EVEN if you already have an electronic security system installed in your home or workplace, there is likely to
be a use for this Door Protector. With any security system, or even with none, it is important that all doors and windows should be protected by bolts, bars, grids or other physical means. It costs relatively little to fix strong bolts or locks to windows and doors, to make it virtually impossible for anyone to gain access without employing drastic measures. Unfortunately, there is nearly always one weak point. This is the Exit Door, the door by which you normally leave the house when you are going out. This is also the door by which you enter the house when you come back home. Other doors (and the windows) are bolted or locked from the inside. Once secured, they can only be opened by someone who is already inside the house. Only physical protection is needed. On the other hand, the Exit/Entry Door has to be openable from outside the house as well. There is a limit to the number of locks that can be fitted, and usually it is not practicable to fit any bolts.
Door Guard
The Door Protector system described here can be set to one of two states:
- Disarmed: After pressing the Disarm button, you may open and close the Exit/Entry Door as often as you like and this has no effect on the siren.
- Armed: You arm the system by pressing the Arm button and then have 20 seconds to leave the house via the Exit Door without making the siren sound. On re-entering the house through the same door, nothing happens for the first 20 seconds but your entry has triggered the system and the siren will start to sound after 20 seconds unless you press the Disarm button. The timings can be altered to suit individual locations. Of course, the function buttons are hidden away so that an intruder cannot quickly find them.
How it works
The circuit is triggered by a switch mounted on the door, and accessible only
from the inside. This may be a microswitch or more conveniently a reed switch that closes when a permanent magnet is near it. Usually the switch is mounted on (or in) the frame of the door and the magnet is mounted on (or in) the door. When the door is open, the magnet no longer has an effect on the switch, which springs open. When the door is closed the magnet comes very close to the switch, causing it to close. As shown in the full circuit diagram for the Door Protector in Fig.1, the door switch S1 is closed whenever the door (with magnet insert) is closed, so pin 9 of IC1a is held at logic low. If the door is opened, even by only a few centimeters and for only a fraction of a second, the input at pin 9 is pulled to logic high, via resistor R1, for long enough to trigger the circuit. If the circuit is in the “disarmed’’ state the other input (pin 8 of IC1a) is at logic low, so the output of the gate at pin 10 remains at logic high, whatever the input to pin 9. Opening and closing the door has no effect on the system. If the system is in the “armed state’’, the input at IC1a pin 8 is high. Then any high level at pin 9 caused by opening the door causes the output at pin 10 to become low. This output goes to a set-reset flip-flop consisting of two NAND gates, IC1b and IC1c. In the reset state, pin 11 of IC1c is high but this goes low (and stays low) when the flip-flop is triggered. The low-going level passes across capacitor C1 and produces a short low pulse that triggers the timer IC2a. The timer output at pin 5 is normally low but now goes high for 20 seconds. The next stage is a pulse generator, formed by IC4a/IC4b which normally has a low output at IC4b pin 11, but produces a short high pulse when the input from the timer goes low, that is, after 20s. The output from the pulse generator goes to another flip-flop, formed this time from a pair of NOR gates IC4c/IC4d. When this receives a high pulse its output at pin 10 goes high and stays high. It turns on transistor TR1, which in turn switches on the siren (WD1). The siren sounds until the system is disarmed or the power is switched off. The remainder of the circuit is concerned with arming and disarming. Pressing the Arm button of switch S2 has two effects. It resets the flip-flop IC1b/IC1c, making its output at pin 11 go high. It is now ready to trigger the timer (IC2a) as already described. The second effect is to trigger another timer, IC2b. The output of this goes high for 20s and, at the end of this period, another pulse generator (IC3a/IC3b) produces a short high pulse. This sets flip-flop IC3c/IC3d, making its output at pin 10 go high. This output is fed back to pin 8 of the input gate IC1a that also received input from the door switch S1. With pin 8 high, pulses from the door switch are passed through to the flip-flop of IC1, so triggering IC2a. The system is now armed, but not until 20s after pressing the Arm button. The Disarm button of pushswitch S3 also has two actions. One function is to produce a low pulse to reset the arm/disarm flip-flop at pin 6 of IC3. The low pulse is also inverted by transistor TR2 and then used to reset the siren flip-flop (IC4c/IC4d) and turn the siren off. If you want to make one or both delay times longer, recalculate the values of the timing capacitor and resistor (R3, C2 or R5, C4), using the formula, t = 1.1RC. The delay time is t seconds, R is in ohms and C is in farads.
