Specifications

CIRCUIT IDEAS
S.C. DWIVEDI
level. At power-on,
the output of
NAND gate N2
goes low. This is
the default state of
the gate. The out-
put of gate N2 goes
low (indicated by
glowing of LED2)
during the follow-
ing situations:
1. Power to the
probe is switched
‘on’.
2. The probe’s
tip is floating, i.e.
when it is neither
in contact with a
point at logic ‘0’ nor at logic ‘1’ state.
3. The probe tip is in contact with a
TTL output that is in high impedance
state.
The LED indications for various tip
levels are summarised in the accompa-
nying table.
This logic probe is very well suited
for use with microcontrollers, micropro-
cessors, EEPROMs, SRAMs etc.
Some points to be noted are:
• Use IC1 of type HD74LS00 only.
• If any other type of IC (e.g.
74HCT00 or 74LS00) is used, diodes D2-
D3-D4 should be added or deleted as
necessary; for example, when using
HD74LS00, one diode D2 is required.
• Use another diode in series with
D2 if the LED indication overlaps the
float indicator.
(EFY Lab note. The probe was
found to work properly only with
HD74LS00.)
A
TTL logic probe is an indispens-
able tool for digital circuit
troubleshooting. Various meth-
ods can be used to design a logic probe.
The most common designs employ op-
amps, logic (OR, NOT, XOR) gates, and
transistors.
The circuit presented here uses
NAND logic gates of Hitachi HD series
IC HD74LS00, which is a quad-NAND
IC. Special technique has been employed
to obtain three-state operation using
just a single IC.
Gate N1 is wired such that when the
output of gate N1 is at logic ‘0’ (i.e. when
its input is at logic ‘1’), LED1 will glow,
to indicate high state of the point being
probed. Gate N3 is wired to light LED3
when the output of gate N3 is high or
when the point being tested is at logic ‘0’
T. SURESH
TTL THREE-STATE LOGIC PROBE
S. No. TIP level Output
1. Ground/logic 0 LED3 ON
2. Vcc/logic 1 LED1 ON
3. Floating/or connected
to high impedance LED2 ON
S.C. DWIVEDI
YUJIN BOBY VU3PRX
In this transmitter we remove the
carrier and transmit only the two side
bands. The effective output of the cir-
cuit is three times that of an equivalent
AM transmitter.
Opamp IC 741 is used here as a
microphone amplifier to amplify the
voice picked up by the condenser mi-
crophone. The output of opamp is fed
to the double balanced modulator
(DBM) built around four 1N4148 diodes.
The modulation level can be adjusted
with the help of preset VR1.
The carrier is generated using crys-
tal oscillator wired around BC548 tran-
sistor T2. The carrier is further amplified
by transistor T1, which also acts as a
buffer between the carrier oscillator and
the balanced modulator. The working
frequency of the transmitter can be
changed by using crystals of different
frequencies. For multi-frequency opera-
tion, selection of different crystals can be
made using a selector switch. The level of
the carrier coupled to the DBM can be
adjusted with the help of preset VR2.
The output of the DBM contains only
the product (of audio and carrier) fre-
quencies. The DBM suppresses both the
input signals and produces double side
band suppressed carrier (DSBSC) at its
output. However, since the diodes used
in the balanced modulator are not fully
matched, the output of the DBM does
AM DSB TRANSMITTER
FOR HAMS
T
his circuit of AM transmitter is
designed to transmit AM (am-
plitude modulated) DSB (double
sideband) signals. A modulated AM sig-
nal consists of a carrier and two sym-
metrically spaced side bands. The two
side bands have the same amplitude
and carry the same information. In fact,
the carrier itself conveys or carries no
information. In a 100% modulated AM
signal 2/3
rd
of the power is wasted in
the carrier and only 1/6
th
of the power
is contained in each side band.
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