Two-Zone Alarm - Circuit Description

This alarm is basically a sequence of four Simple Timers. The Exit Timer is first. It allows you to leave the building. This is followed by an Indefinite Pause - while the alarm waits for an intruder. When one of the normally-closed trigger switches is opened - Pin 11 will activate the Latching Timer. It prevents the intruder from defeating the alarm - by simply closing the door behind him. The Latching Timer output starts the Entry Timer. It gives a legitimate visitor time to switch the alarm off. And finally - there's the Cut-Off Timer. It stops the noise from becoming a nuisance.

Simple Cmos Timers

This circuit uses a Cmos 4093. The 4093 has four two-input NAND gates. However - we're not going to use the NAND logic. =============== Note that each gate has its two inputs connected together. This means that the gates are being used as simple inverters. When the input pins are low - the output pin will be high. And when the input pins are high - the output pin will be low. =============== Our alarm is no more than a series of four timers. Each timer operates in turn. The first in the sequence - is the Exit Timer. It's provided by C3 & R6. =============== The alarm can't be activated until pin 11 goes high. And pin 11 won't go high - until pins 12 & 13 are taken low - by the junction of C3 & R6. This takes about 30 seconds. During this time - you may leave the building without activating the alarm. =============== For this type of application - the length of the delay doesn't need to be precise. So it doesn't really need to be adjustable. If you want a different exit time - change the value of R6. Try a few different values - until you get roughly the delay you require. It won't be exact - but it should be close enough to do. =============== THE LATCHING TIMER =============== The second timer is provided by C2 & R3. When the exit delay expires - pin 11 goes high. And current from pin 11 flows to ground - through R5 - the green LED - and the normally closed loop. =============== The loop diverts the pin 11 current to ground - temporarily. R5 keeps that current below about 5mA. And the green LED tells you that the system is armed - and awaiting an intruder. This is the stage I've described as the "Indefinite Pause". =============== When an intruder breaks the loop - the pin 11 current can no longer flow to ground through the green LED. Instead - it charges C2 rapidly - through D3. =============== The diode creates a one-way path. When the loop is opened - D3 allows pin 11 to charge C2. But it prevents a closed loop - from rapidly discharging C2. Instead - the capacitor has to discharge slowly - through R3. =============== This is the latch. The latch prevents an intruder from defeating the alarm - by simply closing the door behind him. Even if the loop is only open for a fraction of a second - the charge in C2 will hold pins 5 & 6 high - for two to three minutes. =============== THE ENTRY TIMER =============== The entry delay is provided by C1 & R2. When C2 takes pins 5 & 6 high - pin 4 will go low. Then C1 will begin to charge through R2 & D2. =============== After about 30 seconds - the junction of C1 & R2 will take pins 1 & 2 low. So pin 3 will go high. When pin 3 goes high - it's connected internally to the positive line. And it supplies base current to Q1 - through R1. =============== The base current switches the transistor on. The transistor connects the negative side of the relay coil to ground. And the relay energizes. =============== When the relay energizes - its contacts deliver power to the siren. You'll normally switch the alarm off - before this 30 second period expires. That is - before the relay energizes - and the siren sounds. =============== The Entry Timer is almost exactly the same as the Exit Timer. The only difference is in the way the resistor is connected to ground. R6 is connected directly to ground - so the Exit Timer will start to run from the moment the alarm is switched on. =============== But we don't want the Entry Timer to start - until the alarm is activated. That is - we don't want the entry delay to begin - until the loop is opened - and pin 11 charges C2. =============== When C2 takes pins 5 & 6 high - pin 4 will go low. When pin 4 goes low - it's connected internally to the negative line. So pin 4 connects R2 to ground - and the entry delay begins. =============== Again - a fixed resistor is usually adequate. The length of the entry delay doesn't really need to be adjustable. If you want a different entry time - change the value of R2. Try a few different values - till you get roughly the delay you require. =============== If you don't switch the alarm off before the entry delay expires - the junction of C1 & R2 will take pins 1 & 2 low. So pin 3 will go high - the transistor will switch on - and the relay will energize. Then the relay contacts will sound the siren - and raise the alarm. =============== There's nothing special about the BC547. Any small NPN transistor - with a gain (hfe) greater than 100 - and an IC(max) of at least 100mA will do. But always check the pin configuration of your transistor. =============== THE CUT-OFF TIMER =============== The siren Cut-Off Timer won't start to run until the loop is restored. When the pin 11 current is once more diverted to ground - C2 will begin to discharge through R3. Two to three minutes later - pins 5 & 6 will go low - and pin 4 will go high. =============== When pin 4 goes high - it breaks the connection between R2 and ground. This means that R2 can no longer keep C1 charged. And the capacitor will begin to discharge slowly - through R4. =============== After about 8 to 10 minutes - C1 will have discharged sufficiently - to take pins 1 & 2 high. Then pin 3 will go low - the transistor will switch off - the relay will de-energize - and the siren will be silenced. =============== D2 creates a one-way path. It allows a low pin 4 - to charge C1 - in about 30 seconds But it prevents a high pin 4 - from discharging C1 - at a similar speed. In other words - C1 can charge quickly through R2 & D2. But it has to discharge slowly - through R4. =============== When the loop has been restored - and C1 & C2 have both been discharged by their respective resistors - the alarm circuit is reset. Once more - it's paused - and awaiting an intruder. In other words - it's ready to go again - the next time the loop is opened. =============== Cmos Timers =============== Since the logic is irrelevant - you don't have to use a 4093. You could choose from a wide range of Cmos gates, inverters and buffers. But you'll get the best results from the Schmidt-Trigger devices. That is - from the 4093 and the 4106. =============== The alarm is basically a series of four timers. And Cmos Schmidt-Trigger devices have a couple of features - that make them ideal for timer circuits. =============== The first is an output pin that's reluctant to change its state. It holds on till the last possible moment - before it finally lets go. As a result - for any given value of resistor and capacitor - the Schmidt-Trigger device will produce a longer delay. =============== The second feature is an output pin that changes state rapidly. When the time comes for the relay to drop out - it does so quickly and cleanly. With other Cmos devices - the output pin changes state more slowly. As a result - relays will often rattle for a few moments - before they finally let go. =============== If you'd like to know more about Cmos Timers - click on the link below the diagram.

