- Jan 1, 1970
I have some questions about my first basic bandpass filter for an
ultrasonic receiver circuit. It's a Mupltiple feedback second order
bandpass II with Q=13.97, f0=40kHz, Apb=0.5dB, Bpb=1kHz and a gain of
32. All resistors are 1% with 2% ceramic caps. As long as R7 is exact
then the filters gain/frequency graph remains O.K. with the other
resistors as standard values.
I have put a schematic on the net at:
Should I have the bandpass filter on both parts of the dual opamp? It
seems logical that with the filter on the first half the second half
wouldn't need one.
Have I connected it correctly to the second half - C4 and Pin#5?
In addition a problem I also have is without any load I can adjust the
trimmer TR1 down to 0.3V and still keep the comparators output high.
The comparator is attached to the trigger pin of a 555 astable timer
circuit which switches a load. As soon as I attach a load to the 555
things change. If for example I attach a 3V motor or even an LED via a
transistor switch then I have to readjust the trimmer or the load
won't turn off. The trimmer has to be adjusted to about 3.5V out for
the motor and 1.4V out for the LED. This reduces the sensitivity of
the receiver. My next step after the filter is to look into a stable
voltage reference for the LM393 but will that solve this particular
Also is it wise to use a pullup resistor on the comparators output?
Dunno about the filter stuff but most of the comparators that "Sink" their
output will need a pullup resistor if you need a :high".