circuit
ideas
Antisleep Alarm for Students

Suresh Kumar K.B.
T
his circuit saves both time and
electricity for students. It helps
to prevent them from dozing
off while studying, by sounding a beep
at a fixed time interval, say, 30 minutes.
If the student is awake during the beep,
he can reset the circuit to beep in the
next 30 minutes. If the timer is not reset
during this time, it means the student
is in deep sleep or not in the room, and
the circuit switches off the light and fan
in the room, thus preventing the wastage of electricity.
The circuit is built around Schmitttrigger NAND gate IC CD4093 (IC1),
timer IC CD4020 (IC2), transistors
9 8 • M a r c h 2 0 1 0 • e l e c t ro n i c s f o r yo u
BC547, relay RL1 and buzzer.
The Schmitt-trigger NAND gate
(IC1) is configured as an astable multivibrator to generate clock for the timer
(IC2). The time period can be calculated as T=1.38×R×C. If R=R1+VR1=15
kilo-ohms and C=C2=10 µF, you’ll get
‘T’ as 0.21 second. Timer IC CD4020
(IC2) is a 14-stage ripple counter.
Around half an hour after the reset
of IC1, transistors T1, T2 and T3 drive
the buzzer to sound an intermediate
beep. If IC2 is not reset through S1
at that time, around one minute later
the output of gate N4 goes high and
transistor T4 conducts. As the output
of gate N4 is connected to the clock
input (pin 10) of IC2 through diode
edi
s.c. dwiv
D3, further counting stops and relay
RL1 energises to deactivate all the appliances. This state changes only when
IC1 is reset by pressing switch S1.
Assemble the circuit on a generalpurpose PCB and enclose it in a suitable cabinet. Mount switch S1 and the
buzzer on the front panel and the relay
at the back side of the box. Place the
12V battery in the cabinet for powering
the circuit. In place of the battery, you
can also use a 12V DC adaptor. 
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circuit
ideas
Automatic Light Controller
Using 7806

M.K. Chandra Mouleeswaran
V
oltage regulator ICs (78xx series) provide a steady output
voltage, as against a widely
fluctuating input supply, when the
common terminal is grounded. Any
voltage about zero volt (ground) connected in the common terminal is added to the output voltage. That means
the increase in the common terminal
voltage is reflected at the output. On
the other hand, if the common terminal
is disconnected from the ground, the
full input voltage is available at the
output.
This characteristic is utilised in the
present circuit. When the common
terminal is connected to the ground,
the regulator output is equivalent to
the rated voltage, and as soon as the
terminal is disconnected from the
ground, the output increases up to the
input voltage.
The common terminal is controlled by a transistor, which works as a
switch on the terminal. For automatic
control of light, a light-dependent resistor (LDR1) is connected to the base
9 2 • A p r i l 2 0 1 0 • e l e c t ro n i c s f o r yo u
of the transistor. In this way, the voltage regulator is able to operate a light
bulb automatically as per the ambient
light.
To derive the power supply for
the circuit, the 50Hz, 230V AC mains
is stepped down by transformer X1
to deliver a secondary output of 12V,
250 mA. The secondary output of the
transformer is applied to a bridge rectifier comprising diodes D1 through
D4, filtered by capacitor C1 and fed
to the input terminal of the regulator
(IC1).
The common terminal (pin 2) of IC1
is connected to the ground line of the
circuit through transistor BC557 (T1).
The transistor is biased by R2, R3, VR1
and LDR1. The grounding of IC1 is
controlled by transistor T1, while light
is sensed by LDR1. Using preset VR1,
you can adjust the light-sensing level
of transistor T1.
The output of IC1 is fed to the base
of transistor T2 (through resistor R4
and zener diode ZD1) and relay RL1.
LED1 connected across the positive
and ground supply lines acts as a
power-‘on’ indicator.
edi
s.c. dwiv
Normally, the resistance of LDR1
is low during daytime and high during nighttime. During daytime, when
light falls on LDR1, pnp transistor T1
conducts. The common terminal of IC1
connects to the ground and IC1 outputs
6V. As a result, transistor T2 does not
conduct and the relay remains de-energised. The light bulb remains ‘off’ as
the mains connection is not completed
through the relay contacts.
