Originally the burp sound simulator circuit was built as quick lash up but because it was so popular, especially with children, it is still in everyday use.
Exactly what it does is hard to describe fairly and since it is difficult to explain the effect in words and so many components are used the reader may well be put off.
How the Burp Simulator Works
The results of this circuit can only really be appreciated by listening to final results. The circuit comprises three multivibrators, all working at very different frequencies. They are supplied by a common positive supply rail which is deliberately not decoupled and the inclusion of the resistor Rx in the line enables the multivibrators to be coupled together.
A transistor output transformer and speaker are coupled in the collector circuit of Tr6 and it is from this that the output is heard.
All the multivibrators are working together at the same time and they can all have their frequency of operation altered over a wide range controlled by the settings of VR1, VR2 or VR3. Tr3 and Tr/4 are producing a high frequency, Tr5 and Tr6 are producing a very low frequency which does not repeat for several seconds while Tr1 and Tr2 are producing an intermediate frequency.
What may at first appear to happen is that due to lack of decoupling the combined frequencies would appear at the loudspeaker at rate approximately equal to the rate of the third multivibrator. In fact, the results are much more interesting.
What actually happens is that the stages each have an effect on the others and they synchronize each other in an extraordinary way. Due to the three controls an almost infinite variety of sounds can be produced.
For instance a typical output may consist of 1 short burst of tone followed by a second of silence - this is then followed by a series of clicks followed by a low frequency note, then more silence and so. the patterns often do not repeat themselves for several seconds.
The combinations and possible varieties are quite remarkable and on the prototype some of the sounds have sounded uncannily like that of animals or machines.
At first sight it would seem possible to reproduce these burp sounds again and again by noting the settings and duplicating these lately but unfortunately this is rarely possible. Each tiniest part of a turn on the controls changes the pattern and sequence - often to a considerable extent.
Selecting the Right Components
The resistor Rx should be selected for maximum effect - its value will depend on the characteristics and values of the components used and so it is not possible to give a single best value but 47 ohms is a good one to start with. The supply lines are all joined to one point and the addition of further resistors here will also effect the output - here again their values will depend very much on the components used and once again a value of 47 ohms is a good one to start with.
All the transistors specified are shown as BC109's but for this sort of project the surplus transistors which are sold in 50p packs are ideal. Initially tested transistors can be used to ensure that each individual circuit works and once operation established these can be replaced by surplus types.
Understanding Astable Multivibrator
As a circuit configuration the multivibrator is remarkably tolerant with regard to transistor types. There is no need of course to limit the number of multivibrators to three. Four or more will add yet further effects but three will still give plenty of range. If an amplifier is available it is possible to make a simple change which will increase the range.
If T1 is replaced with a 2.2K ohms resistor, a low level audio output can be taken via a 0.1 FF capacitor from the operational side oi Rx to an amplifier, of course a wire should also join the two chassis lines together.
In this way even more burping effects can be heard. It is understandable that anyone seeing the circuit and reading a rather inadequate description of what will be hear may be hesitant but the pleasure and interest that the prototype has created testifies to the fact that this a thoroughly worthwhile project - it is for amusement only but that should be a recommendation, not a criticism.