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More protection for your car with the . . . Keypad Engine Immobiliser This project takes the Engine Immobiliser de­scribed last month and adds a keypad. When you stop your car and turn the engine off, you hit any key to enable the Immobiliser. To start the car again, you must enter the correct 4-digit code, otherwise the car will stall every time it is started. By JOHN CLARKE For good protection against car thieves the Engine Immobi­ liser described last month works well but you do need a concealed switch to operate it and this can be a drawback. Using a keypad to enable the Immobiliser is much more elegant. The design uses a standard 12-button keypad, labelled 62  Silicon Chip from 0 to 9 plus asterisk (*) and crosshatch (#) keys. Four buttons must be pressed in the correct sequence before you turn on the ignition. The car can then be started in the normal way. You can program in any 4-digit code, including the “*” and “#” buttons, by means of links on the PC board. This means that you can set the combination to say, #123, 1223 or whatever. You cannot trick the keypad circuit into disarming the Immobiliser by pressing all keys at once, by disconnecting the battery and reconnecting again or any other jiggery-pokery. The code must be entered in the correct sequence. If you enter the wrong code, you can start again by pressing any key which is not used in the code sequence, followed by the correct code sequence. The Immobiliser is armed by pressing any key which is not used in the combination code. A LED flashes to indicate when the Immobiliser is active and it goes out when the correct code is entered. The keypad can only be used when the ignition is turned off. It does not Fig.1: this circuit has two parts. IC2 and diodes D3, D4 & D5 detect when a key is pressed while IC3, IC4 & IC5 detect when the code is entered in the correct sequence to deactivate the Engine Immobiliser via the output at pin 10 of IC4. respond to any buttons when the ignition is switched on. This means that you can only arm the Immobiliser once you have switched off the engine. Similarly, to disable the Immobiliser, you must enter the correct code before switching on the ignition. The reason for this approach is so that the Immobiliser cannot be activated by the keypad when the car is in motion; if this happened the car could possibly be stopped in a dangerous situation if any of the keypad buttons was inadvertently touched. As with the basic Engine Immobiliser described last month, the keypad version becomes active when power to the ignition is switched on, provided it has already been armed. If the ignition is off, the Immobiliser circuit is off and the only current drain from the battery is that drawn by the keypad Main Features • • • • • • • • Keypad operation to restore normal ignition. 4-digit code entry. Any of 12 keys can be used for the code. Any order, sequence or duplication of code is allowed. LED flashes when ignition is disabled. LED is off when correct code entered to enable normal ignition. Keypad disabled when power to ignition is switched on. Normal ignition cannot be restored by disconnecting and recon­necting    battery supply. • System is armed by pressing a key when the ignition is off (which is not    part of the code). • Can be used in unarmed mode by not pressing a key. January 1999  63 Fig.2: this is the modified circuit of the Engine Immobiliser published last month. Q4 responds to the high signal from the keypad circuit and disables IC1. circuit itself. This draws about 6mA which should not be a problem for the car bat­tery. Circuit details The keypad circuit is shown in Fig.1. The keypad itself has 12-keys which are connected in a matrix of three columns and four rows. As shown on the circuit, the columns are labelled C1, C2 & C3 while the rows are marked R1, R2, R3 & R4. If, for example we press the “1” key, then there will be a connection between row R1 and column C1. Similarly, if the “9” key is pressed, row R3 is connected to column C3, and so on. The keypad circuit has two functions. First, it must detect when buttons are pressed and second, it must detect if they are pressed in the correct order. The first part, detecting when buttons are pressed, is relatively easy and is accomplished with the 4017 decade counter, IC2. This chip is clocked at about 100Hz by a Schmitt trigger oscillator, IC6a, and four of its outputs are connected to the four rows of the keypad matrix. As IC2 is clocked, its outputs cycle high and low and nothing happens until a key is pressed. The column associated with the key is then connected to that key’s row and when that row goes high, perhaps a millisecond later, the key column goes high as well. Each of the three col­umns is monitored by a diode and so the “high” signal is fed via diode D3, D4 or D5 to the “clock enable” line (pin 13) of IC2. This stops IC2 and so the key just pressed will have its column and row both high. IC2 will not start counting again until the pressed key is released. Key detection Four 2-input AND gates, in IC3, are used for key detection. Why only four, considering that there are 12 buttons on the keypad? The reason is that only four digits are used in the code. Each 2-input AND gate has one input connected to a row and one connected to a column, depending on the “hard wire” programming. If we consider IC3a, for example, its inputs are shown connected to row R1 and column C2 and so if key “2” is pressed, both inputs of IC3a will be pulled high and its output at pin 3 will also go high. So far then, we have described how each correct key-press is detected