M- T4 M+ - S1 + COMPONENTS LIST Receiver Resistors: R1 = 470Ω R2,R9 =27kΩ R3,R8 = 10kΩ R4 = 100Ω R5,R6,R7 = Ω Capacitors: C1,C2 = 15pF C3 = 10µF 16V C4,C6 = 100nF C5 = 470µF 16V C7 = 220µF 16V Semiconductors: D1 = LED, red, 3mm D2 = MBR745 T1 = SUP75N03, IRL2203 T2,T3 = BUZ11 Addresses The European branch of CTS Corporation is at High Blantyre Glasgow G72 0XA Tel CTS resistive products are also supplied by Quiller Electronics Ltd. Bournemouth BH6 5EU Tel T4 = IRF4905L (International Rectifier) T5 = BC547 IC1 = 87LPC762BN, programmed, order code IC2 = SFH (Siemens) IC3 = LM2940 Miscellaneous: X1 = 6MHz quartz crystal S1 = on/off switch, 1 make contact JP1,JP2 = 2-way pinheader with jumper 4 servo connector plugs, 3-way UR1 = 433MHz RX2 receiver module from Radiometrix (Farnell) PCB, order code Disk, project software, order code Joystick: Microcontroller: LM2940: LM2940.html MBR745: MBR745.html SUP75N03: RF Module: soldered to point A1 on the PCB to act as an aerial. To make the unit a little more robust, this connection and the RF module should be secured to the PCB with the aid of a little hot glue or silicon adhesive. The current consumption of the receiver unit alone is approximately 40 ma. Diagnostics (C) ELEKTOR Figure 4. Layout and component placement of the receiver PCB. For debugging purposes a diagnostic output is available from pin P0.3 on the receiver controller. This test point can be fitted with a solder or test pin. The receiver controller uses this pin to output a signal every time it detects an error anywhere in the received message. It checks the received message values after the start bit and flags an error when they are not within tolerance. Using this signal it is possible to optimise the receiver siting away from sources of interference. Comparing this signal with the received telegram on a scope will indicate which part of the telegram has been corrupted. ( ) 11/2001 Elektor Electronics 31