from the inside. This may be a microswitch or more conveniently a reed switch that closes when a permanent magnet is near it. Usually the switch is mounted on (or in) the frame of the door and the magnet is mounted on (or in) the door. When the door is open, the magnet no longer has an effect on the switch, which springs open. When the door is closed the magnet comes very close to the switch, causing it to close. As shown in the full circuit diagram for the Door Protector in Fig.1, the door switch S1 is closed whenever the door (with magnet insert) is closed, so pin 9 of IC1a is held at logic low. If the door is opened, even by only a few centimeters and for only a fraction of a second, the input at pin 9 is pulled to logic high, via resistor R1, for long enough to trigger the circuit. If the circuit is in the “disarmed’’ state the other input (pin 8 of IC1a) is at logic low, so the output of the gate at pin 10 remains at logic high, whatever the input to pin 9. Opening and closing the door has no effect on the system. If the system is in the “armed state’’, the input at IC1a pin 8 is high. Then any high level at pin 9 caused by opening the door causes the output at pin 10 to become low. This output goes to a set-reset flip-flop consisting of two NAND gates, IC1b and IC1c. In the reset state, pin 11 of IC1c is high but this goes low (and stays low) when the flip-flop is triggered. The low-going level passes across capacitor C1 and produces a short low pulse that triggers the timer IC2a. The timer output at pin 5 is normally low but now goes high for 20 seconds. The next stage is a pulse generator, formed by IC4a/IC4b which normally has a low output at IC4b pin 11, but produces a short high pulse when the input from the timer goes low, that is, after 20s. The output from the pulse generator goes to another flip-flop, formed this time from a pair of NOR gates IC4c/IC4d. When this receives a high pulse its output at pin 10 goes high and stays high. It turns on transistor TR1, which in turn switches on the siren (WD1). The siren sounds until the system is disarmed or the power is switched off. The remainder of the circuit is concerned with arming and disarming. Pressing the Arm button of switch S2 has two effects. It resets the flip-flop IC1b/IC1c, making its output at pin 11 go high. It is now ready to trigger the timer (IC2a) as already described. The second effect is to trigger another timer, IC2b. The output of this goes high for 20s and, at the end of this period, another pulse generator (IC3a/IC3b) produces a short high pulse. This sets flip-flop IC3c/IC3d, making its output at pin 10 go high. This output is fed back to pin 8 of the input gate IC1a that also received input from the door switch S1. With pin 8 high, pulses from the door switch are passed through to the flip-flop of IC1, so triggering IC2a. The system is now armed, but not until 20s after pressing the Arm button. The Disarm button of pushswitch S3 also has two actions. One function is to produce a low pulse to reset the arm/disarm flip-flop at pin 6 of IC3. The low pulse is also inverted by transistor TR2 and then used to reset the siren flip-flop (IC4c/IC4d) and turn the siren off. If you want to make one or both delay times longer, recalculate the values of the timing capacitor and resistor (R3, C2 or R5, C4), using the formula, t = 1.1RC. The delay time is t seconds, R is in ohms and C is in farads.
Installation of the system
The entire Door Protector system, including the siren, can be located at a convenient point and housed in a single enclosure. However, it makes more sense to mount the siren in a remote and relatively inaccessible place where it can easily be heard and cannot be interfered with. Wiring between the board and siren must be concealed as far as possible. Power leads should likewise be hidden as much as possible. Similarly, you should hide the pushbutton switches, particularly the Disarm one, in a place where they are difficult for an intruder to find but are quick and easy for you to reach. It is not so important to hide the leads to the door switch because, if the intruder finds and cuts these leads, it has the same effect as opening the door; the siren sounds 20 seconds later! In some situations there may be false triggering due to electromagnetic interference picked up in the leads joining the circuit board to the door switch. If this occurs, it can usually be cured by wiring a capacitor between IC1 pin 9 and the 0V rail (C6 in Fig.1).
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