Thursday, October 2, 2014
Car Anti Theft Burglar Alarm Security Circuit
A car thief will avoid a car that causes him trouble and attracts attention. A good anti theft alarm in your car will soon convince a would be thief that there are easier pickings elsewhere.
The present circuit (figure 63), when set, reacts to the opening of a door (via the door switch for the interior courtesy lights). Such switches can also guard the engine compartment and boot. Fitting switches to the rear doors, boot lid, and bonnet makes sense, because thieves aren’t so sporting as to always try to break in via the driver’s or front passenger’s door. The alarm is set by a well hidden switch, S. Once it is set, you have about one minute to leave the car and lock the doors. A lamp La, goes out just before the minute has lapsed. After that, whenever one of the doors is opened even for a short time the alarm goes off unless switch S has been disabled (hopefully by the owner) within 6 seconds from the door opening. The normal car horn is used as alarm and you should therefore check that this is not in easy reach of the thief. lf the horn, for instance, is fitted under the wings of the car, it is fairly easy to cut through the connecting wires. As may be seen from figure 63, the circuit consists of a control stage, the alarm section, and a switching stage which operates the horn via a relay.
Construction hints When the hidden switch S has been closed, the anti—theft alarm is powered, and the following actions take place: a) gates form a NAND gate latch (also called R-S flip—flop) which acts as a (set) (reset)g1ange;Jver switch that can be set to the required state by logic levels via inputs S and R . The convention with these circuits is that they are sg when the O output is high, or reset (= cleared) when the O output is low; the O output is complementary to the O output. The set input (pin 13) is logic high via R2 and R3, while the R input is low. To begin with, capacitor C6 is not charged. The low level at the O output (pg 11) inhibits the clock of IC3 via NAND gates N5 and N6, while the high level at O resets binary counter (C3 to zero. . b) To begin with, the inputs of N4 are logic low, so that its output is ‘l’ and this causes transistor T3 to conduct via D3 and R11: lamp La then lights. At the sang time, T1 is switched on via R9 and short—circuits R2 and R3 which holds the S input logic high. This means that when a door is opened, nothing happens! c) As long as counter IC3 is set to zero, its outputs O and Q7 are logic low, which makes the output of N3 high. As diode D2 is reverse biased, capacitor C6 charges via R5. After about 1 minute (determined by the values of C6 and R5) the trigger level of N4 has been reached, the output of the gate goes low, and La extinguishes. At the same time, C5 charges via R9 so that after six seconds T1 is cut off. The alarm is then set.
Alarm stage
When the alarm is set, and a door—switch is opened, the driver stages, as you have already ascertained, cause a high logic level at pin 11 of IC1 and a low logic level at pin 10. The high level clears the clock generator formed by N5 and N6: the clock pulses are applied to the CLOCK input of IC3. The low level at the CLR (clear) input of the counter releases the handbrake so to speak: IC3 now starts counting the clock pulses. Since outputs O3 and O4 of IC3 are taken through an OR circuit (diodes D7, D8, and resistor R12), the parallel-connected NAND gates N7 and N8 are cleared for a total of four periods after the first four clock pulses: during each of these periods twelve clock pulses are applied to T3. The horn therefore gives twelves short sounds fol- lowed by a pause, then twelve short sounds again, and so on until sixty—four pulses have passed. After the sixty—fourth pulse, both inputs of N3 go high (from O7, O1). The inverted output level of N3 causes the partial discharge of C6 and the reset of flip-flop N1/N2: the counter and clock are then inhibited again. At the same time, N4 causes the base of T1 to go logic high for a brief moment: this allows C5 to discharge. After that, it takes six seconds again for the alarm to become ’live afresh.
Installation
Well, the most difficult part is now behind you: the alarm unit is ready for installation. The best place to fit the alarm is under or behind the instrument panel. Connections of the mating sockets for PLT and PL5 should not present too much of a difficulty, but remember that PLl must be permanently connected to the positive battery line, irrespective of the ignition switch! The positive line is identified in most cars by the code number 30, while earth (chassis = battery negative line) is coded 31. Control lamp La should be fitted in good view of the driver: its terminals should be connected to a socket mating with PL3 and to earth.
The horn relay should be fitted in the engine compartment: the pin—out of a typical horn relay is shown in figure 65. Remember what we said about the location of the horn at the beginning of this chapter: make sure it is not within easy reach of a prospective thief. If it is, a second horn may be the answer, and this should then, of course, be located in a secure position. The door switches in most cars (normally only fitted to the front doos) close when the door is opened and so connect the interior lights to chassis (see lLa in figure 63). lf the socket mating with PL2 is connected to the take—off point of this switch con- tact, the alarm is operational. However, as remarked before, a// doors (if not already factory—fitted), bonnet, and boot lid should be provided with a switch. Microswitches or reed switches which you may already have in stock are perfectly suitable.
As a matter of fact, they are probably better than switches fitted by the car manufacturer: those are invariably of a cheap, ,unprotected type which rusts easily. Obviously, such switches are not very reliable. lt’s therefore better to spend a little more and get g0od·quality switches on which you can depend for reliable operation of your alarm! The real problem is bound to be the location of the alarm switch: thieves seem to have second sight when it comes to spotting non-standard knobs and switches in cars they are interested in. Here you’|l have to use your imagination: on the one hand you dont want to do acrobatics to turn the alarm on, and on the other the switch should be in easy reach known only to yourself. We cannot give you any hints on this, if only because car thieves also read hobby books and will be very grateful for our tips! The terminals of all switches, as well as soldering joints, should be given a good layer of spray—on protective lacquer: this will prevent rattles and corrosion, while connecetions will remain easily undone or unsoldered.
