Latch circuits are often drawn as complete units in their own block symbols, rather than as a collection of individual gates:
This simplifies schematic drawings where latches are used, much as the use of gate symbolism (as opposed to drawing individual transistors and resistors) simplifies the diagrams of more elementary digital circuits.
From the block symbols shown in this question, is there any way to determine which of the S-R latches is built with NOR gates, and which one is built with NAND gates?
The following relay logic circuit is for starting and stopping an electric motor:
Draw the CMOS logic gate equivalent of this motor start-stop circuit, using these two pushbutton switches as inputs:
Make sure that your schematic is complete, showing how the logic gate will drive the electric motor (through the power transistor shown).
One practical application of S-R latch circuits is switch debouncing. Explain what “bounce” refers to in mechanical switches, and also explain how this circuit eliminates it:
Also, show where an oscilloscope could be connected to display any switch “bounce,” and explain how the oscilloscope would have to be configured to capture this transient event.
A student builds this simple S-R latch for their lab experiment:
When the student powers up this circuit, she notices something strange. Sometimes the latch powers up in the set state (Q high and [Q] low), and other times it powers up in the reset state (Q low and [Q] high). The power-up state of their circuit seems to be unpredictable.
What state should their circuit power up in? Did the student make an error building the latch circuit?
Here, a gated S-R latch is being used to control the electric power to a powerful ultraviolet lamp, used for sterilization of instruments in a laboratory environment:
Based on your knowledge of how gated S-R latches function, what is the purpose of the “Lockout” switch? Also, explain how the CMOS latch is able to exert control over the high-power lamp (i.e. explain the operation of the interposing devices between the latch and the lamp).
Now, suppose the lab personnel want to add a feature to the ultraviolet sterilization chamber: an electric solenoid door lock, so that personnel can open the door to the chamber only if the following conditions are met:
- Lamp is off
- “Lockout” switch is sending a “low” signal to the latch’s Enable input
Modify this circuit so that it energizes the door lock solenoid, allowing access to the chamber, only if the above conditions are both true.
An analog-to-digital converter is a circuit that inputs a variable (analog) voltage or current, and outputs multiple bits of binary data corresponding to the magnitude of that measured voltage or current. In the circuit shown here, an ADC inputs a voltage signal from a potentiometer, and outputs an 8-bit binary “word,” which may then be read by a computer, transmitted digitally over a communications network, or stored on digital media:
As the input voltage changes, the binary number output by the ADC will change as well. Suppose, though, that we want to have sample-and-hold capability added to this data acquisition circuit, to allow us to “freeze” the output of the ADC at will. Explain how using eight D latch circuits will give us this capability:
Gated latch circuits often come packaged in multiple quantities, with common gate inputs, so that more than one of the latches within the integrated circuit will be enabled and disabled simultaneously. Examine this logic symbol, representative of the 74AC16373, a 16-bit D-type latch with tri-state outputs:
Note how the sixteen D latches are divided into two groups of eight. Explain the functions of the four inputs at the very top of the symbol (1EN, C1, 2EN, and C2). Which of these input lines correspond to the “Enable” inputs seen on single D-type latch circuits? Also, describe what the “wedge” shapes represent on the 1EN and 2EN input lines.
Suppose you wished to have all sixteen latch circuits enabled as one, rather than as two groups of eight. Show what you would have to do to this circuit in order to achieve this goal.
In many types of digital systems, a set of square-wave signals are phase-shifted from each other by 90o. Such a phase relationship is called quadrature.
Determine the output of a D-type latch for this pair of quadrature signals, applied to the D and E inputs over time:
Then, determine the output of a D-type latch when the phase relationship is reversed, (D leading E by 90o, instead of E leading D by 90o):
This one-way street is equipped with an alarm to signal drivers going the wrong way. The sensors work by light beams being broken when an automobile passes between them. The distance between the sensors is less than the length of a normal car, which means as a car passes by, first one beam is broken, then both beams become broken, then only the last beam is broken, then neither beam is broken. The sensors are phototransistors sensitive only to the narrow spectrum of light emitted by the laser light sources, so that ambient sunlight will not “fool” them:
Both sensors connect to inputs on a D-type latch, which is then connected to some other circuitry to sound an alarm when a car goes down the road the wrong way:
The first question is this: which way is the correct way to drive down this street? From left to right, or from right to left (as shown in the illustration)?
The second question is, how will the system respond if sensor A’s laser light source fails? What will happen if sensor B’s laser light source fails?
A very common form of latch circuit is the simple “start-stop” relay circuit used for motor controls, whereby a pair of momentary-contact pushbutton switches control the operation of an electric motor. In this particular case, I show a low-voltage control circuit and a 3-phase, higher voltage motor:
Explain the operation of this circuit, from the time the “Start” switch is actuated to the time the “Stop” switch is actuated. The normally-open M1 contact shown in the low-voltage control circuit is commonly called a seal-in contact. Explain what this contact does, and why it might be called a “seal-in” contact.
A student decides to build a motor start/stop control circuit based on the logic of a NOR gate S-R latch, rather than the usual simple “seal-in” contact circuit:
The circuit works fine, except that sometimes the motor starts all by itself when the circuit is first powered up! Other times, the motor remains off after power-up. In other words, the power-up state of this circuit is unpredictable.
Explain why this is so, and what might be done to prevent the motor from powering up in the “run” state.
|Don’t just sit there! Build something!!|
Learning to analyze relay circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.
You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:
- Draw the schematic diagram for the relay circuit to be analyzed.
- Carefully build this circuit on a breadboard or other convenient medium.
- Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
- Analyze the circuit, determining all logic states for given input conditions.
- Carefully measure those logic states, to verify the accuracy of your analysis.
- If there are any errors, carefully check your circuit’s construction against the diagram, then carefully re-analyze the circuit and re-measure.
Always be sure that the power supply voltage levels are within specification for the relay coils you plan to use. I recommend using PC-board relays with coil voltages suitable for single-battery power (6 volt is good). Relay coils draw quite a bit more current than, say, semiconductor logic gates, so use a “lantern” size 6 volt battery for adequate operating life.
One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.
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