Alarms

5 Digit Alarm Keypad

5 Digit Alarm Keypad

is is an enhanced 5 digit keypad which may be used with the Modular Alarm System

This switch will suit the Modular Burglar Alarm circuit. However, it also has other applications. The Keypad must be the kind with a common terminal and a separate connection for each key. On a 12-key pad, look for 13 terminals. The matrix type with 7 terminals will NOT do. Choose the five keys you want as your code, and connect them to ‘A, B, C, D & E’. Wire the common to R1 and all the remaining keys to ‘F’. Because your choice can include the non-numeric symbols, almost 100 000 different codes are available. The Alarm is set using the first four of your five chosen keys. When ‘A, B, C & D’ are pressed in the right order and within the time set by C1 and R2 (about 10 seconds), current through R11 switches Q6 on. The relay energizes, and then holds itself on by providing base current for Q6 through R12. The 12-volt output switches from the “off ” to the “set ” terminal, and the LED lights. To switch the Alarm off again it is necessary to press A, B, C, D & E in the right order. The IC is a quad 2-input AND gate, a Cmos 4081. These gates only produce a high output when both inputs are high. Pressing ‘A’ takes pin 1 high for a period of time set by C1 and R2. This ‘enables’ gate 1, so that when ‘B’ is pressed, the output at pin 3 will go high. This output does two jobs. It locks itself high using R3 and it enables gate 2 by taking pin 5 high. The remaining gates operate in the same way, each locking itself on through a resistor and enabling its successor. If the correct code is entered within the time allowed, pin 10 will switch Q5 on and so connect the base of Q6 to ground. This causes Q6 to switch off and the relay to drop out. Any keys not wired to ‘A, B, C, D or E ‘ are connected to the base of Q4 by R9. Whenever one of these ‘wrong’ keys is pressed, Q4 takes pin 1 low. This removes the ‘enable’ from gate 1, and the code entry process fails. If C, D or E is pressed out of sequence, Q1, Q2 or Q3 will also take pin 1 low, with the same result. You can change the code by altering the keypad connections. If you make a mistake entering the code, just start again. If you need a more secure code you can use a bigger keypad with more ‘wrong’ keys wired to ‘F’. A 16-key pad gives over half a million different codes. All components are shown lying flat on the board; but some are actually mounted upright. The links are bare copper wires on the component side. Two of the links must be fitted before the IC.

Veroboard Layout

Radio Wave Alarm

Radio Wave Alarm

Circuit: Rev. Thomas Scarborough
Email:  scarboro@iafrica.com

This simple circuit is sure to have the police beating a path to your door- however, it has the added advantage of alerting you to their presence even before their footsteps fall on the doormat.

The circuit transmits on Medium Wave (this is the small problem with the police). IC1a, together with a sensor (try a 20cm x 20cm sheet of tin foil) oscillates at just over 1MHz. This is modulated by an audio frequency (a continuous beep) produced by IC1b. When a hand or a foot approaches the sensor, the frequency of the transmitter (IC1a) drops appreciably.

Suppose now that the circuit transmits at 1MHz. Suppose also that your radio is tuned to a frequency just below this. The 1MHz transmission will therefore not be heard by the radio. But bring a hand or a foot near to the sensor, and the transmitter’s frequency will drop, and a beep will be heard from the radio.

Attach the antenna to a multiplug adapter that is plugged into the mains, and you will find that the Medium Wave transmission radiates from every wire in your house. Now place a suitably tuned Medium Wave radio near some wires or a plug point in your house, and an early-warning system is set up.

Instead of using the sheet of tin foil as the sensor, you could use a doorknob, or burglar bars. Or you could use a pushbutton and series resistor (wired in series with the 33K resistor – the pushbutton would short it out) to decrease the frequency of IC1a, so activating the system by means of a pushbutton switch. In this case, the radio would be tuned to a frequency just below that of the transmitter.

novel buzzer

This novel buzzer circuit uses a relay in series with a small audio transformer and speaker. When the switch is pressed, the relay will operate via the transformer primary and closed relay contact. As soon as the relay operates the normally closed contact will open, removing power from the relay, the contacts close and the sequence repeats, all very quickly…so fast that the pulse of current causes fluctuations in the transformer primary, and hence secondary. The speakers tone is thus proportional to relay operating frequency. The capacitor C can be used to “tune” the note. The nominal value is 0.001uF, increasing capacitance lowers the buzzers tone.

Water Activated Alarm

Water Activated Alarm

The circuit uses a 555 timer wired as an astable oscillator and powered by the emitter current of the BC109C. Under dry conditions, the transistor will have no bias current and be fully off. As the probes get wet, a small current flows between base and emitter and the transistor switches on. A larger current flows in the collector circuit enabling the 555 osillator to sound.

