Photoelectric isolation circuit between TTL circuit and relay circuit

In this circuit, using a photoelectric coupler can reach 100G? isolation between TTL and relay circuit, it avoids relay noise and peak voltage influencing TTL circuit. When the TTL input singal is high, Q1 turns on, there is no current in LED, and there is no light incident on detecor, photoelectric diode is in 5G? maximum impedance value. So Q2 is nonc..

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