House Security System

Here is a low-cost,
invisible laser circuit to protect your house from thieves or
trespassers. A laser pointer torch, which is easily available in the
market, can be used to operate this device. The block diagram of the
unit shown in Fig. 1 depicts the overall arrangement for providing
security to a house. A laser torch powered by 3V power-supply is used
for generating a laser beam. A combination of plain mirrors M1 through
M6 is used to direct the laser beam around the house to form a net. The
laser beam is directed to finally fall on an LDR
that forms part of the receiver unit as shown in Fig. 2. Any
interruption of the beam by a thief/trespasser will result into
energization of the alarm.

The 3V power-supply circuit is a conventional full-wave
rectifier-filter circuit. Any alarm unit that operates on 230V AC can be
connected at the output. The receiver unit comprises two identical
step-down transformers (X1 and X2), two 6V relays (RL1 and RL2), an LDR,
a transistor, and a few other passive components. When switches S1 and
S2 are activated, transformer X1, followed by a full-wave rectifier and
smoothing capacitor C1, drives relay RL1 through the laser switch. The
laser beam should be aimed continuously on LDR. As long as the laser beam falls on LDR, transistor T1 remains forward biased and relay RL1 is thus in energised condition.

When a person crosses the line of laser beam, relay RL1 turns off
and transformer X2 gets energised to provide a parallel path across N/C
contact and the pole of relay RL1. In this condition, the laser beam
will have no effect on LDR and the alarm will
continue to operate as long as switch S2 is on. When the torch is
switched on, the pointed laser beam is reflected from a definite
point/place on the periphery of the house. Making use of a set of
properly oriented mirrors one can form an invisible net of laser rays as
shown in the block diagram. The final ray should fall on LDR of the circuit. Note. LDR
should be kept in a long pipe to protect it from other sources of
light, and its total distance from the source may be kept limited to 500
metres. The total cost of the circuit, including the laser torch, is Rs
400 or less.

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Radiator Temperature Indicator

This radiator
temperature indicator can be designed using electronic circuit diagram
below. Temperature indicator consists of two special zener diode, D1 and
D2, connected in series to ensure accuracy of 5.96 V Zener voltage at
25 ° C. As long as the radiator temperature does not exceed 50 ° C,
thermal indicator will flash a green LED, one orange will be provided for temperatures of 50 … 75 ° C and a red LED, for temperatures above 75 ° C.

Zener voltage will increase by 20 mV for each temperature increase
of a degree Celsius temperature. Radiator temperature corresponding
voltage level is compared with two reference voltages, IC1 and IC2
using. When the temperature reaches 50 ° C, IC2’s output goes to logic
state “1” so that T3 leads and following ignition with diode D4. At 75 °
C, IC1’s output is in logic state “1” and, therefore, T2 and T3 will,
so that D3 and D4 lights are off.

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Pulse Rate Monitor

This simple circuit
enables you to listen to your heartbeat, for instance, while you are
exercising. The transducer used for detecting the pulse is an electret
microphone, X1 in the diagram. The model used has two (polarized)
terminals. As usual with this type of microphone, it functions via a
series resistor, R1. The potential drop across this resistor is applied
to op amp IC1a via C1. The amplification of the op amp is set to between
´40 and ´1000 with preset P1. Network R4-C3 in the feedback loop of
IC1a is a low-pass filter with a cut-off frequency of 34 Hz. Higher
frequencies are not needed for the present application. A pulse rate of
180* is equivalent to a frequency of 3 Hz.

So as to cater for a wide range of pulse rates, the cut-off
frequency is made just over 11 times as high as that representing the
highest pulse rate. Operational amplifier IC1c, in conjunction with
push-pull am-plifier T1-T2, creates a headphone amplifier, whose output
resistance is equivalent to the value of R9, that is, 47 Ω. This makes
the circuit usable for virtually any kind of headset. The output is
short-circuit-proof. In case of certain headphones, such as that used
with Sony Walkman™ sets, it is best to connect the two earphones in
series. Operational amplifier IC1b is used as an active potential
divider. The voltage across the actual divider, R5-R6, is half the
supply voltage.

Pulse Rate Monitor Circuit

Pulse Rate Monitor Circuit Diagram

This voltage is buffered by IC1b, taken from the low-resistance
output, pin 7, of this op amp and used as reference for IC1a, and as
operating voltage for the electret microphone. The voltage is decoupled
by C4 to remove any interference signals from it. The supply voltage for
the pulse rate monitor is decoupled by capacitor C7, immediately after
polarity protection diode D1. Owing to the use of CMOS
op amps, the current drain does not exceed 10 mA, so that operation
from a 9 V battery is perfectly feasible. A dry alkaline manganese
battery will have a life of about 50 hours.

