Signals from analogue sensors can be converted into logic levels for digital systems using interfaces such as transistors, which turn on or off at a single threshold, comparators, which compares the signal to a reference voltage, and Schmitt inverters
A Schmitt inverter gate has two different input switching levels so that the traditional effect of noise at the switching level causing the gate to switch many times is negated, resulting in a clean switch
Schmitt inverters are needed to connect a noisy signal to a counting system so that supernumerary switchings are not counted
Schmitt inverters can be used to debounce mechanical switches by connecting a capacitor across the switch that suppresses the noise, and using a Schmitt inverter to remove it completely
Outputs
Transistors can be used as switches or amplifiers
When VIN < 0.7 V the voltage across the base & emitter is VBE = VIN and the voltage across the collector & emitter is VCE = supply voltage, and when VIN ≥ 0.7 V, VBE = 0.7 V & VCE = 0.7 V
Because VBE saturates at 0.7 V, a current-limiting resistor must be connected to base
Transistors amplify current according to the formula hFE = ICIB, wherein IC is the current going into the collector and IB is the current going into the base
Every transistor has a distinct current gain and collector current, which must be taken into account for circuit calculations, such as calculating the value of the current-limiting resistor