Siren LFO Synthesiser Instructions

Before diving into the exciting world that is the LFO Siren Kit, it is a good idea to get the VCO kit and look through the instructions which can be found here, as this will explain how VCOs work (we will not be covering that here).

There are several different parts that make the Siren LFO work, but the four most important are the first VCO (acting as an LFO), the adjustment potentiometers, the second tone VCO, and the selection switches.

Starting with the first and most important part, the first VCO (made from U1A and U1B), is the exact same circuit as that found in the VCO kit, with the exception that the input voltage to the VCO is controlled by the potentiometer RV1 instead of an external voltage. This potentiometer is used to change the output frequency of the VCO, but because of the large size of C1, the frequency of oscillation is very slow (all the way from 0Hz to 1kHz), thus making it an LFO. A second potentiometer, RV2, is used to control the output voltage range of the LFO which effectively increases / reduces the amplitude of the output.

Once the LF waveform has been produced, both the triangular and square wave from the oscillator is fed into a selector switch, so you can decide between a rise and fall in output tone, or a sudden switch between two tones. This signal is then fed into a unity gain buffer (U1C), and this output is then fed into a potentiometer RV3, which determines the amplitude of the final output from the whole LFO stage.

This varied amplitude signal from the LFO stage is then combined with the output from another potentiometer RV5. This potentiometer is used to add an offset to the LFO output, so its centre oscillation frequency can be varied. This offset is also key to setting the main tone for the second VCO, meaning that if the first stage LFO was turned off, the offset potentiometer would force the second VCO to produce a specific tone (adjusting the potentiometer will change this tone).

The LFO and offset are added together using the summation amplifier U1D, and the output of this summation stage is then buffered with a unity gain buffer U2A.  By using the buffer, we ensure that the second VCO stage doesn’t affect the performance of our LFO circuit.

The final stage that is used to generate tones is the second VCO, made up of U2A and U2B. Because of the much smaller capacitor (C2), this stage has a much higher frequency, allowing for most audible tones. The input to this VCO is the combination of all the previous stages, meaning that making changes to any of the potentiometers will change the tone,

The final “final” stage is a simple push pull amplifier (Q3 and Q4) which amplifies the output of the VCO so that it can drive a small external speaker.