Door buzzer or alarm (CB20492E)

This solar powered circuit was found in a 1991 documentation. The transistor is a general purpose NPN.

IR remote control emitter (CB7020E)
This circuit was found in 1980's Radio Electronics. The circuit produces a 1500 Hz modulate 35 kHz....
NAB equalization preamplifier (CB4948E)
We found this preamplifier for recording heads in a 1980s documentation. The circuit can be used...
Voltage Variator LM3900 (CB5057E)
In the figure we have a circuit that allows us to obtain a variable voltage of approximately 0.5 to 25 V...