The circuit in this article is a knock detector that also detects other sounds such as talking and tapping. The talking detection does not work well because this circuit is very simple.

The circuit shown is a simple DC (direct current) amplifier. It amplifies the signal from the microphone with the use of three transistors.

Supplies

Microphone, general purpose BJTs (Bipolar Junction Transistors) (NPN and PNP), 1 kohm resistors - 5 (you might need spare), 100 kohm resistors - 3, 10 kohm resistors - 2, bright LEDs - 2, 1 Megohm potentiometer - 1, 9 V battery harness, 9 V battery, matrix board (10 cm by 10 cm), 1 mm metal wire, insulated wire - 1 meter.

Step 1: Design the Circuit

The circuit is designed with two NPN transistors and one PNP transistor. It is a simple transistor DC amplifier. The potentiometer is used to control the brightness of the LED.

Rc1 is used to bias Q2 when Q1 is OFF and Rc2 is used to bias Q3 when Q2 is OFF. Without Rc1 and Rc2 the circuit might not work.

Critics might argue that Q1 and Q2 are not biased in the linear region during circuit operation. A more complicated circuit (such as AC (Alternating Current) transistor amplifiers) would work better. However, that circuit will require the use of capacitors.

The circuit was drawn in PSpice simulation software student edition version 9.1. The LED is modeled with three general-purpose diodes because this old software does not have an LED component.