Code practice oscillator

The inexpensive 7404 hex-inverter has enough amplification to handle a wide range of transducers. Closing the key completes the battery circuit and applies four to five volts to the 7404. Bias for the first two inverter amps (Ula and Ulb) comes from the two resistors, Rl and R2, connected between their inputs and outputs. The capacitor and rheostat (R3/C1) close the feedback loop from the input to the properly-phased output. The signal leaving Ulb drives the remaining four inverter amplifiers, Ulc through Ulf; they, in turn, drive the phones or speakers.
Code practice oscillator  - schematic

The volume control potentiometers, R4-R7, may have any value from 1500 ohms to 10,000 ohms. The smaller values work best when speakers, or low impedance phones, are used.

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