Hi My name is peter I am a Senor design engineer and have a solution for some of you !
You have a 5 v device that is sending some 5V TTL Level out but the PI accepts only 3.3V you think you need a level shifter because everyone is talking about them dont you ?
A Solution out of the profeshional cook book take a 3.3v Zenner diode and connect it to gnd the other end you connect to a 6.8Kilo ohm resistor , the other end of which you connect to your 5 v output from your 5v device , now go to the summing point between the zenner and the resistor if this has changed to 3.3v congratulations you have made a level shifter with 2 components. if something is wrong change the direction of your zenner cathode ring up to summing point , this method is perfect for very fast signal change and most problems on pi forums concern inputs to the pi not outputs or ?
P.s as an old fart I think the guys who developed the pi are Supercalafristicexpialerdocious !!keep up the good work