1 kHz low pass filter (CB7579E)

This filter can be implemented with almost any operational. The power suppply must be symmetrical.

Long interval timer (CB9365E)
This circuit was found in a Popular Electronics from the 90s. The counter circuit is not common...
Overvoltage switch with the CA3140 (CB12145E)
In this circuit the CA3140, an operational amplifier, is used as a comparator with the zener diode...
Common Collector Stage (CB4937E)
This is a typical common collector amplifier step configuration. The transistor can be used...