STRESS METER

STRESS METER

This stress monitor lets you assess your emotional pain. If the stress is very high, it gives visual indication through a light-emitting diode (LED) display along with a warning beep. The gadget is small enough to be worn around the wrist. The gadget is based on the principle that the resistance of the skin varies in accordance with your emotional states. If the stress level is high the skin offers less resistance, and if the body is relaxed the skin resistance ishigh. The low resistance of the skin
during high stress is due to an increase in the blood supply to the skin. This increases the permeability of the skin and hence the conductivity for electric current. This property of the skin is used here to measure the stress level. The touch pads of the stress meter sense the voltage variations across the touch pads and convey the same to the circuit.
The circuit is very sensitive and detects even a minute voltage variation
across the touch pads. The circuit comprises signal amplifier and analogue display sections. Voltage variations from the sensinG pads are amplified by transistor BC548 (T1), which is configured as a common- emitter amplifier. The base of T1
is connected to one of the touch pads through resistor R1 and to the ground
rail through potmeter VR1. By varying VR1, the sensitivity of T1 can be adjusted to the desired level. Diode D1 maintains proper biasing of T1 and capacitor C1 keeps the voltage from the
emitter of T1 steady. The amplified signal from transistor T1 is given to the input of IC
LM3915 (IC1) through VR2. IC LM3915 is a monolithic integrated circuit that senses analogue voltage levels at its pin 5 and displays them through LEDs providing a logarithmic
analogue display. It can drive up to ten LEDs one by one in the dot/ bar mode for each increment of 125 mV in the input. Here, we’ve used only five LEDs connected at pins 14 through 18 of IC1.
LED1 glows when input pin 5 of IC1 receives 150 mV. LED5 glows when the voltage rises to 650 mV and LED5 flashes and piezobuzzer PZ1 beeps when the stress level is high. Resistors R4
and R5 and capacitor C2 form the flashing elements Resistor R3 maintains the LED current
at around 20 mA. Capacitor  C3 should be p l a c e d close to pin 3 for proper functioning
of the IC. Zener diode ZD1 in series with resistor R6 provides regulated 5V
to the circuit. The circuit can be assembled on a small piece of perforated board. Use
transparent 3mm LEDs and a small piezobuzzer for audio-visual indications. Enclose the circuit in a small plastic case with touch pads on the back side. Two self-locking straps can be used to tie the unit around your wrist.
After tying the unit around your
wrist (with touch pads in contact with
the skin), slowly vary VR1 until LED1
glows (assuming that you are in relaxed
state). Adjust VR2 if the sensitivity
of IC1 is very high. The gadget
is now ready for use.

TELEPHONE OPERATED CALLING SYSTEM
 Dual-tone multiple-frequency (DTMF) receiver IC is commonly used in telephone equipment. One common DTMF receiver is Holtek HT9170 used in electronic communication circuits. The
Holtek HT9170 series comprises DTMF receivers integrated with digital decoder and bandsplit filter functions. All HT9170 series ICs use digital counting techniques to detect and decode
all the 16 DTMF tone pairs into a 4-bit code output. This telephone-operated calling circuit
is very helpful for doctors in calling the patients, in banks and in various other situations where persons have to be called or signalled. When you need to call a person amongst
many standing outside your cabin, just lift the telephone handset off the cradle and press the respective number. The number of the person called will be displayed and a bell will sound to inform
the person that it is his turn. The circuit can also be used in quiz
contests and by visually- or hearingimpaired people. It can be used to call a maximum of nine different persons. The circuit is built around DTMF receiver IC HT9170, BCD-to-7-segment
decoder/driver 7447, quad 2-input OR gate and common-anode display. Simple melody generator IC UM66 is used to produce melody sound in the loudspeaker through Darlington-pair
transistors (T1 and T2). The tone pair DTMF generated by pressing the telephone key is converted
into binary values internally in the IC. The binary values are indicated
by the glowing of LEDs at the output of IC1. The output of IC1 is connected to:
1. LEDs connected via resistors R15 through R18 at pins 11 through 14, respectively.
LED1 indicates the LSB and
LED4 indicates the MSB.

2. BCD-to-7-segment decoder/ driver 7447, whose outputs are connected to the common-anode displayfor displaying the pressed number onthe telephone connected in parallel to the circuit.

