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Opto-Isolation / Opto-coupling Between CMOS and TTL
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Optocouplers (also known as opto-isolators) may be used for
interfacing CMOS and TTL gates without physically connecting the output
of the driving gate to the input of the driven gate. Below are
examples of optocoupler circuits for CMOS-CMOS, TTL-TTL, and TTL-CMOS
interfacing.
Figure 1. CMOS-CMOS
Interfacing Using an Optocoupler
Figure 1
shows how an optocoupler
may be used to interface a CMOS gate to another CMOS gate while
isolating the output of the former to the input of the latter. When the input is
low, the output of the input NAND gate is high, and the light-emitting diode does not conduct. With no light
shining on the phototransistor, it is also off, causing the
collector-tapped output (and the output NAND gate's input) to be high.
This causes the circuit's output to be 'low'.
When the input is high, the LED conducts, shining light on the phototransistor.
The transistor turns on and pulls the output NAND's input to ground,
causing the output to go high. Note that R1 is chosen so as not to
overload the output of the input NAND gate when the LED is conducting.
Figure 2. TTL-TTL
Interfacing Using an
Optocoupler
Figure 2
shows
how an optocoupler
may be used to interface a TTL gate to another TTL gate while isolating
the output of the former to the input of the latter. The operation of
this circuit is the same as that of the first example. Thus, when the input is
low, the output of the input inverter is high, and the light-emitting diode does not conduct. With no light
shining on the phototransistor, it is also off, causing the
collector-tapped output (and the output inverter's input) to be high.
This causes the circuit's output to be low.
When the input is high, the LED conducts, shining light on the phototransistor.
The transistor turns on and pulls the output inverter's input to ground,
causing the output to go high. Note that R1 is chosen so as not to
overload the output of the input inverter when the LED is conducting.
Figure 3. TTL-CMOS
Interfacing Using an
Optocoupler
Figure 3
shows
how an optocoupler
may be used to interface a TTL gate to a CMOS gate while isolating the
output of the former to the input of the latter.
The operation of this circuit is the same as the first two examples,
except that the voltage levels on the CMOS (output) side are higher than
the TTL (input) side. This basically shows how an optocoupler may
also be used in level-shifting.
See Also:
Opto-Isolators;
Logic Gates
See
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