Magic Eye New

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OVERVIEWThe Magic Eye is a versatile electronic eye that can

be used for a variety of applications like a burglar alarm, smoke detector or an automatic guest indicator. All these applications make use of a Light Dependent Resistor (LDR), that is installed and implemented differently in each case.

COMPONENT DESCRIPTION

Single Timer IC NE555:The NE555 is a precision timing circuit capable of pro-

ducing accurate time delays or oscillations (pulses). The NE555 usually operates in two modes- mono-stable mode (for producing a single pulse when triggered) and the asta-ble mode (for generating pulses or oscillations, with the desired duty cycle). The threshold (Pin6) and trigger (Pin2) level for the NE555 are normally two-thirds and one-third, respectively, of VCC. However, connecting the trigger input to the threshold input causes the NE555 to run as a multi-vibrator. This is known as the astable mode of operation whereby pulses or oscillations are generated, the frequency

and duty cycle of which can be controlled independently with two external resistors and a single external capacitor (this purpose is served by R1, R2, the LDR and C3 in the Magic Eye circuit).

The reset pin “active low” of the NE555 normally is held high in the Magic Eye circuit. When this reset pin is held high in the astable mode, the output (pin3) of the NE555 becomes high and oscillations are generated at this pin which can be used to drive loudspeak-ers, motors, piezoelectric transducers, etc. The output of this IC can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply many output transducers directly, in-cluding loudspeakers (with a capacitor in series to block the DC and allow the AC to pass).

Light Dependent Resistor (LDR): A light dependent resistor (LDR or

Magic Eyea photo-resistor) is a resistor whose resistance decreases with increasing intensity of incident light. It can also be referred to as a photoconductor. An LDR is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconduc-tor gives it enough energy to conduct electricity, thereby lowering the re-

sistance. This feature of the LDR is smartly combined in the circuit to activate or de-active the alarm generated through the NE555 timer IC.

Resistors:The most basic role of resistors is current limiting i.e.

precisely controlling the quantity of electrical current that is going to flow through a device or a conductor. Resistors can also be used as voltage divider, in other words they can be used to generate any voltage from an initial bigger voltage by dividing it. They are also used as pull-up resis-tors in electronic logic circuits to ensure that inputs to logic systems settle at expected logic levels if external devices are disconnected or high-impedance. They may also be used at the interface between two different types of logic devices, possibly operating at different power supply voltages.

Capacitors:Capacitors store electric charge. They are used with re-

sistors in timing circuits because it takes time for a capacitor to fill with charge. In the fire alarm circuitry, the values of resistors R1, R2 and capacitor C3 decide the frequency of astable oscillations being generated by the NE555 timer IC.

Note: Electrolytic capacitors are polarized and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat

when soldering.

WORKING PRINCIPLEThe Magic Eye circuit employs a sim-

ple mechanism to sound an alarm when a shadow falls on the LDR. In the circuit, the Light Dependent Resistor (LDR) var-ies its resistance corresponding to a vari-ation in the intensity of light falling on it. Typically, when the light intensity is high they offer a high resistance. However, when a shadow falls on the LDR (i.e. the amount of incident light decreases), the resistance of the LDR rapidly falls.

Fig. 1: Block diagram of the Magic Eye

Fig. 2: Pin-out of a NE555 Timer IC

PARTS LISTSemiconductors: IC1 - NE555 Multi-vibrator/

TimerResistors (all ¼-watt, ±5% carbon): R1 - 2.2 kilo-ohmR2 - 100 kilo-ohmLight Dependent Resistor (LDR)Capacitors: C1 - 22µF, 12V ElectrolyteC2 - 0.02µF, Ceramic DiskC3 - 0.1µF, Ceramic DiskMiscellaneous: LS- 4 ohm, 1Watt LoudspeakerPrinted Circuit Board (PCB)

Fig. 3: Representation and Circuit Symbol of an LDR

DESCRIPTION

Parts ListSemiconductors:IC1 - NE555 Multi-vibrator/TimerResistors (all ¼-watt, ±5% carbon):R1 - 2.2 kilo-ohmR2 - 100 kilo-ohmLight Dependent Resistor (LDR)Capacitors:C1 - 22μF, 12V ElectrolyteC2 - 0.02μF, Ceramic DiskC3 - 0.1μF, Ceramic DiskMiscellaneous:LS- 4 ohm, 1Watt LoudspeakerPrinted Circuit Board (PCB)

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Consider two cases:1. In the absence of guest or intruder, light is directly

incident on the LDR and its resistance is high. Since the NE555 timer IC is connected to operate in the astable mode of operation, this high resistance offered by the LDR gener-ates an output at pin3, but not an inaudible one.