3. Gates N1 and N2 to activate thecall bell. Here is how the circuit works: Connect
the telephone and the circuit in parallel to the telephone line. Connect 6V to the circuit. When you press switch S1, DIS1 shows ‘0.’ Lift the handset off the cradle and dial a number, say, ‘1.’ The output of IC1 becomes A3A2A1A0 = 0001. LED1 glows, the display shows ‘1’ and the call bell
sounds.To stop the call bell, put the receiver on the cradle and press switch
S1 momentarily. Now DIS1 shows ‘0’ and LED1 stops glowing. For calling other numbers, follow the same procedure: Lift the handset off the cradle and press the desired number (0 through 9). The respective LED will glow, the number will be displayed on DIS1 and the call bell will sound. Now put the handset on the cradle and press S1 momentarily to
stop the call bell.

Tester For Remote Control

 Here is a simple tester for checking the basic operations of an infrared remote control unit.
It is low-cost and easy to construct. The tester is built around infrared receiver module TSOP1738. Operation of the remote control is acknowledged by a tone from the buzzer. The circuit
is sensitive and has a range of approximately five metres. The integrated IR receiver detects, amplifies and demodulates IR signals from the remote control unit. The piezobuzzer connected at its output sounds to indicate the presence of signal from the remote control unit. As shown in Fig. 1, output pin 3 of IR receiver module TSOP1738 (IRX1) normally remains high and the piezobuzzer
is in silent mode. When the IR module I R X 1 receives an infrared signal, its output goes low and, as
a result, the piezobuzzer sounds to indicate the reception of signal from the remote (such as TV remote control unit). P o w e r supply for the circuit is derived from the mains using a capacitive
potential dropper, a half-wave rectifier, a shunt regulator and associated components. Make sure that capacitor C1 is of X2 type. Use a suitably small enclosure to make the unit handy. Assemble the circuit on a generalpurpose PCB and enclose in a cabinet. Make sure that the IR receiver module
is placed on the front panel of the cabinet so that it can receive the IR signals easily. Before soldering/connecting the shunt regulator and IR module, refer Fig. 2 for the pin configuration. 
Fig. 2: Pin configuration of TL431 and TSOP 1738

TRAFFIC CONTROLLER                                                                         This simple traffic controller can
be used to teach children rudiments
of traffic rules.
The circuit (shown in Fig. 1) uses
readily available components. It mainly
comprises rectifier diodes (1N4001),
a 5V regulator 7805, two timers IC 555,
two relays (5V, single-changeover),
three 15W, 230V bulbs and some discrete
components.
Mains power is stepped down by
transformer X1 to deliver a secondary
output of 9V, 300 mA. The transformer
output is rectified by a full-wave
bridge rectifier comprising diodes D1
through D4, filtered by capacitor C1
and regulated by IC 7805 (IC1).
IC2 is wired as a multivibrator with
‘on’ and ‘off’ periods of approximately
30 seconds each with the component
values selected. As soon as mains
power is switched on, pin 3 of IC2 goes
high for 30 seconds. This, in turn, energises
relay RL1 through transistor T1
and the red lamp (B1) glows through
its normally-open (N/O) contact. At
the same time, mains power is disconnected
from the pole of relay RL2.
As the ‘on’ time of IC2 ends, a
Fig. 1: Circuit of traffic controller
Fig. 2: Construction details of traffic controller unit
high-to-low pulse at its pin 3
triggers IC3 through C5. IC3
is configured as a monostable
with ‘on’ time of about 4
seconds, which means pin 3
of IC3 will remain high for
this period and energise relay
RL2 through driver transistor
T2. The amber lamp (B2) thus
lights up for 4 seconds.
As soon as 4-second time
period of timer IC3 at pin 3
lapses, relay RL2 de-energises
and the green lamp (B3)
lights up for the rest of ‘off’
period of IC2, which is about
26 seconds. The green lamp
is activated through the normally
closed (N/C) contacts
of relay RL2.
So when mains power is
switched on, red light glows for
30 seconds, amber for 4 seconds
and green for 26 seconds.
You can assemble this circuit
on a general-purpose PCB
and enclose in an insulated
box. The box should have
enough space for mounting
transformer X1 and two relays.

