EVEN if you already have an electronic security system installed in your home or workplace, there is likely to
- 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.
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.