Specifications

CIRCUIT IDEAS
MT2, i.e. connect MT1 to the negative
and MT2 to the positive side. For a good
working triac, S2 would not initiate con-
duction in the triac and the LED would
remain off. On the other hand, momen-
tary depression of S5 would initiate con-
duction of the triac and LED1 would glow.
The indication of a leaky triac is simi-
lar to that of an SCR. If, during both the
above-mentioned tests, the LED lights up,
only then the triac is good.
Before connecting any
SCR
/triac in the
circuit, please check its anode/MT1s con-
nection with the case. (Note: A triac is
actually two SCRs connected back to back.
The first accepts positive pulse for con-
duction while the second accepts nega-
tive pulse for conduction.)
You can also check transistors with
this circuit by introducing a resistor
(about 1 kilo-ohm) between the junction
of switches S1 and S5 and point G. The
collector of npn or emitter of pnp tran-
sistor is to be connected to positive (point
A), while emitter of an npn and collector
of a pnp transistor is to be connected to
negative (point K). The base in both cases
is to be connected to point G.
Fig. 2 indicates the conventional current direc-
tion and forward biasing condition for pnp and npn
transistors. If the transistor under test is of npn type,
on pressing S1, the LED glows, and on releasing or
lifting the finger, it goes off, indicating that the tran-
sistor is good. For pnp transistor, the LED glows on
pressing switch S5 and goes off when it is released.
This indicates that the transistor under test is good.
A leaky or short-circuited SCR or transistor would
be indicated by a permanent glow of the LED by
itself, i.e. without pressing switch S1 or S5.
results in increase of frequency, instead of
decrease. When the value of variable re-
sistor is zero, frequency is given by
Now, when the value of variable re-
sistor VR1 is increased, the frequency in-
creases from the value of ‘f as determined
from above formula (with VR1=0).
Lab note: The circuit has been prac-
tically verified with two different values
of timing capacitor C1 and the results
obtained are tabulated in Table I.
E
lectronics For You readers are
very much familiar with the func-
tioning of timer 555 in astable
mode of operation. Traditionally, if at all
there was a need to keep the duty factor
constant and change the frequency con-
tinuously, the method was to use a vari-
able capacitor, rated in picofarads. But in
that case, changing the frequency in tens
of hertz range would become tedious. On
the other hand, sacrificing duty factor,
change of frequency can be done by chang-
ing the values of R1 or R2.
Both of the above-mentioned problems
can be overcome simply by using a vari-
able resistor, as shown in the circuit dia-
gram. Surprisingly, increase of resistance
TABLE I
VR1 Frequency Frequency
() (Hz) (C1=0.1µ) (Hz) (C1=1µ)
10 453 46
33 455 46.5
68 459 47
100 462 48
220 467 50
500 478 52
1000 506 55
2000 585 63
3300 780 81
4700 1300 140
5600 2200 244
Duty Cycle Duty Cycle
=66% =68%
Note: Using higher values of capacitors (e.g.,
10µ) or higher values of resistors (e.g., 6.8k),
the results were found to be erratic.
VYJESH M.V.
A NOVEL METHOD OF FREQUENCY
VARIATION USING 555
RUPANJANA
39