1. INTRODUCTION

In earlier days people used mechanical ON and OFF switch, but now are the days of

touch. The circuit of TOUCH SENSITIVE MUSICAL BELL WITH TIMER helps us to

decrease the effort. It consists of the following components: melody generator IC UM66, CMOS

NAND gates IC CD 4011, transistors SL 100, diodes 1N 4148, resistors, capacitor, touch plates

and a loud speaker.

In this circuit touch plates act as ON switch and OFF switch. When the plates are not

being touched, the circuit is OFF and melody is not generated. When touch plates are bridged by

hand, music is generated.

The output of UM66 is amplified by SL 100 transistor and given to loud speaker. The

loud speaker is responsible for generating the sound. The resistor R2 provides a discharging path

to the transistor and together they are responsible for the time period of the melody.

The three PN junction diodes 1N 4148 are used to reduce the supply voltage by providing

a drop and giving it to the pin 2 i.e. the VCC of UM66.

IC CD4011 contains four two input CMOS NAND gates, When both the inputs to the

NAND gate are logic 1’s only then the output is logic 0. Even if one input is logic 0, the output is

logic 1. In this three of the NAND gates act as inverter.

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2. COMPONENT DESCRIPTION

2.1 MELODY GENERATOR IC UM66

UM66T is a melody integrated circuit. It is designed for use in bells, telephones, toys etc.

It has an inbuilt tone and a beat generator. The tone generator is a programmed divider which

produces certain frequencies. These frequencies are a factor of the oscillator frequency. The beat

generator is also a programmed divider which contains 15 available beats. Four beats of these

can be selected.

  There is an inbuilt oscillator circuit that serves as a time base for beat and tone generator.

It has a 62 notes ROM to play music. A set of 4 bits controls the scale code while 2 bits control

the rhythm code. When power is turned on, the melody generator is reset and melody begins

from the first note. The speaker can be driven by an external npn transistor connected to the

output of UM66.

  Many versions of UM66T are available which generate tone of different songs. For

example, UM66T01 generates tone for songs ‘Jingle bells’, ‘Santa Claus is coming to town’ and

‘We wish you a merry X’mas’.

2.1.1 PIN CONFIGURATION OF UM66

Figure 1 Pin configuration of UM66

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2.1.2 PIN DESCRIPTION OF UM66

Pin No Function Name

1 Melody output Output

2 Supply voltage (1.5V - 4.5V) Vcc

3 Ground (0V) Ground

2.1.3 FEATURES OF UM66

62 note ROM memory

1.3 to 3.3 operating voltage and low power consumption

Dynamic speaker can be driven with an external npn transistor

OSC, resistor is built in

Power on reset, melody begins from first note

2.1.4 PIN DIAGRAM OF UM66

Figure 2 Pin diagram of UM66

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2.1.5 BLOCK DIAGRAM OF UM66

Figure 3 Block diagram of UM66

2.2 CMOS NAND GATES IC CD4011

CD 4011 consists of four two input NAND gates. This IC provides the system designer with

direct implementation of the NAND function and supplements the existing family of CMOS gate. All

inputs and outputs are buffered.

It is provided in 14- lead dual in line ceramic packages.

2.2.1 FEATURES OF CD4011 Propagation delay time = 60 ns

Buffered inputs and outputs

Standardized symmetrical output characteristics

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2.2.2 PIN DIAGRAM OF CD 4011

Figure 4 Pin diagram of CD 4011

Figure 5 CD 4011 DIP package

2.2.3 NAND GATE

Figure 6 NAND gate

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2.3 TOUCH PLATES

Resistance is measured in ohms and when something is a good conductor it has a low

resistance. In other words it has only a few ohms resistance. 

When something is a bad conductor it has a high resistance. In other words it has a resistance of

many ohms, sometimes thousands of ohms or even millions of ohms. 

The resistance between the two tracks on the touch plates (when not being touched) is

many millions of ohms. When you touch the plate, the resistance reduces to about 100,000 ohms.

When you press harder, this decreases to 50,000 or 30,000 ohms. 

When the plate is connected to a circuit, the change in resistance is detected by the circuit

and a certain amount of current flows. This current is very small and the circuit amplifies this

current.