It can be fixed near 230V
circuit
ideas
electronics for you • w w w. e f y m a g . c o m November 2008 • 81
AC, 50Hz power supply or mounted
on the PVC tube used in construction
of the traffic light container.
Construction of the traffic light
container box is shown in Fig. 2. A
stout cardboard box of 30x15x10cm3 is
required for housing the lamps. To ensure
strength, use a 10x45cm2 plywood
plate having 1.5cm thickness and secure
onto it three light sockets and the
box using nuts and bolts or screws.
Make three tubes of thin aluminium
sheet, which is readily available in
hardware shops. The inner diameter of
aluminium tubes should be such that
these can snugly fit on the light sockets.
Using a sharp knife, make holes
opposite the sockets carefully. Wire the
sockets at the back and take the wires
out through the PVC tube.
First, fix three 15W bulbs (B1
through B3) and then press on the
tubes. Support the other ends of the
tubes in the holes made on the front
panel of cardboard box. Sandwich gelatine
papers of the three colours between
two sheets of cardboard and fix over the
tubes. The visibility of red, amber and
green lights improves with their mounting
on the tubulaThis simple traffic controller can
be used to teach children rudiments
of traffic rules.
The circuit (shown in Fig. 1) uses
readily available components. It mainly
comprises rectifier diodes (1N4001),
a 5V regulator 7805, two timers IC 555,
two relays (5V, single-changeover),
three 15W, 230V bulbs and some discrete
components.
Mains power is stepped down by
transformer X1 to deliver a secondary    
output of 9V, 300 mA. The transformer
output is rectified by a full-wave
bridge rectifier comprising diodes D1
through D4, filtered by capacitor C1
and regulated by IC 7805 (IC1).
IC2 is wired as a multivibrator with
‘on’ and ‘off’ periods of approximately
30 seconds each with the component
values selected. As soon as mains
power is switched on, pin 3 of IC2 goes
high for 30 seconds. This, in turn, energises
relay RL1 through transistor T1
and the red lamp (B1) glows through
its normally-open (N/O) contact. At
the same time, mains power is disconnected
from the pole of relay RL2.
As the ‘on’ time of IC2 ends, a
Fig. 1: Circuit of traffic controller
Fig. 2: Construction details of traffic controller unit
high-to-low pulse at its pin 3
triggers IC3 through C5. IC3
is configured as a monostable
with ‘on’ time of about 4
seconds, which means pin 3
of IC3 will remain high for
this period and energise relay
RL2 through driver transistor
T2. The amber lamp (B2) thus
lights up for 4 seconds.
As soon as 4-second time
period of timer IC3 at pin 3
lapses, relay RL2 de-energises
and the green lamp (B3)
lights up for the rest of ‘off’
period of IC2, which is about
26 seconds. The green lamp
is activated through the normally
closed (N/C) contacts
of relay RL2.
So when mains power is
switched on, red light glows for
30 seconds, amber for 4 seconds
and green for 26 seconds.
You can assemble this circuit
on a general-purpose PCB
and enclose in an insulated
box. The box should have
enough space for mounting
transformer X1 and two relays.
It can be fixed near 230V
circuit
ideas
electronics for you • w w w. e f y m a g . c o m November 2008 • 81
AC, 50Hz power supply or mounted
on the PVC tube used in construction
of the traffic light container.
Construction of the traffic light
container box is shown in Fig. 2. A
stout cardboard box of 30x15x10cm3 is
required for housing the lamps. To ensure
strength, use a 10x45cm2 plywood
plate having 1.5cm thickness and secure
onto it three light sockets and the
box using nuts and bolts or screws.
Make three tubes of thin aluminium
sheet, which is readily available in
hardware shops. The inner diameter of
aluminium tubes should be such that
these can snugly fit on the light sockets.
Using a sharp knife, make holes
opposite the sockets carefully. Wire the
sockets at the back and take the wires
out through the PVC tube.
First, fix three 15W bulbs (B1
through B3) and then press on the
tubes. Support the other ends of the
tubes in the holes made on the front
panel of cardboard box. Sandwich gelatine
papers of the three colours between
two sheets of cardboard and fix over the
tubes. The visibility of red, amber and
green lights improves with their mounting
on the tubula

