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This is what you need for remote control of a central locking system. The two-button transmitter provides the lock and unlock functions and a relay on the receiver board can power up a sepa­rate car alarm. By LEO SIMPSON Add central locking to your car Don’t you just envy those swaggering motorists who just park their car, get out and then walk away without having to lock the doors. They just blip their little key remote and the doors all lock themselves automatically. It’s even better for them when it’s raining. None of this fumbling with keys while you get drenched. A S YOU CAN SEE, I get frustrated by motoring’s little tribulations. It’s even worse if you regularly drive two cars, one with central locking and one without (no, I don’t mean at the same time). When driving the car without central locking, it’s all too easy to walk away without locking the car. And then there are the times when you go to open the rear doors to get something off the back seat and the doors are still locked. All of which is a pretty strong incentive to install remote central locking. It’s stops you getting wetter in wet weather and avoids the possibility of strained fingers when trying to open locked doors. Actually, adding central locking to a car doesn’t require any electronics at all. All you need is a set of central door locking solenoids, a wiring diagram, a screwdriver, a free after­noon and a fair bit of patience. The tricky bit is where you have to thread the wires for the solenoids through the door pillars and so on. But simply having central locking is not good enough be­cause you still have to lock and unlock your car with a key. To be truly up to date you need one of those nifty keyring doodads and that’s what this project is all about. In essence, this project provides the UHF remote link; a keyring transmitter with two buttons and a UHF receiver board which operates the central door-locking solenoids. Actually, we should note that they are not solenoids but motor-driven actua­tors. You don’t need to assemble the circuitry of the keyring transmitter. It October 1997  41 Fig.1: the 2-channel transmitter uses diodes to pull pins 12 or 13 low when the pushbuttons are pressed. The result is a 100kHz burst of pulses at 304MHz. button S1 or S2 on the transmitter is pressed, IC1 will detect a valid code at pin 12 or 13 which is signified by that line going low for as long as the transmitter button is pressed. The receiver module also drives Q6, an emitter follower which turns on LED5 whenever a signal is being received. Note that any received signal will be indicated by LED5, whether it is a valid code or not. At other times, LED5 may flicker on or off due to random noise being received. If pin 12 goes low, its output is inverted by gate IC2c to drive transistor Q4 via LED3 and a 3.3kΩ resistor. When Q4 turns on it provides the “unlock” function on the central locking module. If pin 13 goes low, its output is inverted by gate IC2d to drive transistor Q3 via LED4 and a 3.3kΩ resistor. Q3 provides the “lock” function on the central locking module. Alarm switching comes fully assembled. All you need to build is a small PC board with a preassembled UHF receiver module and a handful of other parts. Transmitter circuit Now while you don’t have to build the transmitter since it comes ready built, it is useful to have a look at the circuit in order to understand how it functions. The transmitter circuit is shown in Fig.1. It shows an 18-pin trinary encoder chip, IC1, an A5884, It drives a single transistor connected as an oscillator which runs at 304MHz whenever pin 17 of IC1 goes high. The result is a 100kHz burst of encoded pulses at 304MHz which is radiated by the inductor L2, which is actually just a single loop of track on the PC board. IC1 can deliver two separate encoded pulse trains, depend­ing on which button is pressed. When either button is pressed, power is applied to IC1 via diode D3 or D4 while the encoding option is selected by D2 or D1 respectively. Note that there are many thousands of codes available depending on whether the ad­dress lines are tied high, low or left open circuit. That’s where the word “trinary” applies, because there are three separate encoding options for each address line. LED1 provides a visual indication that the transmitter is operating, a 42  Silicon Chip handy feature if you suspect that battery is dying. A number of the components on the transmitter board are surface mount types so if you do pull it apart you will find that they are rather hard to see and identify unless you have a magni­fying glass. Receiver circuit Fig.2 shows the circuit of the receiver board. This uses a small UHF receiver module to detect the pulses of 304MHz and turn them into a pulse stream which is fed into pin 14 of the matching trinary decoder chip, IC1, an A5885M. Depending on whether Gates IC2a and IC2b are connected together as an RS flip­flop. When pin 13 of IC1 goes low, corresponding to button S1 on the transmitter being pressed, the flipflop is set, with pin 3 of IC2 going high. Being a flipflop, pin 3 stays high even after button S1 is no longer being pressed and it drives transistor Q5 via LED2 and the associated 3.3kΩ current limiting resistor. Q5 operates the relay to supply power to a car alarm, if you have one fitted. Hence this circuit can operate central locking and a car alarm if you wish. When pin 12 of IC1 goes low, corresponding to button S2 on the transmitter being pressed, the flipflop is reset, with pin 3 of IC2 going low. This turns off Q5 and the relay, so that the car alarm is turned off. Operating either of the buttons will cause diode D2 or D3 to conduct and turn on transistor Q2 which operates a buzzer. The buzzer is optional and probably not necessary for most applica­tions. Board assembly This is what the transmitter looks like when you pull it apart. You will need to tweak the trimmer capacitor at the top of the board to set it to 304MHz. Fig. 3 shows the component layout on the PC board. The board assembly is pretty straightforward but we would suggest that the PC stakes and links be installed first. Then insert the resistors, diodes, LEDs and electrolytic capacitors. Make sure that the polarised components are installed the right way