I need to get an LED shining brightly when a photo-diode (in the local store it is sold as such, but of course it could be something else, as suggested in the answer of WhatRoughBeast) receives some light. My idea with an operational amplifier proofed to be a little bit complicated (Opamp constant on). I tried to use the phototransistor (there is only one type at the local store, two legged with internal basis), but its resistance (in conducting state) was too high. (The LED lit, but "shining brightly" is different from that.)
I had the idea that a transistor should be able to manage the task. What properties should the transistor "Q1" have (and why those should it be those properties)?
R1: 95 Ohm, LED D3: 3.1V, D2: has R=1852 Ohm when illuminated (and about 400000 Ohm when nearly not illuminated).
Best Answer
I ended up using the following circuit instead:
There is a transistor inside of the LM393 comparator, even more than one, but it is not necessary to care for their properties.