Avionics News April 2020 - 61

Q1 is on, the Q output is low because all the voltage is
dropping across R1. Q2 will be off because no voltage
can make it to its base. Q1 will remain on because voltage
is applied to the base through R3 and R4. D1 is forward
biased (on) because it has a positive voltage on its anode,
and its cathode is connected through R7 to the collector
of Q1, which is at ground potential. D2 is reverse biased
because its cathode is connected through R8 and R4 to the
supply voltage. Its anode is connected through R6 to ground
and through R2 to the cathode of Q1, which as stated
above, is at ground potential.
Now let's upset this stable apple cart by introducing
a negative going pulse to the trigger input. The negative
voltage causes C3 to charge and the current flow will pass
through the forward-biased D1, which lowers the voltage on
Q1, turning it off. Figure 2(b) shows the state of the circuit
after a trigger pulse.
When Q1 turns off, its collector and the Q output go
high. Now Q2 has enough voltage to turn on, Q NOT goes
low, D2 forward biases, and the circuit has flipped. It will

F R O M

T H E

remain in this state until another trigger pulse is applied,
at which time the base of Q2 goes low, turning it back
off, which allows Q1 to turn back on, and the circuit has
flopped.
Figure 2 shows one trigger pulse. If no more trigger
pulses arrive, the circuit would remain in this stable state
until the power was shut off. In this way, the circuit would
act as a memory, holding a logic 1 or a logic 0.
Bistable multivibrators can be constructed with
transistors, as shown here, with logic gates, operational
amplifiers and even with the 555 timer chip.
In the field, you will find it rare to see the multivibrators
discussed this month, and the monostable multivibrator
discussed last month constructed from individual, or
discreet, components. In a school setting where students
need to learn the operating principles of electronics, you will
see these circuits on breadboards. In addition to learning
electronic theory, students constructing these circuits will
learn building and troubleshooting skills. Multivibrators of
any type are useful in many settings. q

C O C K P I T

T O

Y O U R

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AVIONICS NEWS

APRIL

2020

Avionics News April 2020

Table of Contents for the Digital Edition of Avionics News April 2020