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.,: ·:., .:•.··•t:, . · m. :.:,:·;:::.•. . ·,, . . . _. :_·/i ' ~-~ . . .. .,v ... ' .· ansmI .e , By GREG SWAIN Here's an easy-to-build UHF transmitter that you don't have to adjust for frequency. It uses a SAW filter for high stability and is fully compatible with the UHF Remote Switch described in December 1989. Although designed specifically for the DSE Remote Control Switch described last December, this UHF transmitter can also be used with previous remote switches described in SILICON CHIP (eg, March 1988 & August 1988). However, this new transmitter has several improvements compared to earlier designs. These improvements have been 16 SILICON CHIP wrought mainly by scrapping the LC tuned circuit used in previous designs and replacing it with a Surface Acoustic. Wave (or SAW) filter. A SAW filter offers several advantages over an LC circuit: (1). it locks the transmitter frequency to 304MHz (303.88MHz to be precise); (2). it eliminates the need for transmitter alignment; and (3). it means that there is very little frequency drift due to hand capacitance or temperature variations. Another advantage of this transmitter is that it automatically ceases transmission after about 10 seconds if the button is accidentally (or purposely) held down. This can easily occur if you put the transmitter in a pocket. As well as extending battery life, this feature is now also a DOTC (Department of Transport and Communications) requirement. As can be seen from the photos, the circuit fits in a plastic keyring style case and is powered by a 12V lighter battery. A red LED flashes when the button is pressed to indicate that the unit is operating. In essence, the new transmitter duplicates all the features of the commercial transmitter supplied with the Dick Smith Electronics UHF Remote Control Switch (SILICON CHIP, December 1989). That design also used a SAW filter but unfortunately is now becoming difficult to obtain. The unit described here will be used as a replacement transmitter in future kits or can be built simply to provide additional transmitters for the remote switch. A complete kit of parts for the project will be available from Dick Smith Electronics [see panel). How it works Fig.1 shows the circuit details of the UHF Remote Transmitter. It uses a digital encoder IC to key a UHF oscillator operating at 304MHz [Ql) on and off. ICl is a Motorola MC145026 9-bit trinary encoder and is used to generate a 9-bit code word. Trinary code is similar to binary code except that there are three logic states instead of two [ie, high, low and open circuit). The 9-bit code word is selected by the connections to ICl's nine address pins, Al-A9. In practice, this means that each address pin can be tied to the + 12V rail, to ground or left open circuit, thus giving 19,683 combinations. However, because of the particular decoder IC used in the receiver [MC145028), A9 should only be tied high or low and this reduces the number of coding options to 13,122. In fact, in the DSE receiver (SILICON CHIP, December 1989), A9 is tied to ground which means that A9 in the transmitter must also be tied to ground. The 9-bit serial data is shifted by an internal oscillator in ICl, the frequency of which is set to 770Hz by the RC timing components on pins 11, 12 & 13. Pin 14 is used to control ,the transmission. When S1 is initially pressed, this pin is low and the internal oscillator is enabled to shift the data out. The 33µF capacitor on pin 14 of IC1 now charges via an internal pullup resistor. After about 10 seconds, the voltage on pin 14 100(! + 47k 16 -T 1 2 3 Al 14 + 33 16VW A2 A3 47k 15 1k !Cl MC145026 A5 SAW 6 FILTE{1=--f * J 5.6pFl .,. A6 13 7 * 3.3pF 4 A4 5 .001! - * SEE TEXT 56k A7 12 .01 g AB c<at>e A~K 11 120k VIEWED FROM BELOW 10 A9 .,. HIGH ST ABILITY UHF REMOTE TRANSMITTER Fig.1: the circuit uses an MC145026 trinary encoder IC to key UHF oscillator Ql on and off. The SAW filter sets the output frequency to 303.8MHz while inputs Al-A9 are connected to give the address code. reaches logic 1. When this happens, the internal oscillator stops and data transmission ceases. The 47k0 resistor is there is discharge the 33µF capacitor when S1 is released, so that the circuit is ready for the next transmission. During transmission, the output from pin 15 consists of a series of pulses and these are used to modulate the external oscillator (Ql). The frequency of this oscillator is set to 303.8MHz by the SAW filter in Ql 's base circuit. The 3.3pF capacitor between collector and emitter provides the necessary positive feedback to ensure oscillation. Another capacitor (5.6pF) is used to bypass the 4700 emitter degeneration resistor to increase the oscillator output. This output appears at Ql 's collector and drives a transmission line antenna which actually forms part of the PCB pattern. The PCB fits neatly into a small keyring case and is powered by a 9V lighter battery. The SAW filter locks the output frequency to 303.8MHz and eliminates drift due to hand capacitance. AUGUST 1990 17 PARTS LIST auaa A6A5A4A3A2A1 Fig.2: keep all component leads as short as possible when assembling the PCB. The SAW filter is oriented so that the side closest to its four pins is adjacent to the lkn resistor. Inductor L1 is part of the PC pattern. Fig.3: to code the transmitter, each A1-A9 input is connected to the high rail, the low rail, or left open circuit. If the unit is for use with the DSE receiver, then tie A9 low. Be sure to match the receiver code. 1 transmitter case 1 PC board, code ZA-1432 (copyright Dick