Programmable Control and Logic Gates

Making decisions in a control system

A control system receives information from inputs, processes it and operates outputs. A switch might report whether a door is closed; a sensor might report the temperature. The processing part decides whether to turn on a lamp, sound an alarm or run a motor.

Two ways to carry out this processing are logic gates, which apply fixed rules to digital signals, and microcontrollers, which follow a loaded program. Combining decisions allows a product to respond to several conditions rather than simply switching on whenever one input changes.

Digital signals: LOW and HIGH

A digital signal has two states: LOW, represented by logic 0, and HIGH, represented by logic 1. In the simple circuit model, LOW is usually 0 V and HIGH is the supply voltage. The numbers describe signal states, not quantities of electricity.

An analogue signal, by contrast, can vary continuously over a range. A temperature-sensing circuit can provide an analogue voltage that changes as the temperature changes, whereas a digital input reports one of two states.

Before designing a control system, give each input a clear meaning. For example, if a door-switch input is HIGH when the door is closed, logic 1 means ‘closed’. The processing rule must use that meaning consistently.

The five logic-gate rules

A logic gate produces an output according to a particular rule. A truth table records its output for every possible input combination. Two digital inputs have four possible combinations.

An AND gate produces HIGH only when both inputs are HIGH. This suits a product that should operate only when two conditions are satisfied, such as a start request and a closed safety guard.

An OR gate produces HIGH when either input, or both inputs, are HIGH. It suits a warning that should operate when either of two sensors detects a problem.

An EOR gate, meaning exclusive OR and also called XOR, produces HIGH when its two inputs are different. Exactly one input must be HIGH: unlike OR, EOR produces LOW when both inputs are HIGH.

A NAND gate means NOT AND. First consider the AND result, then reverse it. Its output is therefore LOW only when both inputs are HIGH.

A NOT gate has only one input. It reverses that input, so it is also called an inverter.

Combining gates to make a control rule

The output of one gate can become an input to another. This allows a system to combine conditions such as ‘B is HIGH and A is not HIGH’.

Consider a circuit in which input A passes through a NOT gate. Call the NOT output C. Inputs B and C then feed an AND gate, whose final output is Q.

Complete C first by reversing A. Then find Q using B and C, because these are the actual inputs to the AND gate.

For the third row, A is LOW, so C is HIGH. B is also HIGH, so the AND output Q is HIGH. The whole circuit therefore produces HIGH only when B is HIGH and A is LOW.

The same approach works in reverse when designing a system: describe the required conditions, list all input combinations, decide the required output for each row, then select and connect gates that produce those results.

A microcontroller follows software

A microcontroller is a programmable integrated circuit into which software can be loaded to perform different tasks. Its program reads inputs, makes decisions and changes outputs. Changing the program can change the sequence or decision rules without rebuilding the processing circuit.

Microcontrollers have connection pins with particular functions. Some accept digital inputs; others can be configured for analogue or digital inputs. Some pins can be configured as inputs or outputs. The designer must choose a device with enough suitable connections and check its data sheet rather than assuming every pin can do every job.

Switch and sensing circuits provide signals to input pins. A suitable LED output can be controlled from an output pin with appropriate current limiting. Devices needing more power, such as motors or lamps, require an interface such as a transistor or MOSFET driver. The microcontroller supplies the control signal; the driver switches the load current.

A program can then be downloaded, tested with the actual input and output devices, and modified until the product behaves as intended.

Following a programmed sequence

A flowchart makes the order of a program visible. Arrows show the direction of execution. Rounded boxes mark Start and Stop, parallelograms show input/output actions, rectangles show processes such as delays, and diamonds show decisions with Yes and No routes.

For a warning lamp on a rubbish skip, the start switch is connected to input C5, the stop switch to input C1 and the lamp to output B0. The required sequence is to wait for the start switch, turn the lamp on for 2 seconds, turn it off for 1 second, and repeat until the stop switch is detected.

At the first decision, ‘Is input C5 on?’, the No route loops back to the same test. The program repeatedly checks the start input rather than moving on. The Yes route begins the flashing sequence.

After the two delays and output changes, the program asks ‘Is input C1 on?’ A No answer repeats the flashing sequence; a Yes answer reaches Stop. This combines input testing, output control, time delays and repetition in one system.

The position of a test matters. Here the stop input is checked only after the complete on–off sequence. Pressing the stop switch during a delay does not cause an immediate response; the input must still be active when the program reaches its test.

Fixed rules and programmable behaviour

A combined logic-gate circuit applies the rules built into its connections. A microcontroller applies instructions in software and can include sequences and delays as well as decisions. In either case, successful control depends on matching the input meanings, processing rules and output interfaces to the product’s requirements.

Digital states

• LOW = logic 0; usually 0 V.
• HIGH = logic 1; the supply voltage in the simple model.
• Analogue signals vary continuously; digital signals have two states.

Gate rules

• AND: HIGH only when both inputs are HIGH.
• OR: HIGH when either or both inputs are HIGH.
• EOR/XOR: HIGH when the two inputs differ.
• NOT: reverses its single input.
• NAND: opposite of AND; LOW only when both inputs are HIGH.

Combined logic

Find intermediate outputs first, then use them as the inputs to the next gate. A truth table includes every possible input combination.