Avionics News April 2020 - 59

timing of the output, which will be a square wave. The
diodes isolate the collectors of the transistors.
Figure 1 shows an astable multivibrator circuit in two
conditions. For a description of how this circuit works,
let's start with a condition where Q1 is in saturation,
or turned on, as shown on the left side (a) of Figure 1.
Output Q will be low. Q2 will be off and dropping all
available voltage, causing output Q NOT to be high, at the
Vcc level. D2 will be reverse biased (off), isolating the
collector of Q2 from the charging circuits. At this point,
D1 is forward biased, because Q1's collector is at ground
potential and R2 is connected to Vcc. C1 will be charging
with the plate connected to R3 going positive. The path
for C1's charge will be Q1, D1, C1 (charging), R3, R6 and
R5. At the same time, C2 will be charging through Q1 and
R7. In this circuit, R2 and R7 are lower values than R3,
R4, R5 and R6. As a result, C2 charges quickly and won't
play a role until the circuit switches states. After a period
of time, based on the RC time constants involving C1, the
charge on C1 will be high enough to cause Q2 to turn on,
going into saturation.
When Q2 turns on, as shown on the right side of Figure
1 (b), the collector of Q2 goes low, forcing the Q NOT
output to zero, forward biasing D2, taking the base of Q1
toward ground, and shutting Q1 off. The Q output will go
to Vcc, and D1 will be reverse biased. Now D2 becomes
forward biased, C1 charges quickly, with the positive plate
connected to R2. C2 charges slowly through Q2, D2, R4,
R5 and R6. When the C2 charge gets high enough to turn
Q1 back on, the process will repeat itself.
A Google image search will show plenty of astable
multivibrator circuits without adjustable resistors;

however, potentiometers will allow a technician to change
the frequency of the circuit and the symmetry of the
output. In Figure 1, R5 is in series with charging paths for
both C1 and C2. As a result, R5 will control the frequency
of the entire circuit. As shown in Figure 1(c), as the
resistance of R5 decreases, the frequency of the circuit
will increase because both capacitors will charge faster.
R6 will adjust the symmetry of the output waveform. If
R6 is adjusted so there is less resistance between R3 and
R5 (to the left), there will be more resistance between R4
and R5. This will result in:
* C1 charging faster.
* C2 charging slower.
* Q1 being off for a longer period of time.
* Q2 remaining on for a longer period of time.
* Q output high for a longer period of time.
* Q NOT output low for a longer period of time.
If R6 is adjusted so there is more resistance between
R3 and R5 (to the right), there will be more resistance
between R3 and R5. This will result in:
* C1 charging slower.
* C2 charging faster.
* Q1 being off for a shorter period of time.
* Q2 remaining on for a shorter period of time.
* Q output low for a longer period of time.
* Q NOT output high for a longer period of time.
The astable multivibrator described here is designed
entirely with discreet, analog components. This circuit
could also be constructed from logic gates, an operational
amplifier or a 555 timer integrated circuit. The astable
Continued on following page

Figure 2
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