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When the photo transistor detects the emitter light, the voltage at the emitter of photo transistor is 11.5V. But at the collector of pnp it is 12V. How is like this, since high voltage is at the base of pnp it should not work know?
There's high and then there's not high enough. The pnp transistor Vbe will be about 0.7V hence 11.5 is just enough to keep the pnp transistor on and it's collector at 12V. I would check you have enough light on the transistor to switch the npn hard on. What do you get if the LED is off?