Ramsey Electronics PG13 Manual de instrucciones descargar pdf (Pagina 5)

ECS1• 5

sounds like a cricket chirp. The circuit responds to temperature by way of a

thermistor, a resistor whose value changes with temperature. The thermistor

is used as a timing resistor in U1. When the temperature changes, U1’s

output frequency changes and so does the cricket chirp rate. In order to keep

the circuit simple and inexpensive, some compromises had to be made. The

main one is that the circuit is composed of elements that produce square

waves, not sine waves. For audio guys, this may cause foaming at the

mouth, but you can get some square waves to sound almost the same as a

sine wave if you tweak everything right. To try to match the cricket chirp

waveforms as closely as possible, three 555 timers are used. They are

chained together in such a way that they modulate each other. The output of

the first modulates the second, and the output of the second modulates the

third. Look upon the picture above. Keep in mind that this diagram just shows

the “relative” frequencies generated by the three timers so you can see how

the different waveforms look. In reality the output of U3 is much higher

frequency, but if drawn that way would be too hard to see on this picture. As

you can see the pulses from each 555 build up the final waveform. You will

notice that the final waveform from U3 has groups of 4 pulses not three. This

goes back to what was said about tweaking a circuit that produces square

waves so that it sounds like the original sine wave pattern. By listening to the

sound the circuit made as I adjusted the timings of the 555 timers, I found

that these timings worked best. The circuit has a pot that will let you change

the frequency of U3 so you can tweak the pitch of the cricket sound to your

liking. The last stage in the circuit is an output transistor that drives the

speaker.

U1 Out

U2 Out

U3 Out

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Ramsey Electronics PG13 Manual de instrucciones Pagina 5