We found this circuit in an old documentation, but it is still current. General purpose transistors can be used, and the SCR can be the TIC106. The bell is a transducer with an oscillator (buzzer).
This ultra-low noise amplifier and simple power supply was found in Analog Devices documentation from the...
This interesting bandpass filter using an operational amplifier 741 has its cutoff frequency...
Found in a 1968 EEE, this circuit uses transistors from that time. The output is 300 ns....