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,