Until the three capacitors are completely discharged - the various timers are not completely reset. When the alarm is switched off - the existing circuit components will provide a number of - more or less satisfactory - discharge paths. And R7 will help - by creating a few more.

In normal use - this uncertainty surrounding the precise discharge routes - probably isn't a problem. But an alarm circuit should really operate reliably and predictably - every time. So we're going to improve the design - by providing a dedicated rapid discharge path - for each capacitor.

This circuit is exactly the same as the last - except that we've shortened the discharge times. With the help of three extra diodes - when Sw1 is switched off - each capacitor will discharge rapidly through R7. In each case - the diode speeds up the discharge - by bypassing the capacitor's timing resistor. =============== While Sw1 is in the Off position - R7 is connected across the plates of all three capacitors. Since it's only in circuit when the alarm is switched off - it doesn't use any power. The value of R7 is not too important. But it should be low enough to discharge the capacitors fairly quickly. And it should be high enough to prevent the peak discharge current from damaging the three additional diodes. =============== The three discharge paths only exist when Sw1 is in the Off position. And some of the discharge paths pass through the negative line. Note that if two components are connected to the negative line - they are also connected to each other. For example - R7 & D5 are connected together at the negative line. The same is true of R7 & D6. =============== Try to follow the discharge paths on the drawing. In each case - start at the positive plate of the capacitor in question. C1 will discharge rapidly - when its positive plate is connected to its negative plate - through Sw1, R7 & D5 C2 will discharge rapidly - when its positive plate is connected to its negative plate - through D4, Sw1 & R7 And C3 will discharge rapidly - when its positive plate is connected to its negative plate - through Sw1, R7 & D6 ===============

Here we're going to complete the alarm circuit - by adding an entry buzzer - and an instant alarm zone. We'll use a second transistor to sound the buzzer. And the spare inverter will provide the instant zone. The instant zone will bypass the entry delay - and sound the siren immediately.