Unless you are a young superfit top-class athlete, you should see
your GP immediately when you find you have a pulse rate of 180. As a
general guide, the absolute maximum pulse rate for a young, very fit
person is 180, for a middle-aged person, 160, and for an elderly person,
140. When exercising, the pulse rate of a not very fit person should
not exceed 60% of these maxima.

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Ultrasonic Distant Obstacle Detector

The first sensor a robot
usually gets fitted with is an obstacle detector. It may take three
different forms, depending on the type of obstacle you want to detect
and also — indeed, above all — on the distance at which you want
detection to take place. For close or very close obstacles, reflective
IR sensors are most often used, an example of such a project appears
elsewhere in this blog. These sensors are however limited to distances
of a few mm to ten or so mm at most. Another simple and
frequently-encountered solution consists of using antennae-like contact
detectors or ‘whiskers’, which are nothing more than longer or shorter
pieces of piano wire or something similar operating microswitches.

Ultrasonic Distant Obstacle Detector Circuit

Ultrasonic Distant Obstacle Detector Circuit Diagram

Detection takes place at a slightly greater distance than with IR
sensors, but is still limited to a few cm, as otherwise the whiskers
become too long and hinder the robot’s normal movement, as they run the
risk of getting caught up in things around it. For obstacles more than a
couple of cm away, there is another effective solution, which is to use
ultrasound. It’s often tricky to use, as designers think as if they
needed to produce a telemeter, when in fact here we’re just looking at
detecting the presence or absence of obstacles, not measuring how far
away they are. So here we’re suggesting an original approach that makes
it possible to reduce the circuit required to a handful of cheap,
ordinary components.

Our solution is based on the howlround or feedback effect all too
familiar to sound engineers. This effect, which appears as a more or
less violent squealing, occurs when a microphone picks up sound from
speakers that are connected to it via an amplifier. Feeding back the
output signal from the speaker into the input (the microphone) in this
way creates an acoustic oscillator. Our detector works on the same
principle, except that the microphone is an ultrasound receiver while
the speaker is an ultrasonic emitter. They are linked just by a very
easily-built ordinary amplifier. Feedback from the output to the input
occurs only when the ultrasonic beam is reflected off the obstacle we
are trying to detect.

As Figure 1 shows, the receiver RXUS is
connected to the input of a high-gain amplifier using transistors T1 and
T2. As the gain of this stage is very high, it can be reduced if
necessary by pot P1 to avoid its going into oscillation all on its own,
even in the absence of an obstacle. The output of this amplifier is
connected to the ultrasonic emitter TXUS,
therby forming the loop that is liable to oscillate due to the effect of
feedback. When this takes place, i.e. when an obstacle is close enough
to the ultrasonic transducers, a pseudo-sine wave signal at their
resonant frequency of 40 kHz appears at the amplifier output, i.e. at
the terminals of the transmitting transducer.

This signal is rectified by D1 and D2 and filtered by C3 and, if its
amplitude is high enough, it produces a current in R6 capable of
turning transistor T3 on to a greater or lesser extent. Depending on the
nature and distance of the obstacle, this process does not necessarily
happen in a completely on/off manner, and so the level available at T3
collector may be quite poorly-defined. The Schmitt CMOS
invertors are there to convert it into a logic signal worthy of the
name. So in the presence of an obstacle, S1 goes high and S2 goes low.
Powering can be from any voltage between 5 and 12 V.

The gain, and hence the circuit’s detection sensitivity, does vary a
bit with the supply voltage, but in all cases P1 makes it possible to
achieve a satisfactory setting. Although it is very simple, under good
conditions this circuit is capable of detecting a
normally-ultrasound-reflective obstacle up to around 5 or 6 cm away. If a
smaller distance is needed, you simply have to reduce the gain by
adjusting P1. Building the circuit is straightforward. Both transducers
are 40 kHz types that can be found in any retailers, and the other
components couldn’t be more ordinary.

However, one precaution is needed when wiring up the transducers.
Even though they aren’t strictly speaking polarized as such, one of
their terminals is common with the metal case, and this is the one that
must be connected to the circuit earth, on both emitter and receiver.
The circuit should work at once, and all you have to do is adjust P1 to
set the detection distance you want — but this is also dependent on the
positioning of the transducers. For optimum operation, we recommend you
angle them as shown in Figure 2.