An On/Off switch is provided and remember to use a non-reactive metal for the probe contacts. Gold or silver plated contacts from an old relay may be used, however a cheap alternative is to wire alternate copper strips from a piece of veroboard. These will eventually oxidize over but as very little current is flowing in the base circuit, the higher impedance caused by oxidization is not important. No base resistor is necessary as the transistor is in emitter follower, current limit being the impedance at the emitter (the oscillator circuit).

Modular Burglar Alarm

Modular Burglar Alarm

This circuit features automatic Exit and Entry delays and a timed Bell Cut-off. It has provision for both normally-closed and normally-open contacts, and a 24-hour Personal Attack/Tamper zone. It is connected permanently to the 12-volt supply and its operation is “enabled” by opening SW1. By using the expansion modules, you can add as many zones as you require; some or all of which may be the inertia (shock) sensor type. All the green LEDs should be lighting before you open SW1. You then have up to about a minute to leave the building. As you do so, the Buzzer will sound. It should stop sounding when you shut the door behind you. This indicates that the Exit/Entry loop has been successfully restored within the time allowed. When you re-enter the building you have up to about a minute to move SW1 to the off position. If SW1 is not switched off in time, the relay will energise and sound the main bell. It will ring for up to about 40 minutes. But it can be turned off at any time by SW1. The “Instant” zone has no Entry Delay. If you don’t want to use N/O switches, leave out R8, C8 and Q2; and fit a link between Led 3 and C7. The 24 Hour PA/Tamper protection is provided by the SCR/Thyristor. If any of the switches in the N/C loop is opened, R11 will trigger the SCR and the bell will ring. In this case the bell has no time limit. Once the loop is closed again, the SCR may be reset by pressing SW2 and temporarily interrupting the current flow. The basic circuit will be satisfactory in many situations. However, it’s much easier to find a fault when the alarm is divided into zones and the control panel can remember which zone has caused the activation. The expansion modules are designed to do this. Although they will work with the existing instant zone, they are intended to replace it. When a zone is activated, its red LED will light and remain lit until the reset button is pressed. All the modules can share a single reset button.

Miniature Loop Alarm

Miniature Loop Alarm

This circuit is utterly primitive and consists of two identical transistor switches. Each has its own alarm LED and they’re coupled to a neat 82dB buzzer. The two 1N4148 diodes are used to prevent a signal from one sensor from triggering both LEDs. The sensors used are either wire loops or normally closed reed switches or even a combination of both. You could, for example, tie a wire loop to your suitcase and place a reed switch to the door of your hotel room.

Since this little alarm is intended to be kept in arms reach at all times, there aren’t any provisions for automatic shutdown after a certain period of time. The buzzer will sound until you turn the whole circuit off or connect the wire loop back to the jumpers. The same goes for the two LEDs, each indicating its own zone.

Construction is not critical and there aren’t any traps for the novice. The two 100n capacitors aren’t really necessary, I just included them to make sure that there is no noise interference coming from the long wire loops. For transistors, you can use any NPN general-purpose audio amplifiers/switches (BC 107/108/109, BC 237/238, 2N2222, 2N3904…). Assemble the circuit on perf board. Together with the buzzer and a 9V battery, it should easily fit in a pocket-sized plastic box smaller than a pack of cigarettes. A fresh battery should suffice for weeks of continuous operation.

5 Zone Alarm System

5 Zone Alarm System

This is a complete alarm system with 5 independent zones suitable for a small office or home environment. It uses just 3 CMOS IC’s and features a timed entry / exit zone, 4 immediate zones and a panic button. There are indicators for each zone a “system armed” indicator. The schematic is as follows:

Each zone uses a normally closed contact. These can be micro switches or standard alarm contacts (usually reed switches). Suitable switches can be bought from alarm shops and concealed in door frames, or window ledges.

Zone 1 is a timed zone which must be used as the entry and exit point of the building. Zones 2 – 5 are immediate zones, which will trigger the alarm with no delay. Some RF immunity is provided for long wiring runs by the input capacitors, C1-C5. C7 and R14 also form a transient suppresser. The key switch acts as the Set/Unset and Reset switch. For good security this should be the metal type with a key.
At switch on, C6 will charge via R11, this acts as the exit delay and is set to around 30 seconds. This can be altered by varying either C6 or R11. Once the timing period has elapsed, LED6 will light, meaning the system is armed. LED6 may be mounted externally (at the bell box for example) and provides visual indication that the system has set. Once set any contact that opens will trigger the alarm, including Zone 1. To prevent triggering the alarm on entry to the building, the concealed re-entry switch must be operated. This will discharge C6 and start the entry timer. The re-entry switch could be a concealed reed switch, located anywhere in a door frame, but invisible to the eye. The panic switch, when pressed, will trigger the alarm when set. Relay contacts RLA1 provide the latch, RLA2 operate the siren or buzzer.

