1.1 DIGITAL SIGNALS

 

• read by the computer, and interpreted as ON or OFF.

 

• Examples of Inputs

- contact switches

- proximity switches

- pulse counters

- pulse generators

- optical sensors

 

• Examples of Outputs,

- indicator lights

- solid state relays (e.g. for power to motor)

 

• Within this signal type there are two categories of signal,

- Discrete binary - such as a switch which will be thrown once

- Pulse data - a continuous stream of pulses. The frequency, period, or duty cycle will control the signal value. A pulse from a tachometer for every revolution will indicate the motor speed.

 

• The electrical connections for these signals quite often require some additional circuitry. This includes,

latches - used to capture fast changing values (74163?)

schmitt triggers - pull a signal that may not be 0 or 5V to a reasonable logic level. It also uses a dead zone to reduce random switching. (7414?)

flip flops - we can toggle these (7421?)

inverters - gives the signal strength an buffers (7404?)

opto-isolators - eliminates electrical connections for circuit safety. These are basically an LED and phototransistor in one package.

triacs - allows DC signals to switch AC

relays - allows electrical switching that is electrically isolated with mechanical devices.

line drivers - converts low current signals to higher current (7485?)

op-amps - allow high impedance buffering between inputs and outputs. (741)

RC network - a resistor capacitor pair can be used to pass only high/low frequency signals, and filter out noise/DC.

pull-up resistors - can keep inputs/outputs from floating.

 

• Consider the example below,

 

 

 

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