Author: B. Broussas – Copyright: Elektor Electronics 2007

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Home Made Metal Detector

This homemade metal
detector circuit will help you find objects composed of materials with
relatively high magnetic permeability. It is not suitable for buried
coins discovery that is not sensitive enough but you can detect pirates
treasures!

The metal detector is powered by 2 × 9V batteries, each of it
charges with 15mA. L1 detector coil is part of the sinusoidal oscillator
built around transistor T1. Normally, the center frequency of the
voltage controlled oscillator (VCO) from the PLL
loop that is contained in IC1 is equal to the oscillation frequency of
T1. This changes when entering a metallic object (ferrous or nonferrous)
in the field induced by L1. S1 is a miniature 2-pole switch.

Meter needle deviation is a measure of frequency change, since the
direction of deviation depends on the type of material detected by the
coil.
The meter tool used for this homemade metal detector is zero as central, +-50µA.

Metal detector circuit schematic

Metal detector circuit schematic

Coil L1 consists of 40 turns of enamelled copper wire, wound on a
plastic template with a diameter of about 10 cm. Inductance thus
obtained ensure the functioning of the oscillator at a frequency
approximately equal to the VCO included in the PLL loop.

Use an oscilloscope to check that pin 2 of IC1 delivers sinusoidal
signal with frequency about 75 kHz. Adjust P1 so that fronts rectangular
signal from pin 4 to coincide with the peaks of the sinusoidal signal
from pin 2. Then, adjust P2 in order to obtain 0 on the meter. Since the
neutral zero setting “runs” with the battery’s decreasing voltage it
will be necessary to restore it (zero balancing) from time to time
during use of the metal detector.

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Electronic Gate Keeper

This circuit can do the
job of a Gatekeeper and alerts you if someone passes through the gate.
The alarm can be an AC bell or a Lamp. The alarm turns on for 1 minute
and stops if the light barrier is restored again. Infrared rays are used
as the light barrier to activate the alarm system based on a
Phototransistor. The high gain NPN Darlington phototransistor L14F1 conducts when its face is illuminated with IR rays.

This brings its collector to ground potential. IC1 is used as a
simple voltage comparator with a potential divider R2 and R3 connected
to its inverting input. So that half supply voltage (6 volts) is
available to its inverting input. Its non inverting input is connected
to the collector of the phototransistor. Normally the output of IC1 will
be low since T1 is conducting.

When the IR beam breaks, the collector of T1 becomes high and the
voltage at the non inverting input of IC1 increases above the voltage at
the inverting input and output becomes high. This triggers the relay
driver T2 and relay turns on. Capacitor C1 gives a short delay at the
non inverting input of IC1 to prevent false triggering. Capacitor C2
keeps the base of T2 high for a short time even if the IR rays restore.

Electronic Gate Keeper Schematics

Electronic Gate Keeper Schematics

Settings:

  1. IR rays should be aligned exactly to the Phototransistor so as to keep the alarm off.
  2. Normal range of the circuit is 2 meters. This can be increased to 5 meters if a convergent lens is used in front of the IR LEDs.
  3. A buzzer can be used in the place of the relay, if AC alarm is not required.
  4. Instead of IR LEDs, a Laser pointer can be used to increase the range up to 25 meters.
  5. If Laser is used, take all precautions to prevent direct viewing.
  6. Enclose to the Phototransistor in a small case with an opening in front. This prevents the entry of ambient light.
  7. Fix the IR LEDs on one gate pillar and the Photo transistor on the opposite pillar with exact alignment
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Super Light Sensor

This “Super Light
Sensor” responds to minute fluctuations in light level, auto-adjusting
over the range from about 200 lux up to 60,000 lux (ie, from a modestly
lit room to direct sunlight). It has lots of potential uses – eg,
detecting a car entering a driveway, a person moving in a room, or wind
rustling the leaves of a tree. At the same time, it has a high level of
rejection of natural light variations, such as sunrise, sunset and the
movement of clouds. While it is a “passive” system, it can also be used
as an “active” system – ie, used in conjunction with a light beam.

Its great advantage here is that, since it responds to fluctuations
in light level rather than the crossing of a specific light threshold,
it is much more flexible than other typical “active” systems. It can be
placed within the line-of-sight of almost any light source, including
“vague” ambient light, and simply switched on. As shown, the LDR
is wired as part of a voltage divider so that, between darkness and
full sunlight, its output at “X” varies between about one-quarter and
three-quarters of the supply voltage. A wide variety of sensors may be
used in place of the LDR, including photo-transistors, photo-diodes and infrared and ultraviolet devices.

