For some time, hobbyists have relied on dedicated sound generator ICs, such as the UM3561, UM3562 to build compact alarm and siren circuits. Unfortunately, this device is becoming increasingly difficult to find in the market. The manufacturer of the chip, United Microelectronics Corporation (UMC), is still around. However, they are no longer producing the UM3561 or UM3562. As part of a fun experiment, I decided to design a replacement circuit using only discrete transistors and passive components. This avoids the need for specialized ICs altogether. The result is a simple yet effective transistor-based siren. It can reproduce the familiar rising and falling alarm effects typically associated with police cars, fire trucks, or ambulances.

The design philosophy behind this project was straightforward: recreate the functionality of the classic two-timer based siren circuit (NE556/NE555 pair), but instead of timers or sound generator ICs, use only transistors in a multivibrator and modulation arrangement. By carefully selecting resistor and capacitor values, we were able to replicate the frequency modulation effect that gives sirens their distinctive sweeping tone.

The circuit of this siren can be divided into three functional sections: the LF oscillator section, built around Q1 (BC548) and Q4 (BC558), which generates a slow modulation waveform. The rate of this oscillator, which controls how fast the siren "sweeps" up and down, can be adjusted via the 500k trim-pot (RV1). Capacitors C2 and C6, along with resistors R7 and R8, shape this low-frequency oscillation.

The next most crucial section is the audio frequency oscillator. Built around Q3 (BC558), this section is responsible for generating the actual audio tone. The output frequency is determined by the capacitor selected through jumper J1. By switching between C1, C3, and C5, different frequency ranges can be chosen, producing distinct siren effects. This is conceptually similar to selecting different alarm sound profiles on the UM3561.

The final section amplifies the modulated audio signal and drives a small speaker (16Ω - 32Ω). Diodes D1 and D2 provide bias stabilization for the push-pull transistor stage. This improves linearity and reduces distortion. The coupling capacitor C4 ensures that DC is blocked while allowing the audio signal to pass into the speaker.

This circuit is designed to operate from a 6V to 12V DC power source. The recommended supply voltage is 9V. The output frequency of this siren is heavily dependent on the supply voltage. Therefore, to produce stable output, this circuit needs a regulated "clean" DC power source.

To make the project more accessible, we designed a dedicated PCB for this circuit. The board is compact and uses commonly available through-hole type components. For convenience, standard 2.54mm pin headers are provided for the speaker, power input, and jumper settings.

To complement this project, we also created a short video showing the assembly process and testing of the siren. The demonstration highlights how the different jumper settings produce distinct alarm effects, and how adjusting the rate control potentiometer changes the siren’s sweep speed.

All the resources of this project, including KiCAD design files and schematics, are available to download at https://github.com/dilshan/simple-electronic-siren.