In this circuit touch plates act as ON and OFF switch. In one of the projects the touch pad

is used to turn on a LED. In another project it is used to alter the flash rate of a LED.  

When you press harder on the plate, resistance decreases. Because more of your finger

touches the tracks and more moisture comes out of the pores of your finger. It is the MOISTURE

IN YOUR FINGER (the salts in the moisture) that causes the resistance to reduce. 

The touch plate can also be used as a rain detector. When a drop of water falls on the

grid, it touches the two tracks and reduces the resistance between the terminals. Pure rain water

is non conductive however as the droplets fall through the air they pick up small amounts of

carbon-dioxide and other impurities and this makes the water slightly conductive. 

This type of pad was one of the earliest forms of touch switch and was used in a number

of electronic devices in place of a push button. It was one of the first attempts at a vandal-proof

switch or trouble-free switch; however it suffered from one major problem. If residue was left on

the pad, such as jam, butter or oil from the skin, it became less effective.

In addition, it did not work successfully for all type of users. The effectiveness of the pad

depends on the amount of moisture in the finger and it will not work very effectively with a very

dry finger.  

If your pad does not work as described in any of the experiments, try moistening your

finger slightly and see the results improve. Touch plates have now been replaced with membrane

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switches in most electronic devices. Membranes require only very slight pressure for their

operation and no dirt can enter the sealed switch, but a touch pad is a very good way to show

how the resistance of your skin changes with pressure and moisture content. 

Figure 7 Touch plates

2.4 TRANSISTOR (SL 100)

In this circuit touch sensitive musical bell, we use two SL 100 transistors. One of them

acts as a switch while the other acts as an amplifier.

SL100 is a general purpose, medium power NPN transistor. It is mostly used as switch in

common emitter configuration. The transistor terminals require a fixed DC voltage to operate in

the desired region of its characteristic curves. This is known as the biasing. For switching

applications, SL100 is biased in such a way that it remains fully on if there is a signal at its base.

In the absence of base signal, it gets turned off completely. 

The emitter leg of SL100 is indicated by a protruding edge in the transistor case. The

base is nearest to the emitter while collector lies at other extreme of the casing.

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Figure 8 Pin configuration of SL 100

Figure 9 SL 100

2.5 RESISTORA resistor is a passive two-terminal electrical component that implements electrical

resistance as a circuit element.

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The current through a resistor is in direct proportion to the voltage across the resistor's

terminals. This relationship is represented by Ohm's law:

Where I is the current through the conductor in units of amperes, V is the potential difference

measured across the conductor in units of volts, and R is the resistance of the conductor in units

of ohms.

Resistors are common elements of electrical networks and electronic circuits. Practical

resistors can be made of various compounds and films, as well as wire made of a high-resistivity

alloy, such as nickel-chrome. Resistors are also implemented within integrated circuits,

particularly analog devices, and can also be integrated into hybrid and printed circuits.

The electrical functionality of a resistor is specified by its resistance: common

commercial resistors are manufactured over a range of more than nine orders of magnitude.

When specifying that resistance in an electronic design, the required precision of the resistance

may require attention to the manufacturing tolerance of the chosen resistor, according to its

specific application. The temperature coefficient of the resistance may also be of concern in

some precision applications. Practical resistors are also specified as having a

maximum power rating which must exceed the anticipated power dissipation of that resistor in a

particular circuit: this is mainly of concern in power electronics applications. Resistors with

higher power ratings are physically larger and may require heat sinks.

Practical resistors have a series inductance and a small parallel capacitance; these

specifications can be important in high-frequency applications. In a low-noise amplifier or pre-

amp, the noise characteristics of a resistor may be an issue. The unwanted inductance, excess

noise, and temperature coefficient are mainly dependent on the technology used in

manufacturing the resistor. They are not normally specified individually for a particular family of

resistors manufactured using a particular technology. 

A family of discrete resistors is also characterized according to its form factor, that is, the

size of the device and the position of its leads (or terminals) which is relevant in the practical

manufacturing of circuits using them

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Figure 10 Resistor

2.6 CAPACITORA capacitor (originally known as condenser) is a passive two-terminal electrical

component used to store energy in an electric field. The forms of practical capacitors vary

widely, but all contain at least two electrical conductors separated by a dielectric (insulator);

for example, one common construction consists of metal foils separated by a thin layer of

insulating film. Capacitors are widely used as parts of electrical circuits in many common

electrical devices.

