1. Used as Solid-State Relays
Using optocouplers as solid-state relays offers advantages such as small size, close coupling, low drive power, fast operation, and wide operating temperature range. Figure 3 shows an actual circuit diagram of an optocoupler used as a solid-state relay. The left half of the circuit converts the input electrical signal Vi into an optical signal emitted by the LED within the optocoupler; the right half then uses a phototransistor within the optocoupler to convert the optical signal back into an electrical signal. Therefore, it is an excellent electro-optical and opto-photoelectric combined conversion device. The optocoupler has a current transfer ratio of 20%, a withstand voltage of 150V, and a drive current between 8 and 20mA. In practical applications, because it lacks the physical contacts common in electromagnetic relays, it avoids poor contact and arcing, and is not prone to malfunction due to external forces or mechanical impacts. Therefore, its performance is relatively reliable and its operation is very stable.

2. Application of Optocouplers in Telephone Security Devices
To prevent unauthorized use of telephone lines or telephones, a simple and practical telephone security circuit can be designed using optocouplers. This circuit consists of a polarity conversion circuit composed of VD1 to VD4. Since it is not necessary to distinguish the polarity of the telephone line feedback voltage when connecting this security device to the telephone line, its use greatly simplifies installation.

3. Replacing Audio Transformers with Optocouplers
In linear circuits, audio transformers are commonly used for coupling between two amplifier stages. The disadvantage of this coupling is that some power is lost in the transformer's iron core, and it may cause some distortion. However, using optocouplers to replace audio transformers can overcome these disadvantages. When the input signal Vi is amplified by transistors BG1 and BG2, it drives the LED on the left side of the optocoupler to emit light. The light is then completely absorbed by the phototube on the right side and converted into an electrical signal. This signal is amplified by the subsequent circuit BG3, and a distortion-free amplified signal V0 is output from the emitter of the phototube through capacitor C3. Because this circuit completely isolates the two amplifier stages, it eliminates interference that could be caused by ground loops. Furthermore, its noise reduction function prevents signal distortion. The overall gain of the circuit is expected to exceed 20dB, with a bandwidth of approximately 120kHz.