3-23
Figure 3-29.—Block diagram of a flip-flop with a toggle input.
Figure 3-30.—Flip-flop with three inputs (block diagram).
Remember, a SET input will SET the flip-flop if it is in the CLEAR state, otherwise, it will not do
anything; a trigger at the CLEAR input can only CLEAR the circuit if it is SET; and a trigger applied to
the TOGGLE input will cause the bistable multivibrator to change states regardless of what state it is in.
Q7. In a bistable multivibrator, how many trigger pulses are needed to produce one complete cycle in
the output?
Q8. How many stable states are there for a flip-flop?
Q9. If a voltage (positive or negative) is measured on the "1" output of a flip-flop, what state is it in?
BLOCKING OSCILLATOR
The BLOCKING OSCILLATOR is a special type of wave generator used to produce a narrow pulse,
or trigger. Blocking oscillators have many uses, most of which are concerned with the timing of some
other circuit. They can be used as frequency dividers or counter circuits and for switching other circuits
on and off at specific times.
In a blocking oscillator the pulse width (pw), pulse repetition time (prt), and pulse repetition rate
(prr) are all controlled by the size of certain capacitors and resistors and by the operating characteristics of
the transformer. The transformer primary determines the duration and shape of the output. Because of
their importance in the circuit, transformer action and series RL circuits will be discussed briefly. You
may want to review transformer action in NEETS, Module 2, Introduction to Alternating Current and
Transformers before going to the next section.
Transformer Action
Figure 3-31, view (A), shows a transformer with resistance in both the primary and secondary
circuits. If S1 is closed, current will flow through R1 and L1. As the current increases in L1, it induces a
voltage into L2 and causes current flow through R2. The voltage induced into L2 depends on the ratio of
turns between L1 and L2 as well as the current flow through L1.
