This circuit was found in a 90s Wels' Think Tank. The frequency depends on C2 and R3. The handset or speaker must be at least 80-ohm. Observe the low supply voltage.
The integrated circuit used in this old configuration is not common today. In it we have the way...
In the figure we have a diagram of a thermostat using an NTC and an operational amplifier 741. The...
For those who work with reproduction or development of photographs or simply like to cook their egg...