Tune that dial - Index of

More documents

Recommendations

Info

TECHNOLOGY READER CIRCUITS Wireless Key Compact and secure Gert Baars Secure wireless switching can be useful in a variety of applications, such as activating or deactivating an alarm system, operating a garage door opener, or operating an anti-start device in your car. The circuit described here is compact and quite secure, and it operates in the 433-MHz ISM band. The circuit described here consists of a transmitter and a receiver operating on a frequency in the 433-MHz ISM band. Frequencies in this band can be used without a licence if the transmitter power does not exceed 10 mW (10 dBm). The transmitter is small enough to be fitted in a key fob, and it is powered by a 3-V lithium cell. The transmitter only emits RF signals in short bursts, so the current consumption is no more than 8 mA. As the battery has a capacity of more than 200 mAh, it will last for several years. The receiver operates from a supply voltage of 5 V and can be powered from a simple AC adapter. If it is used with a car battery, a 78L05 must be connected between the battery and the circuit. In the case of a 6-V battery (such as is used with motorcycles and scooters), a lowdrop 5-V regulator must be used. A 24-bit code is used to ensure that the receiver only responds to the proper key transmitter. More than 16 million combinations are possible with such a code. To give you an idea of how big this number is, with a burst length of 300 ms for each transmitted code the transmitter would have to transmit an uninterrupted series of sequential codes for two months in order to work through all possible combinations. Microcontrollers are used in both units (transmitter and receiver) to encode and decode the key. Transmitter The actual transmitter consists of a Maxim MAX1472 (Figure 1). This tiny 8-pin IC contains a crystal-controlled 38 ASK transmitter for frequencies in the 300–450 MHz range. It includes a fixed-ratio PLL and a crystal reference oscillator. This type of design is more precise and more stable than an ISM transmitter based on a SAW filter. The frequency selected for this project is 433.920 MHz. It results from the fact that the PLL multiplies the crystal frequency by 32. Due to the narrow tolerances of crystals, the match between the transmitter and the receiver is always adequate without any need for tweaking, in part because the receiver has a relatively large bandwidth. The crystal has a frequency of 13.560 MHz. It comes from Hong Kong X’tals [1]. The coil in series with the crystal ensures that the circuit oscillates at the series frequency of the crystal. The transmitter IC has a 10-mW output stage, which is good for a range of at least 10 metres indoors (the author only tested the unit in his house). A loop antenna etched on the circuit board is often used in such applications, but a dipole antenna (too short for the actual frequency) proves to give better results. Two radiating elements, each approximately 4 cm long and 0.04” thick are fitted to the transmitter board for this purpose. The transmitter IC has a data input for ASK modulation. Although it can be modulated at up to 100 Kb/s, we have to point out that the reliability of the system is better at lower rates. As only a relatively small amount of data must be transmitted, a bit rate of 100 b/s can be used. It takes only 240 ms to send 24 bits at this rate. The total burst length is 300 ms, including the preamble and start bit. This burst is transmitted when the transmit button is pressed, with a total current consumption of around 8 mA. The microcontroller switches the transmitter off immediately after the burst has been sent and then enters a power-down mode, with the result that the total current consumption drops to less than 2 mA as long as the button is held pressed. This reduces the battery consumption to a minimum. In practice, it means the transmit button must be held down for approximately half a second to transmit all the data. A microcontroller is used to generate the code key and transmit it by modulating the transmitter IC. The selected microcontroller is the Atmel ATtiny15L, which is available in an 8-pin DIP package. This microcontroller has an internal RC clock generator that runs at 1.6 MHz, and it can be set within a tolerance of 1% using a calibration byte. After the flash code memory has been programmed, the calibration byte must read from the internal ‘signature space’ using the programming software and then written to location 1023 ($3FF) of the flash memory. This only has to be done once when the microcontroller is programmed. Each time the microcontroller starts up, it reads the calibration byte from the flash memory and then writes it to the oscillator calibration register to optimise the accuracy of the internal processor clock. Receiver The receiver is also a Maxim IC (Figure 2). In this case it is a MAX1473, which elektor electronics - 12/2006
