I'm working on a 40kHz ultrasonic range finder and I'm having problems with noise in the circuit. The transmitting circuit uses LM555 timers to produce a 40kHz pulse and the receiver circuit uses two LM741 op amps to amplify the return signal. The amplified signal is then rectified using two NTE585 diodes, and passed through a LM339 comparator. The only problem is when the signal gets amplified it also amplifies noise in the form of the 40kHz pulse, and the noise gets amplified greater than the actual return signal so the comparator is getting the noise rather than the signal. All the chips and the 9v DC source are decoupled using ceramic .1uF caps. Any suggestions to get rid of this noise? Thanks for the help!

 

I have not used 741 for many many years now. But are you sure it can be used in single supply mode. I would have checked it out. You should perhaps also use a OPAMP with a larger bandwidth
Can you also post your schematic?

 

To6afre is correct, 741's are only meant to work with +/- 15 volt supplies.