During nighttime, when no light
falls on LDR1, it offers a high resistance at the base junction of transistor
T1. So the bias is greatly reduced and
T1 doesn’t conduct. Effectively, this
removes the common terminal of IC1
from ground and it directs the full
input DC to the output. Transistor T2
conducts and the relay energises to
light up the bulb as mains connection
completes through the relay contacts.
As LDR1 is in parallel to VR1+R3
combination, it effectively applies
only half of the total resistance of
the network formed by R3, VR1 and
LDR1 to the junction at T1 in total
darkness. In bright light, it greatly
reduces the total effective resistance
at the junction.
The circuit is
simple and can be
assembled on a small
general-purpose
PCB. Use a heat-sink
for IC1. Make sure
that LDR1 and the
light bulb are well
separated.
The circuit can be
used for streetlights,
tubelights or any
other home electrical lighting system
that needs to be automated. 
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circuit
ideas
Digital Timer Enhancement
edi
s.c. dwiv
his simple circuit automatically
activates or deactivates an electronic device at the time of alarm
preset in a clock. When the alarm rings,
the tone burst generated at the terminal
of the buzzer triggers the circuit and the
relay energises or de-energises to switch
on or switch off the load.
The circuit is built around ICs
CD40106 (IC1) and CD4017 (IC2) and
a few discrete components. IC1 is a hex
Schmitt trigger, while IC2 is a decade
counter. The circuit works off regulated 6V power supply, while the alarm
clock runs off its own 1.5V battery.
The tone burst generated at the
circuit can be used:
1. You want an appliance or gadget
to switch on automatically at a preset
time
2. You switch on an appliance or
gadget manually at a particular time
and want it to switch off automatically
at a preset time
Let us see how it works when you
want your appliance to switch on at a
preset time, say, 3 pm. Set the alarm in
your clock to 3 pm and slide switch S3
towards Q1. When the alarm sounds
at 3 pm, Q0 output of IC2 advances to
Q1 and relay RL1 energises to connect
the load (appliance) to mains power
supply through its contacts. The load
remains ‘on’ until you reset IC2 by
and relay RL1 de-energises to disconnect the load from mains power supply
through its contacts. At this time, you
need to pause the alarm using pause
switch of the clock.
When you press reset switch S1,
LED1 glows to indicate that the circuit
is ready to work. When you press start
switch S2, LED2 glows to indicate start
mode. Glowing of LED3 means that
the counter has stopped counting and
needs to be reset before use.
When the counter is in stop mode,
Q2 output of IC2 remains high. As this
pin is connected to the clock-enable
input (pin 13) of IC2, the clock input
is inhibited. In this condition, any tone
piezobuzzer is tapped from its connection points. The positive terminal of the
clock buzzer is connected to the base of
transistor T1 and the negative terminal
is connected to ground of the circuit.
When the alarm clock sounds, the
signal from the clock buzzer makes
transistor T1 conduct. As a result, pin
1 of gate N1 goes low and it outputs
high at pin 2. This low-to-high transition clocks the counter (IC2) at pin 14
through diode D1 and gate N2. In this
way, IC2 advances by one at each clock
produced due to the sounding alarm.
There are two situations where this
momentarily pressing S1. At this time,
you need to pause the alarm using
pause switch of the clock.
Now suppose you manually start
the load at 3 pm and want it to stop
automatically at 6 pm. First, reset IC2
by momentarily pressing S1 and slide
switch S3 towards Q2. Set the alarm in
your clock to 6 pm. To start the load,
press switch S2 momentarily at 3 pm.
The Q0 output of IC2 advances to Q1
and relay RL1 energises to connect the
load to mains power supply through
its contacts. When the alarm sounds at
6 pm, Q1 output of IC2 advances to Q2
burst signal arriving from the clock
has no effect on IC2 and therefore the
circuit remains in stop mode. You can
now set the alarm time in the clock.
Assemble the circuit on a generalpurpose PCB and enclose in a small
cabinet. Connect the base of transistor
(T1) to positive terminal of the alarm
clock and negative terminal to ground
of the circuit. Put the alarm clock at a
convenient place. If you do not want to
use a 6V battery, replace it with a 6V
adaptor to power the circuit. Mount
the LEDs and the pushbutton on the
front panel of the cabinet. 

