Numerical control tone generator circuit based on 555

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As shown in the figure, the circuit consists of ICl (555), Rl, R2 and eight-bit data latches, inverters, and Cl-C8 to form a variable time constant oscillator, which constitutes a digital signal controlled tone generator.
lC2 uses the open collector gate 74LS05 as the inverter. IC3 uses an 8-bit data latch. The 8-bit data cable determines which capacitor in Cl-C8 is grounded to change the charging and discharging time constants, thus changing the oscillation frequency.
The formula of the oscillation period is T=0.693 (R1+2R2) (DoC1+D1C2+...+D7C8).
The reset terminal 4 of the ICl can draw a high level signal from the microcomputer to perform gate control, such as the 8085 system, which can be connected to the SOD terminal.