Schematics by the Book – Adjustable Siren

Schematics by the Book – Adjustable Siren

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
Page: 123
KiCad Schematic File:Forrest Mims Adjustable Siren Project File
BOM CSV: Forrest Mims Adjustable Siren BOM CSV

Forrest Mims Adjustable Siren

(Click schematic to enlarge)

Schematic Ref Description MFN MFP Part Link
R1 22K Resistor SEI Stackpole CF14JT22K0
R2 39K Resistor SEI Stackpole CF14JT39K0
R3 15K Resistor SEI Stackpole CF14JT15K0
RV1 50K Potentiometer Bourns PTD902-2015K-A503
SW1 Momentary Switch TE Connectivity 1825910-6
SW2 SP3T Switch C&K R10307RN02Q
C1 22uF Vishay MAL203851229E3
C2 22nF Vishay S223M75Z5UN83J0R
C3 47nF Vishay D473Z39Z5VF6TJ5R
C4 100nF Murata RHEL81H104K1K1A03B
Q1 2N2222 NPN Transistor Infineon 2N2222
Q2 2N2907 PNP Transistor On Semi P2N2907AG
SP1 8Ω Speaker CUI CVS-1508

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