Smith Electronics) 1 PC-mounting pushbutton switch 1 SAW filter (303.8MHz) 1 12V lighter battery (GP23 or equivalent) Semiconductors 1 MC145026 trinary encoder (IC1) 1 PN3563 NPN transistor (01) 1 red LED (LED 1 ) Capacitors Power is derived directly from the 12V battery and is applied to the circuit via push button switch S 1 and LED 1. During transmission, LED 1 flashes due to variations in the current drawn by ICl as it outputs the serial data stream. At the end of the transmission, LED 1 will remain dimly lit due to the circuit's quiescent current consumption. Building it All the parts for the UHF transmitter are mounted on a small PC board. This board is coded ZA-1432 and fits into a plastic keyring-style case. Fig.2 shows the parts placement diagram. You can install the parts in any order but we suggest that you leave the IC and the switch until last. Keep all resistor and capacitor leads as short as possible and use your multimeter to check the resistor values before installing them on the board. Note carefully the orientation of the SAW filter - the edge closest to the four pins goes towards the IC. Transistor Ql is mounted with its leads bent at right angles and with the flat side of its plastic body sitting flush against the PCB. The leads of the 33µ,F tantalum capacitor are also bent at right angles so that the device can lie flat on the board. Install this capacitor so that its positive ( +) lead is closest to the IC. This done, install the switch with the flat side of its body towards the battery, then install the two PC-mounting battery terminals. The LED should be installed so that its top sits about 13mm above the board. That way, it will just pro- 1 1 1 1 1 33µ,F 16VW tantalum .01 µ,F ceramic .001 µ,F ceramic 5.6pF ceramic 3.3pF ceramic Resistors (0 .25W, 5%) 1 120k0 1 56kQ 2 47k0 1 1 kO 1 4700 1 1000 trude through the hole in the lid when the case is assembled. Check that the LED has been correctly oriented (the anode lead is the longer of the two) before soldering its leads. Coding Before testing, the A1-A9 address pins must be connected to match the receiver code. This simply involves tying each pin high, low TABLE 1: RESISTOR CODES □ □ □ □ □ □ □ Value 120k0 56k0 47k0 1k0 470Q 1000 No 1 1 2 1 1 1 4-Band Code (5%) brown red yellow gold green blue orange gold yellow violet orange gold brown black red gold yellow violet brown gold brown black brown gold TABLE 2: CAPACITOR CODES □ □ □ □ □ 18 Value .01 µ,F .001µ,F 5.6pF 3.3pF SILICON CHIP All Value 10nF 1nF IEC Code 10n 1n 5p6 3p3 EIA Code 103 102 5.6 3.3 Before mounting Ql, bend its leads at right angles then install it so that the flat of its body rests against the PCB. The 33µF capacitor is mounted in similar fashion. Why Kill your Computers Memoryl COMPUTER PROTECTOR The transmitter output drives inductor Ll which is a transmission line antenna etched into the PCB pattern. This transmitter has been coded by tying A1-A4, A6 & A9 low and by leaving A5, A7 & AB open circuit. or open circuit. For example, you could connect A1 to the high rail, A2 to the low rail, leave A3 open circuit, bridge A4 low and so on. Note that if the transmitter is for use with the DSE receiver, then A9 (pin 10) must be tied low (ie, connected to ground). Fig.3 shows the A1-A9 address pins on the copper side of the PC board, together with the locations of the high and low rails. Use solder bridges or short lengths of tinned copper wire to connect the selected address pins to the high or low rails. Once coding has been completed, the PCB and battery can be installed in the case and the unit tested with its receiver. Note that it may be necessary to retune the receiver so that it matches the transmitter frequency. The range achieved will depend on whether or not an antenna is fitted to the receiver but distances of up to 25 metres in open space should be possible. Note that the value of the feedback capacitor between collector and emitter of Ql is critical. If the circuit does not oscillate with 3.3pF, increase the value to 3.9pF. How can you tell? Simple - the LED will flash when the button is pressed but the transmitter just won't work. Finally, you will have to slightly modify the receiver circuits of March & August 1988 for use with this transmitter. In each case, change the 39kQ resistor on pin 6 of the MC145028 decoder IC to 56k0 and the 180k0 resistor on pin 1O to 390k0. 1§:;l Where to buy the kit This project was developed by Dick Smith Electronics and is available from all DSE stores or by mail order from PO Box 321 , North Ryde, NSW 2113. You can also order by phone on (02) 888 2105 or, from outside the Sydney area, on (008) 22 6610. The kit consists of all parts including the case and the battery. The price is as follows : UHF Remote Transmitter (Cat. K-3259) ... .... ... ....... ..... ..... ... $29.95 Postal orders should include another $3.00 for packaging & postage. Please quote the catalog number when ordering . Note: copyright of the PCB artwork associated with this project is retained by Dick Smith Electronics. ~~~ Or Risk the Loss of Expensive Data? Available in the following Current Ratings: l Amp 3 Amp 6 Amp 10 Amp Install o COMPUTER PROTECTOR A definite must for all electronic office equipment ..-----■ cmi:mi ■ ~ ~Ls!,Lng~se I SYSTEMS WESTINGHOUSE BRAKE & SIGNAL COMPANY (AUSTRALIA) LIMITED SIGNAL DIVISION 80-86 Douglas Pde. Williamstown. Victoria. (P.O. Box 267, Williamstown. Victoria. 3016) Phone (03) 3971033. Fax (03) 397 1861 Telex : 37 477 Wessys Vic(03) 3971033. NSW (02)5451322, Old (07) 275 3188 SA: (08) 212 3161 . WA:(09) 446 8844 COM/2 AUGUST 1990 19