Raj K. Gorkhali
T
9 8 • M ay 2 0 1 0 • e l e c t ro n i c s f o r yo u
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circuit
ideas
Electronic Candles

Raj K. Gorkhali
H
ere is a simple circuit that
can produce the effect of
candle light in a normal
electric bulb. A candle light, as we
all know, resembles a randomly
flickering light. So, the objective of
this project activity is to produce a
randomly flickering light effect in an
electric bulb.
To achieve this, the entire circuit
can be divided into three parts. The first
part comprises IC1 (555), IC2 (74LS164),
IC3 (74LS86), IC4 (74LS00) and the associated components. These generate a
randomly changing train of pulses.
edi
s.c. dwiv
gate trigger circuit components. It is
basically half-wave AC power being
supplied to the electric bulb.
The third part is the power supply circuit to generate regulated 5V
DC from 230V AC for random signal
generator. It comprises a stepdown
transformer (X1), full-wave rectifier
(diodes D3 and D4), filter capacitor
(C9), followed by a regulator (IC5).
The random signal generator of
the circuit is built around an 8-bit
serial in/parallel out shift register
(IC2). Different outputs of the shift
register IC pass through a set of logic
gates (N1 through N5) and final out-
to provide better flickering effect in
the bulb.
The random signal triggers the
gate of SCR1. The electric bulb gets
AC power only for the period for
which SCR1 is fired. SCR1 is fired
only during the positive half cycles.
Conduction of SCR1 depends upon
the gate triggering pin 3 of IC2, which
is random. Thus, we see a flickering
effect in the light output.
Assemble the circuit on a generalpurpose PCB and enclose it in a suitable
put appearing at pin 6 of gate N5 is
fed back to the inputs of pins 1 and
2 of IC2. The clock signal appears
at pin 8 of IC2, which is clocked by
an astable multivibrator configured
around timer (IC1). The clock frequency can be set using preset VR1
and VR2. It can be set around 100 Hz
case. Fix bulb and neon bulb on the
front side of the cabinet. Also, connect
a power cable for giving AC mains
supply to the circuit for operation. The
circuit is ready to use.
Warning. Since the circuit uses
230V AC, care must be taken to avoid
electric shock. 
Fig. 1: Circuit diagram for electronic candle
Fig. 2: Pin configurations
of C106 and 7805
The second
part of the circuit
consists of SCR1
(C106), an electric
bulb connected
between anode of
SCR1 and mains
live wire, and
1 4 2 • J a n ua ry 2 0 1 0 • e l e c t ro n i c s f o r yo u
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circuit
ideas
Four-Stage FM Transmitter
Pradeep G.
edi
s.c. dwiv
his FM transmitter circuit uses
four radio frequency stages:
a VHF oscillator built around
transistor BF494 (T1), a preamplifier
T
the pre-driver stage. You can also use
transistor 2N5109 in place of 2N2219.
The preamplifier is a tuned class-A RF
amplifier and the driver is a class-C
amplifier. Signals are finally fed to the
class-C RF power amplifier, which de-
frequency generated. You can also use
a 12V battery to power the circuit.
Assemble the circuit on a generalpurpose PCB. Install the antenna prop-
built around transistor BF200 (T2), a
driver built around transistor 2N2219
(T3) and a power amplifier built
around transistor 2N3866 (T4). A condenser microphone is connected at the
input of the oscillator.
Working of the circuit is simple.
When you speak near the microphone,
frequency-modulated signals are
obtained at the collector of oscillator
transistor T1. The FM signals are amplified by the VHF preamplifier and
livers RF power to a 50-ohm horizontal
dipole or ground plane antenna.
Use a heat-sink with transistor
2N3866 for heat dissipation. Carefully
adjust trimmer VC1 connected across
L1 to generate frequency within 88108 MHz. Also adjust trimmers VC2
through VC7 to get maximum output
at maximum range.
Regulator IC 78C09 provides stable
9V supply to the oscillator, so variation
in the supply voltage will not affect the
erly for maximum range.
Coils L1 through L5 are made with
20 SWG copper-enamelled wire wound
over air-cores having 8mm diameter.
They have 4, 6, 6, 5 and 7 turns of wire,
respectively.
EFY note. This transmitter is meant
only for educational purposes. use of
this transmitter with outdoor antenna
is illegal in most parts of the world. The
author and EFY will not be responsible
for any misuse of this transmitter. 