and the four outputs of IC3 will go high if the correct keys are pressed. But the circuit must also detect if the those keys have been pressed in the correct sequence. This is where IC4 and IC5 come into the picture. Sequence detection Fig.3: this is the modified component layout for the Engine Immobiliser, with Q4 and three resistors added in. 64  Silicon Chip IC4 is another 4017 decade counter but it is not clocked in the same way as IC2. It is clocked each time a correct button in the code sequence is pressed. Let’s see how this happens. Say, for example, button 2 is pressed. This will cause the output of AND gate IC3a to go high and pull pin 13 of NAND gate IC5a high as well. At the same time, pin 12 of IC5a will be high because the “0” output of IC4 (pin 3) is high. This will cause pin 11 of IC5a to go low and pull pin 14 (the clock input) low via diode D6. But nothing happens until you take your finger off button 2. This kills the column signal to IC3a, takes pin 3 of IC3a low and so pin 11 of IC5a goes high. It is this “low to high” transi­tion that causes IC4 to be clocked and its “1” output, pin 2, goes high. The next button in our sample 4-bit code is 4. Provided this button is pressed, pin 4 of IC3b goes high, as does pin 9 of IC5b. Its pin 8 will already be high, since it is connected to pin 2 of IC4 and so pin 10 will go low, again pulling pin 14 of IC4 low via diode D7. Again, when button 4 is released, pin 10 goes high and IC4 is clocked by one count, so that its “2” out­put, pin 4, goes high. By now, you should see how the sequence is going. The next button in the 4-bit code is 5 and pressing it causes pin 3 of IC5c to go low and pull pin 14 of IC4 low via diode D8. The end of the correct sequence is when you press button 9 and then take your finger off the button. This again causes IC4 to be clocked and its “4” output, pin 10, goes high. This has two results. First, its high output is fed to the Engine Immobiliser board, to disable its operation. Second, it disables Schmitt trigger oscillator IC6d and LED1 stops flashing. This view shows the Engine Immobiliser PC board with the extra parts added in the bottom lefthand corner. You have to add one transistor and three resistors, with the 10kΩ resistor to the left of the IC replacing a wire link. Invalid keys So far we have seen what happens when you press the correct buttons in sequence. But what happens when someone else has a go and gets it wrong? Previously we noted that each time a key was pressed, a column is connected to a row and when the row output from IC2 went high, one of the three diodes D3, D4 or D5 would feed the high signal to the CE pin and stop the counter while ever the key was pressed. That same high signal is also fed via an RC delay circuit (10kΩ and .01µF) to the reset pin of IC4 but if a correct key has been pressed, this reset signal is suppressed by diode D10 and one of the four diodes D6-D9. The RC delay in the reset signal line ensures that when a “correct” key is pressed, IC4 is not reset. So if keys are pressed in the correct sequence, IC4 is clocked forward with each key press. On the other hand, if a couple of correct keys are pressed and then Fig.4: the component layout for the keypad. The 4-digit code is programmed by installing links on the board to the left of IC2. a wrong key, IC4 will be reset and its “0” output goes high. The correct sequence must now be entered in full for the Immobiliser to be deactivated. Ignition monitoring Transistor Q5 and gate IC6b monitor the +12V line from the ignition keyswitch. With the ignition switch off, Q5 is off and pins 1& 2 of IC6b are high and so pin 15 of IC2 is held low. Hence, IC2 is continually clocked by IC6a and the circuit is waiting for buttons to be pressed. When the ignition is turned on, Q5 turns on and pulls pins 1 & 2 of IC6b low. Thus, pin 15 of IC2 is pulled high, which is the reset condition. IC2 is prevented from clocking and so the cir­cuit cannot respond to any buttons being pressed. By the way, we have used the “2”, “6”, “7” and “3” outputs of IC2 to drive the keypad switch rows and so the rows are not scanned in sequence. The reason for doing this was to make the layout of the PC board more convenient. Power for the circuit is derived from the car battery and this is decoupled via a 39Ω resistor and a 100µF electrolytic capacitor. This effectively filters any hash on the supply line. The 16V zener diode ZD6 clamps any voltage above 16V to protect the ICs from damage. Immobiliser circuit The Engine Immobiliser circuit published last month is modified by the addition of one transistor to make it work with the keypad circuit. The January 1999  65 header for the ribbon cable to the keypad. Next, insert the links which can be made using the tinned copper wire or component pigtails. Before you can insert the links associated with the keypad, you need to decide on the 4-digit code. Have a look at the component overlay diagram in Fig.4. You will notice that there is an area on the board to the left of IC2 which has seven tracks labelled R1-R4 and C1-C3. These correspond to the four rows and three columns of the keypad. Each of the four keys to be programmed has two link connec­ tions, with the lefthand side link connected to one of the four rows and the right-hand connected to one of the three columns. In our example code shown on the circuit, key 2 is pro­ grammed as row 2 and column 1; key 4 key is programmed as row 1 and column 1; key 5 is row 2 and column 2; and key 9 is The keypad board in the prototype was mounted above the Engine Immobiliser board row 3 and column 3. Table 1 in a standard plastic case, with