WIRELESS SWITCH

Normally, home appliances are controlled by means of switches, sensors, etc. However, physical contact with switches may be dangerous if there is any shorting. The circuit described here requires
no physical contact for operating the appliance. You just need to move your hand between the infrared LED (IR LED1) and the phototransistor (T1). The infrared rays transmitted by
IR LED1 is detected by the phototransistor to activate the hidden lock, flush system, hand dryer or else. This circuit is very stable and sensitive compared to other AC appliance control circuits. It is simple, compact and cheap. Current consumption is low in milliamperes. The circuit is built around an IC CA3140, IRLED1, phototransistor and other discrete components. When regulated 5V is connected to the circuit, IR LED1 emits infrared rays, which are received by phototransistor T1 if it is
properly aligned. The collector of T1 is connected to non-inverting pin 3 of IC1. Inverting pin 2 of IC1 is connected to voltage-divider preset VR1. Using preset VR1 you can vary the reference
voltage at pin 2, which also affects sensitivity of the phototransistor. Op-amp IC1 amplifies the signal
received from the phototransistor. Resistor R3 controls the base current of transistor BC548 (T2). The high output of IC1 at pin 6 drives transistor T2 to energise relay RL1 and switch on the
appliance, say, hand dryer, through the relay contacts. The working of the circuit is simple. In order to switch on the appliance, you simply interrupt the infrared rays falling on the phototransistor through your hand. During the interruption, the appliance remains on through the relay. When you remove your hand from the infrared beam, the appliance turns off through the relay.
Assemble the circuit on any general- purpose PCB. Identify the resistors
through colour coding or using the multimeter. Check the polarity and pin
configuration of the IC and mount it using base. After soldering the circuit, connect +5V supply to the circuit. 

Anti collision rear light
During poor visibility, i.e., when there is fog, or at dawn or dusk, or when your ve-hicle gets stalled on a lonely stretch of a highway, this flashing light will pro-vide safety and attract the attention of
people to help you out. It uses high-brightness yellow LEDs. The circuit uses a dual binary
counter CD4520, quadruple 2-input NAND schmitt trigger CD4093, 8-stage
shift-and-store bus register CD4094 and some descrete components. An oscillator is built around gate A, whose frequency can be varied through preset VR1 when required. The output
of the oscillator is fed to IC1 and IC3. When the circuit is switched on, the oscillator starts oscillating, the counter starts counting through IC1 and the data is shifted on positive-going clock
through IC3. As a result, the four groups of LEDs flash one by one. All the LEDs will then glow for
some time and switch off for some time, and the cycle will repeat. Input pins 12 and 13 of the unused gate D must be tied to ground and pin 11 left open. Preset VR1 should be of cermet type and used to change the flashing rate of each group of LEDs. The circuit works off regulated 12V. Assemble it on a general-purpose PCB and house suitably.
                     

9 line telephone sharer
This circuit is able to handle nine independent telephones (using a single telephone line pair) lo-cated at nine different locations, say, up to a distance of 100m from each other, for receiving and making outgo-ing calls, while maintaining conversa-tion secrecy. This circuit is useful when a single telephone line is to be shared by more members residing in different rooms/apartments. Normally, if one connects nine phones in parallel, ring signals are heard in all the nine telephones (it is
also possible that the phones will not work due to higher load), and out of
nine persons eight will find that the call is not for them. Further, one can over-hear others’ conversation, which is not desirable. To overcome these problems, the circuit given here proves beneficial, as the ring is heard only in the desired extension, say, extension number ‘1’. For making use of this facility, the calling subscriber is required to initially dial the normal phone number of the
called subscriber. When the call is estab-lished, no ring-back tone is heard by the calling party. The calling subscriber has then to press the asterik (*) button on the telephone to activate the tone mode
(if the phone normally works in dial mode) and dial extension number, say, ‘1’, within
10 seconds. (In case the calling subscriber fails to dial the required extension num-ber within 10 seconds, the line will be disconnected automatically.) Also, if the dialed extension phone is not lifted within 10 seconds, the ring-back tone will cease. The ring signal on the main phone line is detected by opto-coupler  MCT-2E (IC1), which in turn activates the 10-second ‘on timer’, formed by IC2
(555), and energises relay RL10 (6V, 100-ohm, 2 C/O). One of the ‘N/O’ contacts of the relay  has been used to connect +6V rail to the processing circuitry and the other has been used to provide
220-ohm loop resistance to de-energise the ringer relay in telephone exchange, to cut off the ring.
When the caller dials the extension number (say, ‘1’) in tone mode, tone receiver CM8870 (IC3) outputs code ‘0001’, which is fed to the 4-bit BCD-to-10 line decimal de-coder IC4 (CD4028). The out-put of IC4 at its output pin 14 (Q1) goes high and switches on the SCR (TH-1)
and associated relay RL1. Re-lay RL1, in turn, connects, via its N/O contacts, the 50Hz ex-tension ring signal, derived from the 230V AC mains,  to the line of telephone ‘1’. This
ring signal is available to tele-phone ‘1’ only, because half of the signal is blocked by di-ode D1 and DIAC1 (which do not conduct below 35 volts). As soon as phone ‘1’ is lifted, the ring current in-creases and voltage drop across R28 (220-ohm, 1/2W re-sistor) increases and operates
opto-coupler IC5 (MCT-2E). This in turn resets timer IC2
causing:
(a) interruption of the
power supply for processing
circuitry as well as the ring