Circuit diagram:

Super Light Sensor Circuit

Super Light Sensor Circuit Diagram

Fig.1: light level fluctuations are detected by LDR1 and the
resulting signal fed to comparator stage IC1. IC1 in turn triggers 7555
timer IC2 which is wired as a monostable and this drives transistor Q2
and a relay.

The signal from the sensor is fed to the inputs of comparator IC1
via two 150kO resistors. However, any signal fluctuations will be
slightly delayed on pin 3 compared to pin 2, due to the 220nF capacitor.
As a result, the pin 6 output of the comparator (IC1) switches low
during short-term signal fluctuations and this triggers monostable timer
IC2. IC2 in turn switches on transistor Q2 which activates Relay 1. It
also lights LED1 via a 1.5kO current-limiting resistor. Trimpot VR2
allows the monostable period to be adjusted between about 3s and 30s.

As with all such circuits, the Super Light Sensor may not work as
well under AC lighting as under natural lighting. If AC lighting does
prove a problem, a 16µF (16V) electrolytic capacitor can be connected
between the sensor output and ground to filter the signal to the
comparator. When pin 3 of IC2 goes high, FET
Q1 also turns on and pulls pin 2 of IC2 high. This transistor remains on
for a very short period after pin 3 goes low again due to the 100nF
capacitor on its gate. This “blanking” is done to allow the circuit time
to settle again after the relay disengages (and stops drawing current).

LDR placement:

The LDR should be installed inside a black tube, as shown here

the LDR should be installed inside a black tube, as shown here.

The “blanking” also makes it possible to run external circuits from
the same power supply as the Super Light Sensor, without upsetting the
circuit. The current consumption is less than 10mA on standby, so that
battery operation (eg, 8 x AA batteries) is feasible. After building the
circuit, switch on and wait for the circuit to settle. It’s then just a
matter of adjusting VR1 so that the circuit has good sensitivity
without false triggering. With some experimentation, it’s possible to
set the circuit to change seamlessly from natural to AC lighting. If
maximum sensitivity under natural lighting false triggers the circuit
under AC, then adjust VR1 to give maximum sensitivity under AC (and vice
versa).

In daylight, the Super Light Sensor will typically detect a single
finger moving at a distance of 3m, without the use of any lenses. It
will also detect a person crossing a path at a distance of more than
10m, again without lenses. And when used as an “active” system, it will
typically detect a person walking in front of an ordinary light source
(eg, a 60W incandescent light-bulb) at more than 10m. Note that these
ranges are achieved by placing the LDR (which
is used as the light sensor) in a black tube, as shown in Fig.2. A
single lens will double these distances, while the use of two lenses in
an “active” system will multiply the basic range by 6 or 7.

Author: Thomas Scarborough – Copyright: Silicon Chip Electronics Magazine

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Infra-Red Proximity Detector

The detector is intended
for the recognition of obstructions at distances of a few millimeters
to a few centimeters. Similar detectors are used in the industry and
health services, for instance, to open a water tap via a magnetic valve.
The sensor, IC2, is a Type SFH900 optoisolator from Siemens or similar.
A phase-locked loop (PLL) in decoder IC1
compares the frequency of the input signal from IC2 with that of an
internally generated signal. When the two signals fall within the same
band, the output, pin 8, of IC1 changes state (from high to low). The
internal oscillator generates a signal at a frequency of about 4.5 kHz
(determined by time constant R1-C1). Its rectangular signal at pin 5
switches on the light-emitting diode in IC2 via T1.

Circuit diagram:

Infra Red Proximity Detector Circuit

Infra-Red Proximity Detector Circuit Diagram

The diode then transmits an infra-red light signal pulsed at 4.5
kHz. When the infra-red light is reflected by a nearby object, the photo
transistor in IC2 provides a signal to pin 3 of IC1 If the frequency of
this signal lies within the same band as that of the internal
generator, pin 8 is connected to earth, whereupon diode D1 lights. The
comparison by the PLL prevents the circuit
reacting to stray light. The sensitivity of the detector may be varied
with P1. The detector with components as specified draws a current of
10–30 mA.

As stated earlier, the optoisolator may be one of several types. It may also be built from a discrete LED
and photo-transistor, but great care should then be taken to ensure
that the photo transistor cannot receive light transmitted by the LED.
A suitable solid-state relay at the output enables larger loads to be
switched. Circuit IC1 can switch currents of up to 100 mA to earth.
Diode D1 should then be omitted.