contains a complete superheterodyne receiver. Here again the reference frequency, in this case the local oscillator frequency, is provided by a PLL oscillator. As the PLL multiplication factor is 32 and the IF is 10.7 MHz, the crystal frequency must be 13.2256 MHz (see [1]). A remarkable feature of this IC is the integrated image rejection mixer, which effectively means that the mixer stage also provides adequate image frequency suppression, so there is no need for a preselection filter at the input. As a result, the required number of external components can be reduced to around 20. The receiver does not have to be tuned, and it boasts a sensitivity of better than approximately 1 µV. The receiver IC also has an audio filter that is intended to reduce noise and interference. A ‘data slicer’ after the filter provides automatic operating point adjustment to provide data that is as reliable as possible even under weak signal conditions. The receiver unit is also fitted with an ATtiny15L microcontroller. The data slicer output of the receiver IC is connected to an input of the microcontroller, which decodes the burst signals from the transmitter. If the decoded data contains the correct key, it activates two outputs. Here OUT1 provides a 1-second pulse if the correct key is received. It is intended to be used to drive a mechanical latch opener, which is usually fitted with a solenoid. The second output (OUT2) is an alternate-action switch. OUT2 L1: 8mm L2: 5mm 7mm 4mm switches to +5 V when the key is received once and then toggles to 0 V the next time it is received. It can thus be used to switch something on or off, such the control unit of an alarm system or an anti-start device in a car. Software The software consists of two separate programs – one for the transmitter and one for the receiver. At somewhat more than 400 lines each, these programs are fairly modest in size. At the transmitter end, after starting up the software looks for a particular value (a ‘signature’) in the EEPROM of IN FL1 C7 10n C3 1n IN C1 10n X1 GND 10,7MHz OUT C8 1n 2 7 14 AVDD AVDD DVDD 27 11 1 PWRDN IRSEL XTAL1 VDD5 AGCDIS XTALSEL 24 15 16 28 3 12 XTAL2 LNAIN MIXOUT IC1 LNAOUT MIXIN1 PDOUT DATAOUT 6 8 26 25 18 IFIN2 17 IFIN1 9 MIXIN2 L2 4 LNASRC 22nH the microcontroller to see whether a valid key has already been generated. If it hasn’t, the software immediately generates a 24-bit code and stores it in the EEPROM memory along with the signature so it will not have to generate a new key the next time. A random number generator that provides an arbitrary 24-bit value is needed to generate the key. This is implemented using a capacitor connected to pin 2 of the microcontroller. The algorithm used for this purpose is based on timing. Pin 2 is first configured as an output and held low for a certain length of time to discharge the capacitor. After that the pin is config- IC2 1 RESET PB0 5 7 PB2 PB4 2 3 PB3 PB1 6 ATTiny15L 12/2006 - elektor electronics 39 C1 10n IC1 5 PB0 RESET 1 2 PB4 PB2 7 6 PB1 PB3 3 ATTiny15L C2 100n MAX1473 DSP DFFB DSN DFO OPP AGND AGND DGND 5 10 13 23 22 20 19 21 C4 1n C9 100p C2 10n C11 100n L1 Ø 15mm C5 2p7 R3 220k C10 47n C6 220p 1M 10k R1 R2 DATA JP1 LEARN Figure 2. The receiver is somewhat larger, primarily due to the extensive circuitry around the MAX1473. 8 4 L1 * 3p3 4cm 4cm * VDD GND PAGND C4 10n 5 EN XTAL1 1 IC2 X1 6 DATA MAX1472 4 PAOUT XTAL2 8 13.560MHz C3 * 7 2 3 +5V 8 4 S1 050364 - 11 C12 100n BT1 CR2032 3V see text * Figure 1. The transmitter consists of only two ICs and can be built in a very compact form. 820 Ω R4 D1 LOW CUR 820 Ω 050364 - 12 R5 D2 OUT1 OUT2
Page 1 and 2: free supplement Tune that dial shor
Page 3 and 4: Digital Storage Oscilloscope Up to
Page 5 and 6: CONTENTS 88 A New Flowcode A new ve
Page 7 and 8: 12/2006 - elektor electronics 7
Page 9 and 10: RFID Quest extended To give readers
Page 11 and 12: Mid-power 24-Vin maxi modules Vicor
Page 13 and 14: CD-ROM Software Toolbox 2 This CD-R
Page 15 and 16: 12/2006 - elektor electronics Talki
Page 17 and 18: Pleasant scents You know the type:
Page 19 and 20: 3. Console enclosures with a tailor
Page 21 and 22: film is printed on the reverse side
Page 23 and 24: milling hands-on next month in elek
Page 25 and 26: ture becomes 180 MHz. Normally the
Page 27 and 28: lated signal with exactly the same
Page 29 and 30: 3 3k9 22k Two BA482 switching diode
Page 31 and 32: ues for the base resistors as shown
Page 33 and 34: Adjustments There are nine trim poi
Page 35 and 36: ations were thought to be operating
Page 37: LED/Light Kits 6 LED V.U. Meter £3
Page 41 and 42: copies the timing of the transmitte
Page 43 and 44: ow SENSOR C1 47n 12/2006 - elektor
Page 46 and 47: This Issue: 24-PAGE ‘E-TRIXX’ S
Page 48 and 49: 4 Did someone ring? If you’re exp
Page 50 and 51: Photo: www.energizer.com 6 M1 Empty
Page 52 and 53: BT1 12V 8 D1 BAT85 1M 47k R1 R2 1M
Page 54 and 55: 10 Temperature-controlled switch It
Page 56 and 57: 12 Check Out Your LEDs You have to
Page 58 and 59: 10k R6 14 R7 220k C1 220p R8 470k S
Page 60 and 61: 16 devices (transistors and diodes)
Page 62 and 63: 18 is combined with the current sou
Page 64 and 65: 20 Musical saw Usually it’s the p
Page 66 and 67: 22 Luminous house number During the
Page 68 and 69: £ 65,99 THREE-IN-ONE LAB UNIT �
Page 70 and 71: know-how rf WLAN Antenna Desi Incre
Page 72 and 73: know-how rf Figure 3. Quarter-wavel
Page 74 and 75: know-how rf Antennas in practice Th
Page 76 and 77: know-how wlan Where am I - and W Po
Page 78 and 79: know-how wlan Advertisement signal
Page 80 and 81: technology radio remote control Rad
Page 82 and 83: technology radio remote control Fig
Page 84 and 85: hands-on fpga As you have probably
Page 86 and 87: hands-on fpga DIY experiments Examp
Page 88 and 89:
hands-on e-blocks A New Flowcode Ma
Page 90 and 91:
hands-on e-blocks What’s new & be
Page 92 and 93:
infotainment modding & tweaking Int
Page 94 and 95:
hands-on propulsion A Wire with Tot
Page 96 and 97:
TECHNOLOGY LAB TALK 96 Smaller is n
Page 100 and 101:
infotainment retronics SSB receiver
Page 102 and 103:
E L E K T O R S H O W C A S E To bo
Page 104 and 105:
Electronic Project Labs An electron
Page 106 and 107:
Order now using the Order Form in t
Page 108 and 109:
info & market sneak preview Overclo
Page 110 and 111:
ORDERING INSTRUCTIONS, P&P CHARGES
show all

Tune that dial - Index of

Create successful ePaper yourself

Delete template?

Save as template?