Audio Circuits

This group of projects lists some siren circuits along with some information about simple audio amplifier chips.

These are several circuits which create either a tone or siren type sound. They create the sounds using three different techniques.

This first siren circuit uses 555 timers which were used in some of the simple DC-DC voltage converters. Here is the circuit diagram:

The first 555 timer just generates a 1.25KHz square wave output. This then goes through the pot and diode into the second 555 timer. This fluctuating signal goes into the comparator pin on the second 555 timer. The comparator pin is usually held at 2/3Vcc internally so depedning on how the 10k pot is adjusted the voltage at this pin will differ thus causing a high or lower frequency oscillation based on the input and also on the voltage in the capacitor. Removing the capacitor from pin 5 makes a 2 tone instead of multitone output. IF there is no input into the second then a constant 781Hz output results. The transistor on the ouput serves as a way to provide the power needed to the large speaker. However, in my case the speaker could not handle much power and instead a 5k pot was placed inline with the speaker and connected directly to the output of the second 555 timer. It produced a very nice siren effect with adjustablilty through the pot.

This siren circuit is actually one from an old radioshack electronics kit. It was poorly built years ago and was rebuilt and investigated now.

This circuit is powered from 3 AAA batteries. I do not know the workings of the circuit at all but here is the output of the circuit which the speaker sees.

This circuit is called a light therevin because it changes frequency based on detected light. A normal therevin is a musical instrument which detects changes in capacitance and thus changes the output frequency and can be used to make songs. This one just uses light instead for the detection mechanism.

This circuit works by creating an oscillator through the two transistors and the capacitor. There is some feedback but I am not sure of the operating principles of oscillators. However, the frequency is controlled by the cds or photocell. The cds (cadmium sulfide) cell is a resistor that varies with light. The more intense the light the more carries that are created due to electron excitement in the material. As the light intensity increases the resistance decreases and the oscillation frequency increases. A sufficiently small cell had to be found so that it would have high enough resistance. If the resistance wasn't high enough then the tone generated wouldn't be audible in normal lighting conditions.

Several simple amplifiers from National Semiconductor were looked at and investigated for their operation. They were the LM380, 384, and 386.

The LM386 has several versions such as the -1 and -4. They are made to work with different voltage ranges and provide different driving power. Details about these can be found in the datasheet. One nice thing about this chip is the lower voltage and power requirements. It is possible to power this chip from 5v, though it doesn't give the loudest or best quality. 9v and higher give a rather good sound quality and intensity, thus it can easily be used for a low power portable audio amp as was suggested.

The circuit found which originally sparked interest in the circuit is:

It turns out that this is actually the same circuit as was found in the datasheet: LM386

This chip was later used to actually make a simple audio amp to power some old computer speakers who amplifier recently died. [picture of the box]

This chip works much like the LM386 except for the power rating and it needs slightly different external components. This chip can provide up to 5W of power at the specified input voltage. The problem with this however is that the chip requires a heat sink when it is being used to provide this much power. Several configurations can be found in the datasheet:LM384

This chip was not able to be successfully tested. As it seems to have a similiar structure to the LM384 it was tested using the same circuit, however no output resulted. The datasheet suggests using two of them to create a bridge amplifier which was not attempted. datasheet:LM380