Maker Pro
Maker Pro

Another Bat Detector

Raven Luni

Oct 15, 2011
798
Joined
Oct 15, 2011
Messages
798
New design, not going to do a full project log for this one - just wanted some quick feedback on this new design. For the unfamiliar, the basic idea of this circuit is to take the vocalisations of bats (usually around 40-60 kHz for the more common species) and produce an audible representation by feeding the zero crossings to a counter / divider.

I'm hoping to keep background noise to a minimum by with the following measures:
- Pre-amp gain control
- Adjustable hysteresis in the compatator
- High pass filter (second order, calculated for around 20 kHz with a gain of 5)

Unfortunately I have no way to test these except in the field (I've had some success with earlier versions but the designs were still pretty crappy, had bad calculations etc).

I'm also not sure about the output volume control but couldnt think of any other way to do that.

bdfd3.gif
 

duke37

Jan 9, 2011
5,364
Joined
Jan 9, 2011
Messages
5,364
That looks reasonable except that Q3 has no current flowing through it.
 

Arouse1973

Adam
Dec 18, 2013
5,178
Joined
Dec 18, 2013
Messages
5,178
I think the output stage needs a bit of work. The counter is going to output digital and your caps are going to block this so all you will get is a nice under damped ringing after the leading edge of the output of the counter signal. You need to remove the cap on the output, and what is D1 for?

Thanks
Adam
 

duke37

Jan 9, 2011
5,364
Joined
Jan 9, 2011
Messages
5,364
C9 blocks any current through the emitter of Q3, I would take this out.
D1 will limit the bottom end of the potentiometer to 0.6V. I see no reason for this. You are not making a hi-fi amp, just blasting out pulses.

I would put a capacitor before the volume control so that when there is no sound, Q3 is off, this would reduce battery drain.
 

Raven Luni

Oct 15, 2011
798
Joined
Oct 15, 2011
Messages
798
The idea of C9 was to save power. During any period of inactivity the output of the 161 is going to be in a constant high or low state. If it's high then the battery is going to be dead in well under 2 hours (I read somewhere that the typical amp hour rating of a 9v is about 400mAh).

Constant current through Q3 is definitely NOT wanted (then again the capacitor would also need a way to discharge).

The diode seemed like a mad idea - my reasoning for that was because I wanted a volume control, but placing a pot at the speaker would limit current to the point of it being useless, so the only other place to put it woudl be on the base of the transistor. The diode was intended to prevent a 'useless zone' where the pot voltage is below vbe.
 

Arouse1973

Adam
Dec 18, 2013
5,178
Joined
Dec 18, 2013
Messages
5,178
Because of the low impedance on the output, when you drive it with a square wave the only part that you will see is the fast rising edge of the pulse and when this stops, the inductor will quickly pull the out put to 0V and as the magnetic field collapses you will have a fly back voltage that bounces back and forth until it dies this is the difference between the neper frequency (R/2L) and the resonant frequency of the speaker 1/sqrt*LC, showing an under damped response. Only by increasing the outputs time constant will you see this square wave suddenly appear. If it were a sine wave then that's a different matter.
Adam
 

Raven Luni

Oct 15, 2011
798
Joined
Oct 15, 2011
Messages
798
To hell with it - just feeding the base of Q3 with a variable resistor (10K inverse log) - dont care if it doesnt go to 0 because its not exactly high tech nor does it require high quality (and I need to stop getting hung up on stupid details like that).

And the capacitor is coming out - been testing how that works with a 555 tone setup - just kills the volume.
 
Top