How could I get output of a tapeplayer (from headphones) to logical level 5v DC - 0v DC??
I have done a project where I use an 7w Velleman amp-transistor-relay circuit to turn on lights/whatever with the signal from the tapeplayer FED THROUGH the amp. I don't want to have to mess with a "powerful amp like such because for this new project I dont want speakers, just the signal of the tapeplayer feeding into a shift register data pin as DC. I feel if I use a normal audio amp without a speaker it will overheat.
So, any suggestions on how to get the signal from hphones to dc logic level (without using a audio amplifier)? Maybe diodes and an opamp??

 

The output of a tape player used to be a storage medium. Your problem isn't so much using the signal off the tape as it is recording the control signal.

Not to disappoint, but the tape head produces a very low-level signal. It will still have to be amplified. A standard amplifier tends to have capacitors in the signal path specifically to eliminate DC offsets, though.

Rather than designing an amp that can reproduce DC levels and output a TTL level, you might use the "original" methodology, which was to use tones. One for "0" and another for "1". Then you need a circuit with filters to detect which tone is present, but that is easier to do than designing a custom amplifier.

 

I have never used an opamp but I was hoping I could use an op amp to bump the signal then switch on a transistor with 5v connect to collector load

 

As I suggested, the means by which tape got used for digital data was in using two non-harmonically realted tones - one for logic 0 and the other for logic 1.

The tape signal needs to be passed through an amplifier and then presented to a pair of filters circuits. The outputs indicate if a 0 or 1 is detected off the tape.

Recording will require some designing. You will need two gated oscillators to record the tones on the tape. Think of following in the grand tradition of the Apple II, which used just that kind of tape storage.