Schematics By The Book is a series highlighting educational and practice circuits from our favorite electronics books. Each post contains a link to the book, a copy of the schematic, and a full current BOM of through-hole parts you can use for prototyping.
Sometimes you want to scare your friends. Sometimes you want to announce your car. Sometimes you just want to make noise.
This schematic and associated components is a simple way to get started making an adjustable siren. SW1 enables current flow down through the R1/R2 divider. This equivalent “resistor” pairs with C1, setting up a slowly ramping signal. That signal is then passed through R3+RV1 and paired with one of three capacitors (C2, C3 or C4) using the 3 way switch. This then determines the frequency which pulls down on the base of Q1 (an NPN transistor) which then activates Q2, turning that switch on and off. The resulting current that flows through the collector and out of the emitter of Q2 creates a slightly higher power signal which pulls the speaker in different directions. The speaker then moves and pushes air back and forth…making sound!
Publication: Electronic Formulas, Symbols & Circuits
Author: Forrest Mims
Circuit: Adjustable Siren
KiCad Schematic File:Forrest Mims Adjustable Siren Project File
BOM CSV: Forrest Mims Adjustable Siren BOM CSV
(Click schematic to enlarge)