the keypad attached to the lid. Alternatively, you can shows the coding needed for mount the keypad separately on the dashboard. all keys. Having installed all the modified circuit is shown in Fig.2. the Immobiliser PC board is shown links to program the 4-digit code, you The circuit operation is as de- in Fig.3 while the component layout can now install the resistors, followed scribed last month, since the addi- for the keypad PC board is shown in by the diodes. Then install the 16V Fig.4. This board is the same size as zener diode and the transistor. tional transistor is off at all times unless a valid 4-digit code has been the Engine Immobiliser board and is The ICs must be inserted with the fed into the keypad. When that hap- coded 05401991. correct polarity as shown and make pens, the base of Q4 is pulled high sure that you insert the correct type in Construction and it turns on to pull pin 4 of IC1 each position. Finally, the capacitors low. This causes IC1 to stop oscillatYou can begin construction by can be installed, taking care that the ing and its output at pin 3 goes low. checking the PC board for shorts electrolytics are oriented with the This causes all transistors, Q3 to Q1, between tracks, breaks in the tracks, correct polarity. The 0.1µF capacitors to turn off and the Immobi­liser circuit or undrilled holes. Fix any defects (if may be marked as “100n” or “104” then has no further effect on the car’s any) and then fit PC stakes into the while the .01µF capacitor may be ignition system. holes for the external wiring points marked as “10n” or “103”. The modified wiring diagram for on both boards. We used a 7-way pin The assembly procedure for the En- Resistor Colour Codes       No. 1 1 9 1 1 66  Silicon Chip Value 220kΩ 100kΩ 10kΩ 2.2kΩ 39Ω 4-Band Code (1%) red red yellow brown brown black yellow brown brown black orange brown red red red brown orange white black brown 5-Band Code (1%) red red black orange brown brown black black orange brown brown black black red brown red red black brown brown orange white black gold brown Parts List 1 plastic case, 130 x 67 x 43mm 4 M3 screws x 6mm 2 15mm long tapped spacers 1 1m length of heavy duty black automotive hookup wire 1 1m length of heavy duty red automotive hookup wire 1 1m length of light duty red automotive hookup wire 1 1m length of heavy duty yellow automotive hookup wire 1 150mm length of hookup wire Fig.5: actual size artwork for the PC board. Table 1: Programming Links Key 1 2 3 4 5 6 7 8 9 * 0 # Row 1 1 1 2 2 2 3 3 3 4 4 4 Column 1 2 3 1 2 3 1 2 3 1 2 3 gine Immobiliser board was featured last month and we expect that most users will assemble and get it going on its own before making it work with the keypad board. Installation The two boards can be housed in several ways. We stacked the PC boards in a plastic case measuring 130 x 67 x 43mm and mounted the keypad onto the lid. However, you could mount both boards underneath the dash and mount the keypad on the dash itself. We’ll leave that up to you. If you want to take our approach, the PC boards are stacked on top of each other using 2 x 15mm spacers. Note that the inte­gral side ribs in the case will need to be removed using a chisel and a hole drilled in the end of the box for the wiring. The keypad was secured to the lid with four small (M2.5) screws or selftappers. Note also that if the keypad is mounted on the lid as shown in the photo you will need to cut slots for its mounting feet in the sides of the case, so that the lid can later be placed in position. Wiring & testing The boards can be wired up using automotive hookup wire. We used light duty wires for all wiring except for the wires to the ignition coil and ground. Connect the circuit boards to a 12V battery or DC supply. Check that the LED flashes at a one second rate and that the LED stops when the 4-digit code is entered into the keypad. Press any other key (ie, not one included in the code) and check that the LED flashes again. The keypad should now be inop­erative. Connect up the ignition wire to the supply positive. You can verify that the high voltage transistor Q1 comes on by measuring the resistance between its emitter and collector. The transistor will be on when the resistance is low. If the circuit operates properly you are now ready to in­stall it into your vehicle. Find a suitable position under the dashboard to mount the unit and then locate the fused side of the ignition circuit and the fused side of the battery supply. The wiring to these points should be made using automotive connectors. Also you will need a chassis point to connect the ground supply of the circuit to the battery negative Keypad 1 PC board, code 05401991, 106 x 60mm 1 12-switch keypad with 4-row and 3-column addressing 1 5mm LED bezel 6 PC stakes 1 7-way pin header 1 60mm length of 7-way rainbow cable 1 400mm length of 0.8mm diameter tinned copper wire Semiconductors 2 4017 decade counters (IC2,IC4) 1 4081 quad 2-input AND gate (IC3) 1 4011 quad 2-input NAND gate (IC5) 1 4093 quad 2-input Schmitt NAND gate (IC6) 1 BC337 NPN transistor (Q5) 1 16V 1W zener diode (ZD6) 8 1N4148, 1N914 signal diodes (D3-D10) 1 5mm red LED (LED1) Capacitors 2 100µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 1 0.1µF MKT polyester 1 .01µF MKT polyester Resistors (0.25W, 1%) 1 220kΩ 1 2.2kΩ 1 100kΩ 1 39Ω 9 10kΩ terminal. This can be an existing screw in the metalwork or a separate self-tapping screw which secures the eyelet terminal for the ground lead in place. The connection to the ignition coil should be made with an eyelet terminal. This wire should be concealed as much as possible. SC January 1999  67