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e l e c t ro n i c s f o r yo u • M a r c h 2 0 1 0 • 1 0 3
circuit
ideas
NUMERIC WATER-LEVEL INDICATOR
Daniyal Syed
ost water-level indicators
for water tanks are based
upon the number of LEDs
that glow to indicate the corresponding level of water in the container.
Here we present a digital version of the
water-level indicator. It uses a 7-seg-
terminal of the sensor must be kept at
the bottom of the container (tank). IC
74HC147 has nine active-low inputs
and converts the active input into active-low BCD output. The input L-9
has the highest priority.
The outputs of IC1 (A, B, C and
D) are fed to IC2 via transistors T1
through T4. This logic inverter is used
when the water level reaches L-1 position, the display shows ‘1,’ and when
the water level reaches L-8 position,
the display shows ‘8.’ Finally, when
ment display to show the water level
in numeric form from ‘0’ to ‘9.’
The circuit works off 5V regulated
power supply. It is built around priority encoder IC 74HC147 (IC1), BCD-to7-segment decoder IC CD4511 (IC2),
7-segment display LTS543 (DIS1) and a
few discrete components. Due to high
input impedance, IC1 senses water in
the container from its nine input terminals. The inputs are connected to +5V
via 560-kilo-ohm resistors. The ground
to convert the active-low output of IC1
into active-high for IC2. The BCD code
received by IC2 is shown on 7-segment display LTS543. Resistors R18
through R24 limit the current through
the display.
When the tank is empty, all the
inputs of IC1 remain high. As a result,
its output also remains high, making
all the inputs of IC2 low. Display
LTS543 at this stage shows ‘0,’ which
means the tank is empty. Similarly,
the tank is full, all the inputs of IC1
become low and its output goes low
to make all the inputs of IC2 high.
Display LTS543 now shows ‘9,’ which
means the tank is full.
Assemble the circuit on a general-purpose PCB and enclose in a
box. Mount 7-segment LTS543 on the
front panel of the box. For sensors L-1
though L-9 and ground, use corrosionfree conductive-metal (stainless-steel)
strips. 

M
1 0 4 • F e b r ua ry 2 0 1 0 • e l e c t ro n i c s f o r yo u
edi
s.c. dwiv
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circuit
ideas
Security System Switcher

T.K. Hareendran
A
n audio signal can be used as
a form of input to control any
security system. For example,
an automatic security camera can be
configured to respond to a knock on
the door. The circuit described here
allows the security system to automati-
of small signal preamplifier built
around transistor T1. Biasing resistor
R1 determines to a large extent the
microphone sensitivity. A microphone
usually has an internal FET which
requires a bias voltage to operate. The
sound picked up by the microphone
is amplified and fed to input pin 2 of
IC1 (LMC555) wired in monostable
Indicator LED1 is provided to display
the relay activity. Any AC/DC operated security gadget is activated or
deactivated through a security switch.
Thus, the security switch of the gadget
configuration.
IC2 (CD4538B) is a dual,
precision monostable multivibrator with independent
trigger and reset controls. The
output of IC1 is connected to
the first trigger input pin 4 of
IC2(A) through switch S1. If
an intruder opens or breaks
the door, IC1 is triggered by
sound signals; the timer output pin 3 of IC1 goes high and
enables first monostable multi
vibrator IC2(A). IC2(A) provides a time period of around
5 to 125 seconds, which is adjusted
with preset VR1.
Another monostable multivibrator
IC2(B) also provides a time period of
around 25 to 600 seconds, which is
adjusted with preset VR2. The output
of IC2(B) is used to energise relay RL1.
is connected in the n/o contacts of
the relay. you can also operate highpower beacons, sirens or hooters in
place of the security switch for any
AC/DC operated security gadget.
Assemble the circuit on a general-purpose PCB and enclose it in
a cabinet as shown in Fig. 2 along
with 5V adaptor for powering the
circuit. Connect the security switch
according to the circuit diagram and
use appropriate AC/DC power supply required to operate the security
gadget.
Warning! All relevant electrical
safety precautions should be taken
when connecting mains power supply
to the relay contacts. With the help
of single pole double throw (SPDT)
switch S1, internal or external trigger input (active high signal) can be
selected. 
edi
s.c. dwiv
Fig. 1: Security system switcher
+5V ADAPTOR
FOR POWER
SUPPLY
CONNECTOR
FOR
SECURITY
GADGET
Fig. 2: Proposed cabinet
cally switch on when a master switch
is in on state. It uses a transducer to
detect intruders and a 5V regulated
DC power supply provides power to
the circuit.
