Optocouplers for TTL-CMOS Logic Level Shifting
Here I'll illustrate using optocouplers to perform voltage logic shifting between TTL and CMOS devices. Optocouplers also isolate output transistor voltages from low-voltage digital logic.
See the following related pages:
Fig. 2 PC817 Optocoupler.
An optocoupler is usually an infra-red LED emitter on the input and a photo detector on the output. Here I'm concerned with the most common type with a phototransistor.
Fig. 2 is the PC817 optocoupler. Transistor rating is 80-volts at 30mA. The collect-emitter voltage and current ratings are the main limitation of the device.
Fig. 3 is pin connection for 4N25, 4N26, 4N27, 4N28 optocouplers. The output transistor rating is 70-volts at 50mA.
Functionally it is no different from PC817 other than lower collector current.
When calculating current limit the current to 75% of maximum.
Fig. 4 uses a 4N25 optocoupler to form non-inverting TTL to CMOS logic level shifter.
One should understand the concepts of source and sink.
The switching device will either sink the current, or create a path to ground. The other end of the load if connected to the often positive supply voltage or +Vcc.
In the source configuration the switching device will connect the load to +Vcc.
In Fig. 4 the input is a TTL 5-volt source connection. The output is also a source switching setup.
A HIGH or 5-volts on the input produces a HIGH 15-volts on the output. This is non-inverting - HIGH in is HIGH out. We merely shifted the voltage level.
Fig. 5 uses a 4N25 optocoupler to form non-inverting CMOS to TTL logic level shifter.
Here we have a source input from a 15-volt CMOS circuit to source 5-volt TTL circuit. Again this is non-inverting - HIGH in is HIGH out.
Input is source, output is source.
Fig. 6 uses a 4N25 optocoupler to form non-inverting CMOS to TTL logic level shifter.
We have a sink CMOS input that when LOW turns on the LED. The output is 5-volt TTL goes LOW when the transistor is turned in. This is non-inverting - LOW input is a LOW output.
Input is sink, output is sink.
Fig. 7 uses a 4N25 optocoupler to form inverting TTL to CMOS logic level shifter.
Fig 7 is identical to Fig. 6 other than output circuit we changed the resistor and phototransistor. The logic level is inverted - LOW input produces a HIGH output.
The input is sink, output is source.
Fig. 8 uses a 4N25 optocoupler to form inverting TTL to CMOS logic level shifter.
Input is source and output is sink.
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