[sdiy] 555 Timer VCO problem

[Jimmy Gogas]

Hey guys,

I just recently finished building a Littleman (found at 
http://www.itn.liu.se/~nikro/diy/littleman/littleman.pdf). After a couple of 
days of testing and debugging, I got every module to work (seperately).When 
time came to join everything together, everything was working for a split 
second, until the VCO went down. I lost complete signal from it.

I spent yesterday trying to figure out why, but I couldn't get it
working.I even tried rebuilding it on another board, but it still didn't 
work. The problem that I think, is that the 555 timer is not trigerring 
itself, as it is supposed to. In that configuration (pin2 connected to 
pin6), it should start oscillating right away when the power is turned on. I 
tested the 555 separately, following the guidelines for astable operation, 
and guess what, it worked perfectly. So the problem is not a defective 
timer. I replaced the op amps, but this didn't help either. I found that 
when the power is turned on, there is a positive spike followed by a 
negative spike really quickly. I think the problem is the circuit 
configuration, it's not stable.

I was wondering if anyone had any tips I should follow. The circuit is good 
when it works (and it has already worked... three days ago it was working 
perfectly), but recently it hasn't been working for me.

Is there a way anyone could give me an clear explanation about each part of 
the circuit. For instance, what each op-amp does, what is actually happening 
at the 555 timer (how this waveform is actually being produced), what each 
capacitor does. I think this would be the best solution to solve this 
problem. By understanding the circuit better, I can troubleshoot it better.

Thank you in advance.




_________________________________________________________________
Send and receive Hotmail on your mobile device: http://mobile.msn.com