Make: Volume 93

PROJECTS: Electronics Fun & Fundamentals

To receive the signal, you use a phototransistor,

which has a very high effective resistance that

goes low in response to light. You can wire it in

an open collector circuit, as on the left side of

Figure

B

, or in an emitter follower circuit, on

the right. Partly because the emitter-follower

option creates some risk of overloading the

phototransistor if the load has a low resistance,

the open-collector version is more common, and

I will be using it in this project. A typical resistor

value is 10K, but you can adjust this up and down

to find the widest range of voltage values across

the load when you ...