2. Conversely, when the shadow of a guest or an intruder is cast upon the LDR its resistance decreases. This change in the resistance of the LDR correspondingly alters the duty cycle of the oscillations generated at the output pin (pin3) of the NE555. The frequency of oscillations generated is now in the audible range and this is fed to the loudspeak-er through the capacitor C1. The loudspeaker is basically a transducer that converts this oscillating electrical signal into sound vibrations. Hence, an alarm sound is generated by the loudspeaker to alert about the guest or intruder at the door.

ASSEMBLING THE KITPreparing the soldering iron:

• Place the soldering iron in its stand and plug in. The iron will take a few minutes to reach its operating tem-perature of about 400°C.

• Dampen the sponge in the stand. • Wait a few minutes for the soldering iron to warm

up. You can check if it is ready by trying to melt a little sol-der on the tip.

• Wipe the tip of the iron on the damp sponge. This will clean the tip. Melt a little solder on the tip of the iron.

Soldering the components:• Use the component overlay on the PCB to insert

the components and solder them in the following order:1. Resistors 2. Capacitors 3. IC socket4. Light Dependent Resistor5. Loudspeaker6. Connection to Supply• Hold the soldering iron like a pen, near the base

of the handle. Touch the soldering iron onto the joint to be made. Make sure it touches both the component lead and the track.

• Hold the tip there for a few seconds and feed a little solder onto the joint. It should flow smoothly onto the lead and track to form a volcano shape. Apply the solder to the joint, not the iron.

• Remove the solder, then the iron, while keeping the joint still.

• Once all the components have been assembled onto the PCB, check the soldering for breaks. Inspect your work carefully under a bright light. The solder joints should have a ‘shiny’ look about them. Check that there are no solder bridges between adjacent pads. Connectivity in circuit can

be tested with the help of a multi-meter.

Precautions to be taken:• It is preferable to use multi-thread wires for con-

nection with the loudspeaker, rather than single thread copper wires since they tend to break upon soldering.

• The pins of the IC sockets may have to be adjusted and bent in order to insert them into the drill-holes prior to soldering. (The sockets and not the ICs are soldered on the PCB, this is to make the mounting and dismounting of the ICs easy while testing and trouble-shooting)

• IC holders/sockets should be soldered with the notch at the correct end.

• Please note that the LDR does not have polarity specific connections, just like any other resistor

PRE-TESTDo not insert the NE555 IC into the socket yet. First,

check the 6V power supply that is to be applied and its po-larity with a multi-meter. Next, connect this +6V DC supply

Completed prototype after assembling the components and soldering

Component identification

Loudspeaker

CapacitorC3

Light Dependent Resistor (LDR)

CapacitorsC2, C3

Resistors (R1, R2)

Printed Circuit Board (PCB)IC Socket

NE555 (IC1)

LoudspeakerConnections to +6V DC Supply

LDR


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and measure the voltage across pin 8 (+V cc) and pin 1 (GND) of the NE555. It should read approximately 6V. If this reading is ok then remove power and insert the IC. Take care to not to bend any of the IC pins while doing so. Accidentally applying a voltage larger than 16 volts to the circuit can burn the IC or some other sensitive component in the circuit.

TESTINGNow, apply the supply voltage.

There should be no alarm sound in the normal state. To perform a test on the working of the Magic Eye, cover the LDR with your hand or with a pen-cap so as to shadow it from the light incident on it. As the hand blocks the light falling on the LDR (i.e. the amount of incident light decreases), the resistance of the LDR rapidly falls and the NE555 generates oscillations of audible frequency. Hence, an alarm sound will be generated by the loudspeaker.

TROUBLESHOOTINGIf the circuit is not working as desired then proceed as

below:• It is preferable to begin trouble-shooting process

from output stage onwards and then proceed to the input side. To begin, check for voltage change at pin3 (output pin) of the NE555. When the LDR is not shadowed, the volt-

age level of this pin should be “low”. However when the LDR is covered with a pen-cap or by hand, the voltage at this pin should go from low to high (approximately 1.12V). If this kind of switching action is taking place then your IC is working. If voltage changes do not take place then there may be problem in the IC.

• Remove the IC from the IC socket and check that no IC pins are bent up under the body of the ICs. This can sometimes happen when inserting ICs into sockets. Check for voltages at

characteristic socket-pins. Typical values can be checked for by referring to the datasheets. If the IC appears to have heated then replace it.

• Next check the polarity of the electrolytic capaci-tor and whether all the resistances being used are of the required value. Verify that you have the right components in the right place.

Once the circuit problem has been resolved your Magic Eye is ready to be installed and used!

CONTACT DETAILSFor full range of available kits, take a look at our web-

site: www.kitnspares.com

Documented by Aditi Goswami

Fig. 4: Circuit Diagram of the Magic Eye

Notes

Magic Eye New

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