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Dub siren – Intermediate/Difficult

 

Building the circuit:

 

 

 

To build this circuit you will need:

 

Ω      Two 555 timers

Ω      One LM386 Low Voltage Audio Amplifier

Ω      One LM741 op-amp

Ω      Three 10K resistors

Ω      Two 560R resistors

Ω      Five 47K (or 50K)potentiometers

Ω      Two 2.2K resistors

Ω      One headphone jack

Ω      One 220uF capacitor

Ω      One 47uF capacitor

Ω      One 100nF capacitor

Ω      One 10uF capacitor

Ω      One 47nF capacitor

Ω      Some stripboard

Ω      Some hook-up wire

Ω      One SPDT switch

Ω      One push switch


Circuit Diagram:

Dub Siren Circuit Diagram

Overview/How it works:

This circuit works using a technique called modulation. This is where you take one signal, and use this to change another signal. In this case this is the signal from one oscillator which is being used to control the pitch of a sound. When the first oscillator is outputting a low level, the sound is lower than when it is outputting a high level. This creates two tones that repeat over and over again, and simply by changing the frequency of the oscillator and the duty of the oscillator, we can control when the tone changes. We can also change the pitch of the note to change the overall pitch of both tones, and the amount we modulate it by (which in our case corresponds to the smoothness of transition between the notes).

We do this by using the output from one 555 timer (the leftmost one) to modulate the second 555 timer (the rightmost one). The frequency, duty cycle, and smoothness of modulation are all controlled by the three potentiometers sitting closest to this 555 timer. The output from this is fed through an LM741 which is an operational amplifier. The 741 is hooked up with a negative feedback loop which keeps the voltage it is supplying exactly the same as the voltage it is getting in. Now you may say, if the voltage is the same, why use an op amp at all? Why not feed the signal directly into the second 555 timer? And the answer is because the op-amp does not draw very much current into its inputs (we call this a high impedance input), and by doing this we avoid affecting the first 555 timer with the second. Instead the signal is replicated with current and voltage from our supply which can be drawn into the second 555 timer without having any effect on the first.

The second 555 timer is the one generating the tone that we hear. This tone is changed by the first 555 timer repeatedly hitting the reset pin of the second (causing the waves to be further or closer together and so changing the note). The output of all of that is then fed, through a volume control and a momentary on/permanent on switch, to a low voltage audio amplifier. This amplifies the signal by 20x so we can hear it and feeds it out to a headphone jack.

Images:

Below are the stripoard layouts for the design we used, but feel free to make a different one if you feel like doing that instead!

Strip board layout of the Dub Siren

Strip board layout of the Dub Siren

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