around. Then insert the transistors and Fig.2: a UHF receiver module drives the trinary decoder to oper­ate transistors for the lock and unlock functions. The relay can be used to switch a car alarm, if desired. note that there is a trap for young (and old) players at this point. We have seen these C8050 transistors supplied with varying pinouts. The board is designed to take transistors with the conventional EBC pin sequence, as shown on Fig.2. However they can sometimes be supplied with the ECB pinout sequence, so you should always check the labelling on the plastic encapsulation. If it is dif­ferent, you will need to bend the transistors’ pins to match the PC board. Next, install the two ICs and then the receiver module. Do not solder the address pins of IC1; ie, pins 1-8 and 10 & 11. These may be soldered later when you custom code the transmitter and decoder. Check your work carefully against Fig.2 and Fig.3. Test & alignment Now apply power from a 12V DC source and operate the but­ tons on the transmitter. Each time a button is The central locking kit comes with two master and two slave actuators, a control unit, the loom and mounting brackets for the actuators. The control unit is linked up to the decoder board for full remote control. pressed, LED5 (orange) should come on brightly. LED3 should light when button S1 is pressed and LED4 should light when button S2 is pressed. Fur- thermore, LED2 should light when S1 is pressed and go out when S2 is pressed, showing that the alarm functions are correct. October 1997  43 Fig.3: the component overlay for the PC board. Install the UHF receiver module as the last step in assembly. If these functions are not operating correctly, go back and double-check all your work. The most common problems are missed solder joints or a component installed at the wrong position. You are not likely to have damaged an IC unless you installed it the wrong way around. Speaking of missed solder connections, 10 pins on the decoder IC should not have been soldered at this stage. If you accidentally tied one or more of these pins to the adjacent positive or negative bus-bars, even by a solder splash, then it will not acknowledge the transmitter even though LED5 may light each time one of the buttons is pressed. Supposing that everything is working so far, the tasks of alignment and coding still remain to be done. Alignment? What alignment? It works, doesn’t it? Well we stated earlier that the UHF transmitter and the receiver module operate at 304MHz. We lied. They are supposed to operate at 304MHz but as supplied they operate at 318MHz. To obtain the correct frequency, you need to tweak the adjustable capacitor in the transmitter and add a capacitor across the coil on the receiver board. To do both of these tasks, you will need super duper eyes with microscopic vision or at least, good lighting, a very good magnifying glass and a steady hand. Let’s do the receiver modification first. You need to iden­tify the 10pF Parts List 1 keyring transmitter with two buttons and LED indicator 1 PC board, 117 x 48mm 1 UHF receiver module 1 12V relay with SPDT contacts 6 PC pins 1 buzzer (optional; see text) 3 100µF 16VW electrolytic Semiconductors 1 A5885 trinary decoder (IC1) 1 4093 quad 2-input NAND Schmitt trigger (IC2) 5 C8050 NPN transistors (Q1-Q5) 2 1N4148 small signal diodes (D2,D3) 3 GIG power diodes (D1,D4,D5) 1 6.2V zener diode (ZD1) 3 green LEDs (LED1, LED3) 3 orange LEDs (LED2,LED4,LED5) Resistors (0.25W, 1%) 1 100kΩ 2 1kΩ 7 3.3kΩ 1 82Ω Where To Buy The Parts The PC board and other parts for this design are avail­able from Oatley Electronics who own the design copyright. Their address is PO Box 89, Oatley, NSW 2223. Phone (02) 9584 3563; fax (02) 9584 3561. The prices are as follows: UHF remote control with two-button transmitter.....................................$35 Additional two-button transmitter............................................................$15 Central locking kit, two masters, two slaves plus loom ..........................$60 Please add $5 for postage and packing. 44  Silicon Chip ceramic capacitor which is connected in parallel with a slug-tuned coil. It and the coil are surrounded by wax so you will need to look very closely. Now solder a 2pF ceramic capacitor across the 10pF capacitor. Set the transmitter and receiver board close to each other and apply power to the receiver board. Press one of the transmit­ter buttons and slowly rotate the trimmer capacitor anticlockwise until LED5 comes on brightly. You will need to use a metal-tipped alignment tool when doing this adjustment, to minimise the ef­fects of stray capacitance. Do not use a small screwdriver – it is just not workable. You will need to do this adjustment repeatedly, to get maximum range. Each time you do the adjustment, the transmitter should be moved further away from the receiver. You will need an assistant to note when the various LEDs on the receiver board light. Ultimately, you should be able to get a range of more than 10 metres and while the system is capable of more range than that, there is not a great deal of point in doing so. After all, do you really want your central locking operable from more than 10 metres? Coding the system The final step in the electronic work for this project is to code the transmitter and receiver. Both must be coded exactly the same way otherwise the system cannot work. If you connect pin 2 of the transmitter chip to 0V, then pin 2 on the decoder chip must also be connected to 0V. Note that while the circuit of Fig.1 shows both positive and negative busbars for coding, and the same on Fig.2, the transmitter board actually only has the 0V track available for easy coding. If you want to tie some pins high, you will need to wire a small link on the back of the PC board. If you take this approach, you must be careful that the board can still sit flush in the bottom of its case. If it does not, you will not be able to close the case up without having one of the but­tons permanently pressed. Once the system is coded and operating as it should, it can be fitted into a case or a large piece of heatshrink tubing and installed underneath the dash panel of your car. Make sure the central locking system is working exactly as it should before hooking it up to the remote control receiver. SC