When there is a potential difference (voltage) across the conductors, a static electric

field develops across the dielectric, causing positive charge to collect on one plate and

negative charge on the other plate. Energy is stored in the electrostatic field. An ideal

capacitor is characterized by a single constant value, capacitance, measured in farads. This is

the ratio of the electric charge on each conductor to the potential difference between them.

The capacitance is greatest when there is a narrow separation between large areas of

conductor, hence capacitor conductors are often called plates, referring to an early means of

construction. In practice, the dielectric between the plates passes a small amount of leakage

current and also has an electric field strength limit, resulting in a breakdown voltage, while

the conductors and leads introduce an undesired inductance and resistance.

Capacitors are widely used in electronic circuits for blocking direct current while

allowing alternating current to pass, in filter networks, for smoothing the output of power

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supplies, in the resonant circuits that tune radios to particular frequencies, in electric power

transmission systems for stabilizing voltage and power flow, and for many other purposes.

Figure 11 Capacitor

2.7 LOUD SPEAKER

A loudspeaker (or "speaker") is an electro acoustic transducer that produces sound in

response to an electrical audio signal input. The most common form of loudspeaker uses a paper

cone supporting a voice coil electromagnet acting on a permanent magnet, but many other types

exist. Where accurate reproduction of sound is required, multiple loudspeakers may be used, each

reproducing a part of the audible frequency range.

Miniature loudspeakers are found in devices such as radio and TV receivers, and many

forms of music players. Larger loudspeaker systems are used for music, sound reinforcement in

theatres and concerts, and in public address systems

FEATURES:

Small size

Power rating : 0.5 W

Impedance : 8 ohm

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Figure 12 Loud speaker

2.8 DIODE 1N4148

The 1N4148 is a standard silicon switching diode. It is one of the most popular and long-

lived switching diodes because of its dependable specifications and low cost. Its name follows

the JEDEC nomenclature. The 1N4148 comes in a DO-35 glass package and is useful in

switching applications up to about 100 MHz with a reverse-recovery time of no more than 4 ns.

The 1N4148 replaced the 1N914, which had a much higher leakage current (5 micro

amps vs. 25 nano amps). Since leakage is almost never a desirable property, today manufacturers

produce the 1N4148 and sell it as either part number. These device types have an enduring

popularity in low-current applications.

2.8.1 SPECIFICATIONS

VRRM = 100 V (maximum repetitive reverse voltage)

IO = 200 mA (average rectified forward current)

IF = 300 mA (maximum direct forward current)

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VF = 1.0 V at 10 ma.

IFSM = 1.0 A (pulse width = 1 sec), 4.0 A (pulse width = 1 µsec) (non-repetitive peak

forward surge current)

PD = 500 mW (power dissipation)

TRR < 4 ns (reverse-recovery time)

Figure 13 Diode 1N 4148

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3. CIRCUIT DESCRIPTION AND WORKING

3.1 CIRCUIT DIAGRAM

Figure 14 Circuit diagram of Touch sensitive musical bell

3.2 DESCRIPTION

CD 4011 is a 14 pin CMOS NAND gate IC. Pin 14 is given to VCC and pin 7 is

grounded. It consists of four two input NAND gates, N1, N2, N3, N4. N1 acts as an inverter with

pins 1 and 2 as inputs and pin 3 as output. Here 1 and 2 pins are shorted. These input pins are

given to touch plate.

NAND gate N2 has 5,6 pins as inputs and 4 as output. The input to pin 5 is the output of

inverter N1.Output of NAND gate N3 is given as input at pin 6 of N2. N3 is used as inverter with

pins 8 and 9 as inputs and pin 10 as output. The input to N3 is output of capacitor C1.

NAND gate N4 acts as inverter with 12, 13 pins as inputs and pin 11 as output. Input to

N4 is the output of N3.

When N2 is ON the capacitor C1 (22 micro F) is charged to +3v exponentially. It can be

discharged through resistor R2 (1M ohm) to ground. Output of N4 drives the transistor T1 (SL

100), which acts as a switch.

