Alarm Circuits

Police Siren

Police Siren

This circuit produces a sound similar to the police siren.
It makes use of two 555 timer ICs used as astable multivibrators. The frequency is controlled by the pin 5 of the IC.
The first IC (left) is wired to work around 1Hz. The 47uF capacitor is charged and discharged periodically and the voltage across it gradually increases and decreases periodically.
This varying voltage modulates the frequency of the 2nd IC. This process repeats and what you hear is the sound remarkably similar to the police siren.

Two presets VR1 and VR2 are provided to vary the siren period of repetition and the tone of the siren.
By varying VR1 you can set how fast the siren changes from high freq. to low freq.
VR2 sets the siren frequency. Adjust VR1 and VR2 to suit your taste.

Brakelight Flasher

Brakelight Flasher

This is basically a flasher circuit modified to turn on and off a bulb instead of a LED. It uses a 555 timer IC working as an astable multivibrator. The flashing rate can be varied from very fast to a maximum of once in 1.5 sec by varying the preset VR1.
The ON time of the circuit is given by:
TON= 0.69xC1x(R1 + VR1) second

and the OFF time is:
TOFF= 0.69xC1xVR1 second

You can increase the value of C1 to 100uF to get a slower flashing rate of upto once in 10 sec.

Water Level Alarm

Circuit : Andy Collinson
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Description:
A circuit that offers visual indication of fluid level in a vessel, with a switchable audible alarm. Example uses would be to monitor the level of water in a bath or cold storage tank.

The Conductance of Fluids
Conductance is the reciprocal of resistance. The conductance of fluids vary with temperature, volume and separation distance ofthe measurement probes. Tap water has a conductance of about 50 uS / cm measured at 25°C. This is 20k/cm at 25°C. See this site for more details about the conductance of fluids.

Circuit Notes
This circuit will trigger with any fluid with a resistance under 900K between the maximum separation distance of the probes. Let me explain further. The circuit uses a 4050B CMOS hex buffer working on a 5 volt supply. All gates are biased off by the 10M resistors connected between ground and buffer input. The “common” probe the topmost probe above probe 1 in the diagram above is onnected to the positive 5 volt supply. If probe 1 is spaced 1 cm away from the common probe and tap water at 25 C is detected between the probes (a resistance of 20k) then the top gate is activated and the LED 1 will light. Similarly if probe 2 at 2 cm distance from the common probe detects water, LED 2 will light and so on. Switch 1 is used to select which output from the hex buffer will trigger the audible oscillator made from the gates of a CMOS 4011B IC.

Placement of Probes
As 7 wires are needed for the probe I reccommend the use of 8 way computer ribbon cable. The first two wires may be doubled and act as the common probe wire. Each subsequent wire may be cut to required length, if required a couple of millimetres of insulation may be stripped back, though the open “cut off” wire end should be sufficient to act as the probe. The fluid and distance between probe 6 and the common probe wire must be less than 900k. This is because any voltage below 0.5 Volt is detected by the CMOS IC as logic 0. A quick potential check using a 900k resistance and the divider formed with the 10M resistor at the input proves this point:
5 x (0.9 / (0.9+10) = 0.41 Volt

As this voltage is below 0.5 volt it is interpreted as a logic 0 and the LED will light. If measuring tap water at 25 C then the distance between top probe and common may be up to 45 cm apart. For other temperatures and fluids, it is advisable to use an ohmmeter first. When placing the probes the common probe must be the lowest placed probe, as the water level rises, it will first pass probe 1, then 2 and finally probe 6.

5 Zone Alarm System

Circuit : Andy Collinson
Email  :

Description:
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:

 
Please Note: This diagram is drawn with Relay and Switch Contacts labeled as in my Practical Section

Wheres the Parts List ? On the diagram click here for more info.

 
Circuit Notes
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 suppressor. The key switch acts as the Set/Unset and Reset switch. For good security this should be the metal type with a key.

Operation
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.

Gate Alarm Circuit

Circuit : Rev Thomas Scarborough
Description:
A cheap and simple gate alarm made from a single CMOS Integrated Circuit.

Circuit Notes
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 Water Sensor 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

Rain Water Sensor Alarm