We found this code key or coded keyboard in a publication from the 90s. The circuit uses components that are still common today and is powered by a 12 V voltage according to the relay used.
This circuit has a frequency range of 30 Hz to 100 kHz with an output signal of 1 Vrms. We found...
In the figure we have a second order filter with very high input impedance, using an operational...
Using the HA2400 this circuit can generate signals of a few megahertz depending on R and C. The...