Parts List:

There are 6 100k resistors, R1, R3, R5, R7, R9, R14.
6 1k resistors, R2, R4, R6, R8, R10, R12.
1 220k resistor R11
1 10k resistor R13. All resistors will be 1/4 watt at 5 or 10& tolerance.
5 100n capacitors, C1 to C5. These may be polyestor or disc ceramic, 50V working or higher.
C6 1 100uF capacitor. This should be electrolytic have a working voltage of at least 25Volts or higher.
C7 1 1uF capacitor. Electrolytic as for C6.
3 1N4148 Diodes, D1, D2, D3 which should be readily obtainable.
1 1N4001 diode D4.
5 LED’s LED1 – LED5. Colour is not important but you may like to use the same colour for zones 2 to 5 and a different colour for zone 1, the entry, exit delay.
1 2N3904 transitor, Q1.
1 4050B CMOS IC for IC1. Note that CMOS 4050BE may also be used.
1 4072B CMOS, IC2
1 4082B CMOS IC, IC3. Note that unused inputs on IC’s 2 and 3 should be connected to earth and that the power pins must be connected.
1 Relay with 2 changeover contacts. The coil needs to match the circuit, i.e. 12V coil, the relay contacts must be suitable for the load. As the load is a piezo buzzer, there will be little load current, so a miniature or sub-miniature relay may be used.
1 keyswitch
1 NO PBS (for the panic switch).
1 reed relay ( for the re-entry switch).
5 NC contact switches. These can be bought from alarm shops and fitted to doors or windows etc.

That completes the full list of components for this circuit. It is an easy matter, and can be applied to any circuit. With experience you just look at the circuit and order all the components required.

Gate Alarm Circuit

Gate Alarm Circuit

A cheap and simple gate alarm made from a single CMOS Integrated Circuit.

Figure 1 represents a cheap and simple Gate Alarm, that is intended to run off a small universal AC-DC power supply.

IC1a is a fast oscillator, and IC1b a slow oscillator, which are combined through IC1c to emit a high pip-pip-pip warning sound when a gate (or window, etc.) is opened. The circuit is intended not so much to sound like a siren or warning device, but rather to give the impression: “You have been noticed.” R1 and D1 may be omitted, and the value of R2 perhaps reduced, to make the Gate Alarm sound more like a warning device. VR1 adjusts the frequency of the sound emitted.

IC1d is a timer which causes the Gate Alarm to emit some 20 to 30 further pips after the gate has been closed again, before it falls silent, as if to say: “I’m more clever than a simple on-off device.” Piezo disk S1 may be replaced with a LED if desired, the LED being wired in series with a 1K resistor.

Figure 2 shows how an ordinary reed switch may be converted to close (a “normally closed” switch) when the gate is opened. A continuity tester makes the work easy. Note that many reed switches are delicate, and therefore wires which are soldered to the reed switch should not be flexed at all near the switch. Other types of switches, such as microswitches, may also be used.

Rain Alarm

Rain Alarm

This circuit gives out an alarm when its sensor is wetted by water.
A 555 astable multivibrator is used here which gives a tone of about 1kHz upon detecting water.
The sensor when wetted by water completes the circuit and makes the 555 oscillate at about 1kHz.

The sensor is also shown in the circuit diagram.
It has to placed making an angle of about 30 – 45 degrees to the ground. This makes the rain water to flow through it to the ground and prevents the alarm from going on due to the stored water on the sensor.
The metal used to make the sensor has to be aluminium and not copper. This is because copper forms a blue oxide on its layer on prolonged exposure to moisture and has to be cleaned regularly.
The aluminium foils may be secured to the wooden / plastic board via epoxy adhesive or small screws.
The contact X and Y from the sensor may be obtained by small crocodile clips or you may use screws.

Theft preventer alarm

Theft preventer alarm

 This circuit utilising a 555 timer IC can be used as an alarm system to prevent the theft of your luggage, burglars breaking into your house etc. The alarms goes ON when a thin wire, usually as thin as a hair is broken.
The circuit is straightforward. It uses a 555 IC wired as an astable multivibrator to produce a tone of frequency of about 1kHz which gives out a shrill noise to scare away the burglar.
The wire used to set off the alarm can be made of a thin copper wire like SWG 36 or higher.
You can even use single strands of copper form a power cable.

The circuit operates on a wide range of voltages from 5V to 15V.
The speaker and the circuit could be housed inside a tin can with holes drilled on the speaker side for the sound to come out.