Author: K. Hagen
Copyright: Elektor Electronics 1998

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Earth Fault Indicator

The security of many
electrical devices depends today on the availability of an earthed mains
outlet. We should remember that these are connected to the frame or to
the metal housing of the equipment and so it routes to the protective
earth (PE) connections. In this setup, mains voltage, however small,
will cause the differential circuit breaker to trip. The circuit breaker
is part of any modern electrical installation. This type of security
device may however become defective due to common corrosion as we have
seen many times on various older household devices, as well as on
construction sites.

Actually, since these devices are frequently in wet conditions, the
screw and/or lug used to connect the earth wire to the device frame
corrodes gradually and ends up breaking or causing a faulty contact. The
remedy is then worse than the problem because the user, thinking that
he/she is protected by earth, does not take special precautions and
risks his/her life. However, all that’s needed is an extremely simple
system to automatically detect any break in the earth connection; so
simple that we ask ourselves why it is not already included as part of
all factory production for appliances that carry any such risk, as we
have discussed above.

We propose it as a project for you to build using this schematic.
The live wire (L) of the mains power supply is connected to diode D1
which ensures simple half-wave rectification which is sufficient for our
use. The current which is available is limited to a very low value by
resistor R2. If the appliance earth connection to which our circuit is
installed is efficient, this current is directed to earth via resistor
R1 and the rest of the circuit is inactive due to insufficient power. If
the earth connection is disconnected, the current supplied by D1 and R2
charges up capacitor C1.

Circuit diagram:

Earth Fault Indicator Circuit

Earth Fault Indicator Circuit Diagram

When the voltage at the terminals of the capacitor reaches about 60
volts, neon indicator light La1 is turned on and emits a flashing light
which discharges capacitor C1 at the same time. This phenomenon is
reproduced indefinitely as long as the earth connection has not been
restored, and the neon light continues to flash to attract attention in
case of danger. Building the project is not particularly difficult but,
since it is a project aimed at human safety, we must take the maximum of
precautions concerning the choice of components utilised. Therefore, C1
must have an operating voltage of at least 160 volts while R2 must be a
0.5-watt resistor, not for reasons of power dissipation, but in order
to maintain the voltage.

The neon light can be any type, possibly used, or it may be part of
an indicator light to make it easier to attach to the protected
appliance. In the second case, we must obviously get rid of its series
resistor which would prevent proper operation here. During installation
of the circuit in the appliance to be protected, we should also clearly
mark Live (L) and Neutral (N) (for example, seek Live with a simple
screwdriver) because inverting these two wires at this point will
disable proper operation. The final point, which is self-evident
considering the principle used here: the earth connection for our setup
must be hooked up to the frame of the appliance to be protected at a
different point than where the normal earth wire is connected.

Author: Christian Tavernier – Copyright: Elektor Electronic Magazine

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Fastest Finger First Indicator

Quiz-type game shows are
increasingly becoming popular on television these days. In such games,
fastest finger first indicators (FFFIs) are
used to test the player’s reaction time. The player’s designated number
is displayed with an audio alarm when the player presses his entry
button. The circuit presented here determines as to which of the four
contestants first pressed the button and locks out the remaining three
entries. Simultaneously, an audio alarm and the correct decimal number
display of the corresponding contestant are activated. When a contestant
presses his switch, the corresponding output of latch IC2 (7475)
changes its logic state from 1 to 0. The combinational circuitry
comprising dual 4-input NAND gates of IC3
(7420) locks out subsequent entries by producing the appropriate
latch-disable signal. Priority encoder IC4 (74147) encodes the
active-low input condition into the corresponding binary coded decimal (BCD) number output.

The outputs of IC4 after inversion by inverter gates inside hex inverter 74LS04 (IC5) are coupled to BCD-to-7-segment decoder/display driver IC6 (7447). The output of IC6 drives common-anode 7-segment LED display (DIS.1,
FND507 or LT542). The audio alarm generator comprises clock oscillator
IC7 (555), whose output drives a loudspeaker. The oscillator frequency
can be varied with the help of preset VR1. Logic 0 state at one of the
outputs of IC2 produces logic 1 input condition at pin 4 of IC7, thereby
enabling the audio oscillator. IC7 needs +12V DC supply for sufficient
alarm level. The remaining circuit operates on regulated +5V DC supply,
which is obtained using IC1 (7805). Once the organiser identifies the
contestant who pressed the switch first, he disables the audio alarm and
at the same time forces the digital display to ‘0’ by pressing reset
pushbutton S5. With a slight modification, this circuit can accommodate
more than four contestants.

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