I say "supposed to" because making both the S and R inputs equal to 1 results in both Q and not-Q being 0. The Q and not-Q outputs are supposed to be in opposite states. To create an S-R latch, we can wire two NOR gates in such a way that the output of one feeds back to the input of another, and vice versa, like this: The simplest bistable device, therefore, is known as a set-reset, or S-R, latch. Typically, one state is referred to as set and the other as reset. The most interesting and widely used multivibrators are of the bistable variety, so we'll explore them in detail now.Ī bistable multivibrator has two stable states, as indicated by the prefix bi in its name. The astable multivibrator mentioned previously, with only one "vibrator," is more commonly implemented with multiple gates, as we'll see later. Henry Ford's engineers also employed the buzzer/transformer circuit to create continuous high voltage for operating the spark plugs on Model T automobile engines:īorrowing terminology from the old mechanical buzzer (vibrator) circuits, solid-state circuit engineers referred to any circuit with two or more vibrators linked together as a multivibrator. The buzzer or vibrator circuit thus formed was used extensively in early radio circuitry, as a way to convert steady, low-voltage DC power into pulsating DC power which could then be stepped up in voltage through a transformer to produce the high voltage necessary for operating the vacuum tube amplifiers. If implemented with relay logic, the resulting oscillator will be considerably slower, cycling at a frequency well within the audio range. The result is a high frequency (several megahertz) oscillator, if implemented with a solid-state (semiconductor) inverter gate: That 0 output gets fed back to the input as a 0, and the cycle repeats itself. When the input is 1, the output switches to 0. That 1 output gets fed back to the input as a 1. When the input is 0, the output switches to 1. There are also monostable multivibrators, which have only one stable output state (that other state being momentary), which we'll explore later and astable multivibrators, which have no stable state (oscillating back and forth between an output of 0 and 1).Ī very simple astable multivibrator is an inverter with the output fed directly back to the input: It is called "bistable" because it can hold stable in one of two possible output states, either 0 or 1. The example we just explored with the OR gate was a very simple example of what is called a bistable multivibrator. When A is 0, the output could be either 0 or 1, depending on the circuit's prior state! The proper way to complete the above truth table would be to insert the word latch in place of the question mark, showing that the output maintains its last state when A is 0.Īny digital circuit employing feedback is called a multivibrator. Since the output feeds back to one of the OR gate's inputs, and we know that any 1 input to an OR gates makes the output 1, this circuit will "latch" in the 1 output state after any time that A is 1. If A is "low" (0), however, we cannot guarantee the logic level or state of the output in our truth table. Such is the nature of an OR gate: any "high" (1) input forces the output "high" (1). We know that if A is 1, the output must be 1, as well. However, if we alter this gate circuit so as to give signal feedback from the output to one of the inputs, strange things begin to happen: Whether we're dealing with a multitude of cascaded gates or a single gate, that output state is determined by the truth table(s) for the gate(s) in the circuit, and nothing else. Take the truth table of an OR gate, for instance:įor each of the four possible combinations of input states (0-0, 0-1, 1-0, and 1-1), there is one, definite, unambiguous output state. With simple gate and combinational logic circuits, there is a definite output state for any given input state. Lessons In Electric Circuits - Volume IV Chapter 10 MULTIVIBRATORS
Lessons In Electric Circuits - Volume IV (Digital) - Chapter 10