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When the output of N4 is 0, T1 acts as OFF switch. When output of N4 is 1, T1 acts as

ON switch.

T1 provides the ground path for melody generator IC UM66. Pin 3 of UM66 is connected

to ground through T1. Pin 2 of UM66 is connected through diodes (1N4148) to VCC.

Three silicon diodes connected in series between pin 2 of UM66 and positive 3 volt rail

keep voltage applied to pin 2 of UM66 below 3v because of drop across them.

Pin 1 of UM66 is the output pin which is given to loud speaker through the amplifier T2

(SL 100)

By changing values of R2 and C1, the time period of melody can be generated.

3.3 WORKING

The working of the circuit ‘TOUCH SENSITIVE MUSICAL BELL WITH TIMER’ can be

divided into two parts:

1. When touch plates are left untouched

2. When touch plates are bridged by hand

When touch plates are not being touched

Touch plates are touch sensitive devices. When they are left untouched, they act as open

circuit. So supply does not reach input of N1, which is an inverter. So output of N1 is logic 1.

Initially the capacitor is uncharged so input to N3 is logic 0, so output of N3 is logic 1

(N3 acts as an inverter). The output of N3 is connected to one of the input of N2, while the other

input is from the output of N1. Here two inputs of N2 are 1 and 1. According to truth table the

output is logic 0. Hence capacitor never charges.

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Input to N4 is 1 and output is logic 0. So transistor T1, which acts as a switch is OFF and

the melody signal does not find the closed path. Hence output is not generated.

When touch plates are bridged by hand

Initially touch plates are not conducting, so resistance is very large. When touch plates

are bridged by hand, the moisture in the hand decreases the resistance and hence touch plates are

in conducting mode. Here touch plates act as closed circuit.

The input to NAND gate N1 is logic 1, and the output is logic 0 (since it acts as an

inverter). If any one of the inputs to NAND gate is logic 0, the output is logic 1. So the output of

N2 is logic 1. Because of this capacitor starts charging to the supply voltage i.e. 3v. The output

of capacitor is given as input to N3, hence output of N3 is logic 0.

The input to N4 is logic 0 so the output is logic 1. Transistor T1 is ON. Hence pin 3 of

UM66 is grounded and melody is generated. T2 acts as amplifier, so it amplifies the output of

melody generator and gives to the loud speaker.

When capacitor voltage is equal to supply voltage, it starts to discharge through R2

towards ground. The input of N3 is now logic 0, so the output is logic 1. When this is given as

input to N4 its output becomes logic 0.

Transistor T1 acts as an OFF switch, so output of UM66 melody generator is turned OFF.

Hence music is stopped.

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4. ADVANTAGES AND DISADVANTAGES

ADVANTAGES

1. We can adjust the time period of the melody by changing resistance R2 and

capacitance C1.

2. It is cheaper

3. The circuit is very simple

4. The components are readily available in the market

5. Noise is very less.

DISADVANTAGES

1. Moisture must be present on the hand to make the touch plates work

2. Because of direct application of supply, melody is automatically generated once

3. All the components are temperature sensitive

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5. BREAD BOARD CONNECTIONS

Figure 15 Bread board connections

Figure 16 Bread board connections

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6. APPLICATIONS

Instead of conventional switch we use touch plates, where just have to touch the plates in

order to make the circuit work. The following are the applications of TOUCH SENSITIVE

MUSICAL BELL:

It can be used in toys where children just touch the plates and melody can be generated.

We can change the melody by changing the IC.

The circuit can also be used in musical greeting cards.

It can be used as door bell where the visitors just have to touch the plates instead of using

a mechanical ON and OFF switch.

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7. CONCLUSION

The circuit for TOUCH SENSITIVE MUSICAL BELL has been constructed, successfully

implemented and desired results have been obtained. This circuit works using touch plates which

are used in place of mechanical ON and OFF switch. The major advantage is that time period of

melody can be adjusted and the type of melody can be adjusted. So several melodies can be

generated using same circuit. This is a commercially viable product. This product finds great

scope futuristically, as part of an environment where human restricts mechanical actions and

performs day –to-day activities on a touch basis.

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REFERENCES

1. www.electronicsforu.com

2. www.alldatasheet.com

3. www.wikipedia.org/wiki

4. www.redcircuits.com

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