This circuit was found in the Linear Applications Handbook of the National Semiconductor of 1994. Some of the components used are not easy to obtain, but equivalents can be used.
This circuit was obtained in an old documentation, but it is still current by the used components....
This circuit makes use of a transconductance operational amplifier. It generates signals with TTL...
This circuit is from a 90s Popular Electronics. The transistors are of general use and the buzzer...