# Photo transistor circuit help

Hi,
I have given my circuit here.

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?

Thanks,

As far as I can see (your drawing is too low resolution to make anything much out of ) the circuit looks like this (I don’t know the resistor values, so left them at the default 220 ohms.) In this circuit the emitter of the optocoupler output transistor will indeed be around 11.2V and the collector of the pnp will be 12V when the phototransistor is off as it is in this drawing (there is no current in the optoisolator LED to provide light!)

The reason is that the base current in of the pnp transistor is clamped at about 0.6v ( 1 diode drop) and the pnp transistor as a result is turned on and its collector is therefore at 12V. If it does not go off when power is applied to the diode on the left of the optoisolator, that will be because the optotranslstor in the optoisolator has not turned on hard enough to reduce the collector-emitter voltage of the optoisolator output transistor to less then .6V which would then turn off the pnp transistor and cause the pnp collector voltage to go to ground as the pnp shuts off. There are two possible causes of this: there isn’t enough light shining on the phototransistor turn it on sufficiently to drive the pnp transistor’s base to less than 11.2V or that the value of resistor R1 is too low and the photo transistor can’t sink sufficient current to bring the base of the pnp transistor above 11.2V and cause the pnp transistor to shut off. Easiest solution would be increase the value or R1 until the voltage at the pnp collector drops to about 11V then reduce the R1 value by 10% or so so the pnp transistor is fully on with no light. Now the photo transistor needs the minimum amount of light to make the switch and you need to drive the diode on the optoisolator (by reducing the value of the resistor limiting its current) until the photo transistor can switch the pnp transistor.

Peter