This means that at 80deg.c (not unreasonably hot) it's 64 times higher then at room temperature, and it's 256 times higher at 100C. It usually doubles with each 10degree c temperature rise. Leakage current is also highly dependent on temperature. If Q4 has a leakage current that is higher then the turned off Q3, then the difference comes from the output, even though the output is "high", and the extra pullup resistor allows for some leakage current before anything goes trough the optocoupler. In the picture of the totempole TTL output wraper posted, you can see that Q3 is turned off at some point. The IR leds in optocouplers have a lower voltage then LED's in the visual spectrum (Voltage drop = energy gap = photon energy = light frequency) IR LED's typically have a voltage drop around 900mV. Because of this large ratio even a small parasitic current though the LED of the opto coupler results in a relatively large voltage over R2. The problem is made worse by the large ratio betwen R1 (150R) and R2 (10k).
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