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Page 60
LOGIC PROBE
IC U206
is a quad single-supply comparator
de-
signed for level detection and low-level sensing ap-
plications. However, for the logic probe application,
only two of the comparators (1 and 2) are actually
used.
With the logic probe not being used, a voltage divid-
ing network consisting of R206 and R207 establishes
a bias of approximately 1.5 volts on pins 5 and 6
of comparators
1 and 2 respectively. Likewise, a
voltage divider consisting of R208 and R209 biases
pin 4 of comparator 1 at approximately 1 volt, while
voltage divider R204 and R205 biases pin 7 of com-
parator 2 at approximately 2 volts.
Comparator 1 is connected as a noninverting device.
Thus, in the quiescent state (no input on the logic
probe), there is a high output at pin 2 of comparator
1. Recall that when the signal applied to a com-
parator's noninverting input (pin 5) is more positive
than the signal applied to the inverting input (pin
4), the output is high. With a high output from com-
parator 1, the green LED can not conduct.
Comparator 2 is connected as an inverting device.
Again, with no input from the logic probe, there is
a high output at pin 1 of comparator 2. This is be-
cause the signal applied to the noninverting input
(pin 7) is more positive than the signal applied to
the inverting input (pin 6). Thus, with a high output
from comparator 2, the red LED will not conduct.
However, when the logic probe is connected to a
high logic level (in excess of 2 volts), the inverting
input (pin 6) of comparator 2 becomes more positive
than noninverting input (pin 7). Because this is an
inverting device, a low will be felt at the output.
This low at the output will allow the red LED to
conduct, showing there is a high logic level present.
At the same time, the high logic level is also applied
Heathkit
to the noninverting
input of comparator
1. This
causes pin 5 to become more positive than in the
quiescent stage. Thus, the output of comparator 1
will remain high and the green LED will not con-
duct.
If the logic probe is connected to a low logic level
(less than .8 volt), the noninverting input at pin 5
of comparator 1 will become less positive than the
inverting input at pin 4. This causes a low to be
felt at the output, which in turn causes the green
LED to conduct, indicating a low logic level is pre-
sent. Also, the low logic level on the probe will
cause the inverting input at pin 6 of comparator 2
to become less positive than the noninverting input
at pin 7. Hence, the output of comparator 2 will be
positive and the red LED will not conduct.
POWER SUPPLIES
The voltage from the secondary winding of tran-
former T201 is rectified by diodes D201, D202, D203,
and D204. The voltage is filtered by capacitor C201
and regulated by IC U201 to produce the +5 volt
DC supply.
The voltage from one of the center-tapped secondary
windings of T201 (green leads) is rectified by diodes
D205 and D206, filtered by C204, and regulated by
U202. This produces the +12 volt supply. The volt-
age from the same winding is rectified by diodes
D207 and D208, filtered by capacitor C207, and regu-
lated by IC U203 to provide the — 12 volt supply.
The voltage from the center-tapped winding (red
leads) is rectified by diode D209 and D211, filtered
by capacitor C211, and regulated by IC U204. Resis-
tor R141 is used to vary the voltage from +1.2 to
+20 volts. The voltage from the same winding is
rectified by diode D212 and D213 and filtered by
capacitor C214. Resistor R142 is used to vary the
voltage from — 1.2 to — 20 volts.
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