This circuit uses a photo-transistor to trigger a circuit sourcing a square signal for the output. The trigger point is adjusted by the trimpot. The NPN transistors can be the BC548 or other general purpose NPN type.
![](/images/sampledata/cb/cb864_0001.png)
This circuit uses a photo-transistor to trigger a circuit sourcing a square signal for the output. The trigger point is adjusted by the trimpot. The NPN transistors can be the BC548 or other general purpose NPN type.