Specifications

CIRCUIT IDEAS
‘zero cal’. Now increase mV input to 21.85
(corresponding to 400
o
C). Then vary VR2
till +out terminal is at 430mV (temp.
+RT). This is ‘gain cal’. Now, as VR1 and
VR2 are interdependent, you may have
to repeat ‘zero cal’ and ‘gain cal’ a few
times till you get the above values.
Properties of J thermocouple and
design aspects of gain block used in the
temperature measurement instrument
are summarised below:
J Thermocouple (ANSI Symbol ‘J’)
1. J is a thermocouple made of iron
(positive) and constantan (negative).
2. Constantan is an alloy of copper
and nickel.
3. Full range of use is from – 200
o
C
to +700
o
C.
4. It is practical to use it only from
0
o
C to 400
o
C.
5. It is useful in reducing and alka-
line atmosphere.
6. It corrodes/rusts in acidic and
oxidising atmosphere.
7. Colour codes of wires are negative-
red and positive-white.
8. J type is popular because of low
price and high mV output.
9. J type TC is used in rubber/plas-
tic forming and for general purpose.
Design of gain block
1. Minimum input from thermo-
couple is as low as 1 to 2 mV. Hence
ultra-low offset (<100µV) op-amp OP07
is used.
2. Inputs may be subjected to wrong
connections or high voltage. Use of resis-
tor R2 limits current and zener ZD1
clamps voltage to a safe level.
3. Gain required is 400mV/21.8mV,
which is approximately 18 at 400ºC.
Gain Av = (Rf+Ri)/Ri. Here Rf is R7
and Ri = R5+R6+VR2 (in circuit-value).
Design of TH1 cold junction com-
pensation copper thermistor
1. J Type TC output changes by
0.052mV per
o
C as per Table I. Copper
has a temperature coefficient of 0.0042
ohm per ohm/
o
C. For example, for a cop-
per wire of 12 ohms, it is 12x0.0042=0.05
ohm/
o
C.
2. For R1 of 5k, current through TH1=
5V/5k=1mA. Change of voltage across
TH1 with temperature is 0.05x1mA =
0.05 mV/deg.
3. This rate is the same as that of J
type thermocouple and hence it simu-
lates cold junction.
Lab Note. During lab testing the
value of VR1 had to be very much in-
creased. However, as per author, it
should be kept at 1 kilo-ohm only.
T
his circuit consists of two parts
as shown in the figure. The up-
per circuit should be assembled
in a box along with regulated 9V power
supply (not shown in figure), while lower
circuit may be assembled on a small gen-
eral-purpose PCB and fixed inside the
doorbell switch enclosure. Connect points
A and B of one module to the similar
points of the other, using a simple 2-
core electric cable. The polarity need not
be adhered to, because the bridge recti-
fier used inside the switch circuit auto-
matically ensures proper polarity.
In the normal condition, any voice
or sound in the vicinity of the door,
where the lower circuit (module 2) is
installed, will be heard on module 1
inside your home. However, as soon as
the door bell switch is pressed by some-
one, a distinct bell sound will be heard
in the loudspeaker, inside the house.
With switch S1 in open condition,
module 2, which is a simple condenser
mic amplifier, amplifies the sound/au-
dio in its vicinity and the audio output
is available across points A and B. In
module 1, this audio is developed across
preset VR1, which acts as a volume
control. The audio from the wiper of
the preset is coupled to the input of
low-power (1.2-watt) audio amplifier
TBA820M, which after amplification is
fed into a 4-ohm loudspeaker. (The com-
bination of resistor R9 and capacitor C7
introduced by EFY Lab in the path of
Vcc pin 6, during actual testing, helps
in noise reduction and limits the power
dissipation in IC2.)
Transistor T1 is normally conduct-
ing due to its base pulled to the posi-
tive supply rails via resistors R3, R5,
and R4. Therefore collector of transis-
tor T1 is at near ground potential, and
VOICE BELL
S.C. DWIVEDI
MUKESH KUMAR SONI
187