As shown in Fig. 1, a condenser
microphone is connected to the input
1 4 0 • J a n ua ry 2 0 1 0 • e l e c t ro n i c s f o r yo u
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circuit
ideas
SPY EAR
Fig. 1: Circuit for spy ear
is designed for operation
with power supplies in
the 4-15V DC range. It
is housed in a standard
8-pin DIL package, coned components amplify the
sumes very small quiessound signals picked up by
cent current and is ideal
the condenser microphone
for battery-powered
(MIC). The amplified signal
portable applications.
from the preamplifier stage
The processed outis fed to input pin 3 of IC
put signal from capaciLM386N (IC1) through Fig. 2: Compact unit of spy ear
tor C2 goes to one end
capacitor C2 (100nF) and
of volume control VR1.
volume control VR1 (10-kilo-ohm log).
The wiper is taken to pin 3 of LM386N
A decoupling network comprising reaudio output amplifier. Note that the
sistor R5 and capacitor C3 provides the
R6-C4 network is used to RF-decouple
preamplifier block with a clean supply
positive-supply pin 6 and R8-C7 is an
voltage.
optional Zobel network that ensures
Audio amplifier IC LM386N (IC1)
high frequency stability when feeding
an inductive headphone load.
Capacitor C6 (22µF, 16V) wired
between pin 7 and ground gives additional ripple rejection. The output of
LM386N power amplifier can safely
drive a standard 32-ohm monophonic
headphone/earphone.
Assemble the circuit on a small
general-purpose PCB and house in
a suitable metallic enclosure with an
integrated battery holder and headphone/earphone socket as shown in
Fig. 2. Fit the on/off switch (S1), volume control (VR1) and power indicator
(LED1) on the enclosure. Finally, fit the
condenser microphone (MIC) on the
front side of the enclosure and link it to
the input of the preamplifier via a short
length of the shielded wire. 
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e l e c t ro n i c s f o r yo u • A p r i l 2 0 1 0 • 9 3

T.K. HAREENDRAN
W
hat binoculars do to improve your vision, this
personal sound enhancer
circuit does for listening. This lightweight gadget produces an adjustable
gain on sounds picked up from the
built-in high-sensitivity condenser
microphone. So you can hear what you
have been missing. With a 6V (4×1.5V)
battery, it produces good results.
As shown in Fig. 1, a small signal
amplifier is built around transistor
BC547 (T1). Transistor T1 and the relat-
edi
s.c. dwiv
circuit
ideas
Versatile Probe

Raju R. Baddi
edi
Y
ou can use this versatile probe
for continuity testing and identification of transistor type and
transformer windings. The n-side or
p-side of a transistor can be identified
quickly in one go. You can make two
contacts with the probe in one hand
Fig. 1: Circuit of versatile probe
s.c. dwiv
while the other hand is
free.
Fig. 1 shows the circuit
of the probe. The operation
of the circuit is simple.
It is driven by an alternating current flowing through two LEDs
(LED1 and LED2). So
one LED corresponds
to forward direction of
current flow, while the
other shows reverse direction of current flow.
This helps to detect orientation of the p-n junction with respect to the
probes. The LEDs can
be arranged near the
probes to glow either for
the p-side or the n-side
as per your choice.