5-BAND GRAPHIC EQUALISER
                                    

                                              This equaliser uses low-cost op-amps. Good-quality opamps powered by a single voltage supply are readily available in the market. The op-amp should have a noise density  of less than 24nV/√Hz, slew rate of more than 5V/μs and gain bandwidth product greater than 3MHz. The NE5532 or LM833 used in this circuit meets these requirements Equaliser circuits typically divide the audio spectrum into separate frequency bands and have independent gain control for each band. The output of each band is mixed at IC4(A) and then fed to an  audio power
 amplifier Proper quality factor (Q) needs
to be selected to avoid overlap in adjacent
bands as this introduces
colouration into the audio signal.
We have used the multiple-feedback
bandpass filter topology shown
in left-most corner at the bottom of the
figure. This is a circuit for single-channel
bandpass filter. If the capacitors areof the same value, the calculations are
fairly simple. For calculating the component
values, use the following formulae:
Centre frequency (fo) : 1/2πC√(Ra||Rb)Rc
Bandwidth (B) : 1/πCRc
Quality factor (Q) : fo/B = πfoCRc
Gain (A) : –Rc/2Ra
These can be combined to give the
following formulae:
Ra = Q/2πfoAC
Rb = Q/2πfoC (2Q2–A)
Rc = Q/πfoC
Begin the calculations by choosing
a large value of capacitance (~0.1F) and
smaller value of resistances. Increasing
the capacitance decreases resistances
(Ra, Rb and Rc). Care must be taken to
avoid overloading on the input buffer
op-amp. Note that stray capacitances
on the board reduces the value of ‘C.’
The bandwidth and gain do not depend
on Rb. Hence, Rb can be used to
modify the mid-frequency without affecting
the bandwidth and gain.
For equalisers, there are standard
mid-frequencies that are normallythe octave division, application and
some degree of manufacturers’ preference,
but nearly all share the basic octave
boundaries that are based on a
centre frequency of 1000 Hz.


A balance between the number of
filters and bandwidth need to be observed.
It is possible to use a wider
bandwidth and fewer filters, or narrower
bandwidth and more filters.
Anything narrower than 1/3 octave is
rare, since the complexity of the filters
increases for higher values of ‘Q.’ This
can get rather expensive and in reality
is of limited use for most applications
in audio systems.
National Semiconductor lists the
following mid-frequencies for a 10-
band graphic equaliser: 32, 64, 125, 250,
500, 1k, 2k, 4k, 8k and 16k. It also recommends
a ‘Q’ of 1.7 for equalisers.
The table lists the component values
for different centre frequencies of
the equaliser. We used ‘Q’ of 1.7 and
gain (A) of 4.
The circuit for the 5-band equaliser
uses IC1 (A) LM833 as the buffer stage
for the equaliser. It is a non-invertingamplifier with a gain of ‘2.’ The input
signal is divided by ‘2’ by the resistive
network comprising R3 and R4. Hence
the net gain of this amplifier is unity.
Two 100k resistors (R1 and R2) are
used as a voltage divider and the junction
voltage is fed to its
positive input through R6. This
divider has enough power to feed all
other op-amps directly. Resistor Ro
(R8=R12=R16=R20=R24=R28=R30=100Ω)
has the dual function of noise reduction
and resistive isolation of capacitive
load. It may be varied between 50
and 150 ohms depending on the noise
in the circuit. the pot metrs [VR1 through VR5] are in the signal path hence should be of the
best quality possible. wrap the body of the pots with bare copper wire and solder the other end
of the wire to the ground.since the filters are very sensitive,allresistance should be metal film
type and the capacitor should be polyester type.
  Each stage of the op-amp needs to be capacitively coupled to the next stage so that the DC does not get propagated and amplified. For a good low-frequency response, this coupling capacitor should be greater than 1 μF. A 10μF, 16V capacitor is used in each stage of the circuit here. The circuit is powered by a 12V DC regulated supply. A well-regulated supply using 7812 is recommended. Ground the Vcc pin of each op-amp with a 0.1μF ceramic disk capacitor to