This part of the bat detector is used to filter and amplify the relatively weak signal from the detector in order to send it to a speaker or to headphones. The filter has the purpose of removing all unwanted high-frequency noise and distortion products.
The filter can be implemented by a cascade of simple RC circuits. The amplification is easily implemented with a LM386 (pdf) audio amplifier.

Another part that I like to try out once, is the TDA7052 from Philips (DIP8). It uses the principle of two bridged amplifiers. That has the advantage that the speaker can be directly coupled to the IC, without any fat coupling capacitors. Also more power can be delivered to the speaker. The quiescent current is the same as for the LM386: 4 mA.

LM386 amplifier

end amplifier with LM386 This schematic shows the basic layout of an end-amplifier. I used something like this in my frequency division detector. First the signal is attenuated by the voltage divider consisting of R2 and logarithmic potmeter R1, to set the volume. Then it is fed into the LM386 by a capacitor of 10 uF. The value of the capacitor is not very critical, 1 uF probably works too. The LM386 has a gain from 20 (with no external components) to 200 (with a 1 uF capacitor connected between pins 1 and 8). In this case 20 is set. The signal comes out again through a capacitor and is now strong enough to drive a small speaker or a set of headphones.
This page was last Monday, June 26, 2000