The frequency is determined by capacitor
C1 and preset VR1 connected between gates G1
Fig. 2: Constructional detail of versatile probe
Testing Results for Different Components
Component
Probe D
Probe C
Red LED
Green LED
1st terminal
1st terminal
2nd terminal
2nd terminal
Off
On
On
Off
Transistors
Any type pnp or npn
C
E
E
C
X
X
X
X
npn-type transistor
B
B
E
C
On
On
Off
Off
p-n junction
p-n junction
Result: ‘p’ is common, so npn transistor
pnp-type transistor
B
B
E
C
Off
Off
On
On
n-p junction
n-p junction
Result: ‘n’ is common, so pnp transistor
Primary terminal 1
Primary terminal 2
Glow with
low intensity
Glow with
low intensity
Both LEDs glowing with low intensity
Result: Primary side
Secondary terminal 1
Secondary terminal 2
Glow with
high intensity
Glow with
high intensity
Both LEDs glowing with high intensity
Result: Secondary side Connect with
LEDs probe
X
On
Off
Diode
Step-down transformer
Continuity
1 0 6 • F e b r ua ry 2 0 1 0 • e l e c t ro n i c s f o r yo u
Result
Probe D side is anode (p) and probe C
side is cathode (n)
Probe D side is ‘n’ and probe C side is ‘p’
Unused pin is base
Unused pin is base
Indicates shorting
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circuit
ideas
and G2. The frequency can be varied
using preset VR1. Higher frequency
results in more sensitivity to inductive
reactance. The preset is trimmed so that
when the probes are shorted, both the
LEDs glow equally.
Fig. 2 shows the probe arrangement
for testing. Most of the battery power is
consumed only when the LEDs glow.
The probes have been constructed to
provide a good grip on the components
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under testing. One probe’s tip has been
widened. (Drop the empty refill of a
ball-pen from some height to remove
the ball, then insert a sharp needle or
something similar into the tip. Slowly
push the needle inside and widen the
tip so that a component lead can be
inserted into it during testing.) Slightly
unequal probe lengths help to make
easy contacts.
Assemble the circuit on a general-
purpose PCB which is as compact as
possible and put it inside a glue stick
tube (whose inner mechanism has been
removed) at its centre. The metallic
disk and metallic strips can be cut out
from any tin container. For the probes,
use the spring mechanism of gel ball
pens. Probes C and D are the points
representing the probe terminals.
Two button cells (CR2032) are used to
power the probe circuit. 
e l e c t ro n i c s f o r yo u • F e b r ua ry 2 0 1 0 • 1 0 7
circuit
ideas
Water-Level Indicator
using 7-segment display

Riju Thazhathu Veettil
his water-level indicator uses
a 7-segment display, instead
of LEDs, to indicate the water
level (low, half and full) in the tank.
Moreover, a buzzer is used to alert you
of water overflowing from the tank.
The circuit shows the water level by
displaying L, H and F for low, half and
full, respectively.
The circuit uses five sensors to
sense the different water levels in the
T
a high voltage at the input pin of the
NOT gate, it outputs a low voltage.
Similarly, for a low voltage at the input
pin of the NOT gate, it outputs a high
voltage.
When the tank is empty, the input
pins of IC 7404 are pulled high via a
1-mega-ohm resistor. So it outputs a
low voltage. As water starts filling the
tank, a low voltage is available at the
input pins of the gate and it outputs a
high voltage.
tank. Sensor A is connected to the
negative terminal (GND) of the power
supply. The other four sensors (B
through E) are connected to the inputs
of NOT gate IC 7404. When there is
When the water in the tank rises
to touch the low level, there is a low
voltage at input pin 5 of gate N3 and
high output at pin 6. Pin 6 of the gate is
connected to pin 10 of gate N9, so pin
w w w. e f y m ag . co m
edi
s.c. dwiv
10 also goes high. Now as both pins 9
and 10 of gate N9 are high, its output
pin 8 also goes high. As a result, positive supply is applied to DIS3 and it
shows ‘L’ indicating low level of water
in the tank.
Similarly, when water in the tank
touches the half level, pins 4 and 5
of AND gate N8 become high. As a
result, its output also goes high and
DIS2 shows ‘H’ indicating half level of
water in the tank. At this time, pin 9 of
gate N9 also goes low
via gate N4 and DIS3
stops glowing.
When the water
tank becomes full,
the voltage at pin 1
of gate N1 and pin 3
of gate N2 goes low.
Output pin 3 of gate
N7 goes high and
DIS1 shows ‘F’ indicating that the water
tank is full.
When water starts
overflowing the tank,
pin 13 of gate N6 goes
low to make output
pin 12. The buzzer
sounds to indicate
that water is overflowing the tank and
you need to switch off
the motor pump.
Assemble the circuit on a general-purpose PCB and enclose
in a suitable box. Use a non-corrosive
material such as steel strip for the five
sensors and hang them in the water tank
as shown in the circuit diagram. Use
regulated 5V to power the circuit. 
e l e c t ro n i c s f o r yo u • M ay 2 0 1 0 • 1 0 1