Specifications

low-resistance path to them. When tran-
sistor Q7801 is biased into conduction, it
provides a much lower path to ground than
the diode network, and the diode network
is effectively removed from the circuit.
10. Schmitt Trigger Circuit
(fig.7)
The Schmitt trigger circuit consists of a
dc amplifier, a Schmitt trigger, a dc switch,
and a pair of indicator lamps. The circuit
is used in all positions of selector switch
S7902.
a. Transistor Q7802 is an emitter-fol-
lower, dc amplifier. Transistors Q7803
and Q7804 form the schmitt trigger. Tran-
sistor Q7805 is used as a dc switch.
b. In the absence of an input signal (de)
or when a small input signal is present,
transistor Q7804 conducts and transistors
Q7802, Q7803, and Q7805 are cut off.
Transistor Q7804 conducts and a relatively
low-resistance path to ground is estab-
lished through R7811, Q7804, and RED
indicator 1amp DS7902, causing lamp
DS7902 to light.
C. When a large input signal (de) is ap-
plied to the base of Q7802, the transistor
s biased into conduction. Since the base of
Q7803 in the Schmitt trigger is directly
connected to the emitter of Q7802, tran-
sistor Q7803 is also biased into conduction.
When Q7803 conducts, the voltage at the
base of Q7803 decreases. This decreased
voltage, which is applied to the base of
Q7804 through R7809, causes Q7804 to cut
off. When transistor Q7804 is cut off, the
voltage at the base of dc switching tran-
sistor Q7805 is increased. Transistor
Q7805 starts conducting and causes a
relatively low resistance path to ground
through R7815, CR7802, Q7805, and GREEN
indicator lamp DS7901 and causes DS7901
to light.
d. Resistors R7806 and R7808 and lamps
DS7902 and DS7901 are the collector loads
for Q7802 through Q7805 respectively.
Resistor R7807 is the emitter-base bias
resistor for Q7802. Resistor R7811 es-
tablishes the emitter bias for Q7803 and
Q7804. Resistors R7809 and R7812 estab-
lish the base bias for Q7804, and R7813
and R7814 establish the base bias for
Q7805. Resistor R7815 establishes the
emitter bias for Q7805. Diode CR7802 pre-
vents any small signal present at the base
of transistor Q7805 from biasing the
transistor into conduction while Q7804 is
conducting. When Q7805 is conducting,
diode CR7802 presents a very small im-
pedance to the current.
11. Line Voltage Sensing Circuit
(fig. 8)
The line voltage sensing circuit consists
of a voltage divider, a pulse generator, and
a dc switch which senses the minimum
limit of the power source voltage for the
TS-1777/VRM-l. The circuit is used when-
ever power is applied to the TS-1777/
VRM-1.
Figure 6. Voltage supply limits sensing circuit, schematic diagram.