The present circuit (figure 63), when set, reacts to the opening of a door (via the door switch for the interior courtesy lights). Such switches can also guard the engine compartment and boot. Fitting switches to the rear doors, boot lid, and bonnet makes sense, because thieves aren’t so sporting as to always try to break in via the driver’s or front passenger’s door. The alarm is set by a well hidden switch, S. Once it is set, you have about one minute to leave the car and lock the doors. A lamp La, goes out just before the minute has lapsed. After that, whenever one of the doors is opened even for a short time the alarm goes off unless switch S has been disabled (hopefully by the owner) within 6 seconds from the door opening. The normal car horn is used as alarm and you should therefore check that this is not in easy reach of the thief. lf the horn, for instance, is fitted under the wings of the car, it is fairly easy to cut through the connecting wires. As may be seen from figure 63, the circuit consists of a control stage, the alarm section, and a switching stage which operates the horn via a relay.
Construction hints When the hidden switch S has been closed, the anti—theft alarm is powered, and the following actions take place: a) gates form a NAND gate latch (also called R-S flip—flop) which acts as a (set) (reset)g1ange;Jver switch that can be set to the required state by logic levels via inputs S and R . The convention with these circuits is that they are sg when the O output is high, or reset (= cleared) when the O output is low; the O output is complementary to the O output. The set input (pin 13) is logic high via R2 and R3, while the R input is low. To begin with, capacitor C6 is not charged. The low level at the O output (pg 11) inhibits the clock of IC3 via NAND gates N5 and N6, while the high level at O resets binary counter (C3 to zero. . b) To begin with, the inputs of N4 are logic low, so that its output is ‘l’ and this causes transistor T3 to conduct via D3 and R11: lamp La then lights. At the sang time, T1 is switched on via R9 and short—circuits R2 and R3 which holds the S input logic high. This means that when a door is opened, nothing happens! c) As long as counter IC3 is set to zero, its outputs O and Q7 are logic low, which makes the output of N3 high. As diode D2 is reverse biased, capacitor C6 charges via R5. After about 1 minute (determined by the values of C6 and R5) the trigger level of N4 has been reached, the output of the gate goes low, and La extinguishes. At the same time, C5 charges via R9 so that after six seconds T1 is cut off. The alarm is then set.
Alarm stage
When the alarm is set, and a door—switch is opened, the driver stages, as you have already ascertained, cause a high logic level at pin 11 of IC1 and a low logic level at pin 10. The high level clears the clock generator formed by N5 and N6: the clock pulses are applied to the CLOCK input of IC3. The low level at the CLR (clear) input of the counter releases the handbrake so to speak: IC3 now starts counting the clock pulses. Since outputs O3 and O4 of IC3 are taken through an OR circuit (diodes D7, D8, and resistor R12), the parallel-connected NAND gates N7 and N8 are cleared for a total of four periods after the first four clock pulses: during each of these periods twelve clock pulses are applied to T3. The horn therefore gives twelves short sounds fol- lowed by a pause, then twelve short sounds again, and so on until sixty—four pulses have passed. After the sixty—fourth pulse, both inputs of N3 go high (from O7, O1). The inverted output level of N3 causes the partial discharge of C6 and the reset of flip-flop N1/N2: the counter and clock are then inhibited again. At the same time, N4 causes the base of T1 to go logic high for a brief moment: this allows C5 to discharge. After that, it takes six seconds again for the alarm to become ’live afresh.
Installation
Well, the most difficult part is now behind you: the alarm unit is ready for installation. The best place to fit the alarm is under or behind the instrument panel. Connections of the mating sockets for PLT and PL5 should not present too much of a difficulty, but remember that PLl must be permanently connected to the positive battery line, irrespective of the ignition switch! The positive line is identified in most cars by the code number 30, while earth (chassis = battery negative line) is coded 31. Control lamp La should be fitted in good view of the driver: its terminals should be connected to a socket mating with PL3 and to earth.
The horn relay should be fitted in the engine compartment: the pin—out of a typical horn relay is shown in figure 65. Remember what we said about the location of the horn at the beginning of this chapter: make sure it is not within easy reach of a prospective thief. If it is, a second horn may be the answer, and this should then, of course, be located in a secure position. The door switches in most cars (normally only fitted to the front doos) close when the door is opened and so connect the interior lights to chassis (see lLa in figure 63). lf the socket mating with PL2 is connected to the take—off point of this switch con- tact, the alarm is operational. However, as remarked before, a// doors (if not already factory—fitted), bonnet, and boot lid should be provided with a switch. Microswitches or reed switches which you may already have in stock are perfectly suitable.
As a matter of fact, they are probably better than switches fitted by the car manufacturer: those are invariably of a cheap, ,unprotected type which rusts easily. Obviously, such switches are not very reliable. lt’s therefore better to spend a little more and get g0od·quality switches on which you can depend for reliable operation of your alarm! The real problem is bound to be the location of the alarm switch: thieves seem to have second sight when it comes to spotting non-standard knobs and switches in cars they are interested in. Here you’|l have to use your imagination: on the one hand you dont want to do acrobatics to turn the alarm on, and on the other the switch should be in easy reach known only to yourself. We cannot give you any hints on this, if only because car thieves also read hobby books and will be very grateful for our tips! The terminals of all switches, as well as soldering joints, should be given a good layer of spray—on protective lacquer: this will prevent rattles and corrosion, while connecetions will remain easily undone or unsoldered.
