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Basic astable multivibrator built from logic gates with inverter output like 74HC04, 74HC01, 74HC02, 74HC04, CD4001, CD4011 etc. Useful as clock generator in circuits without a need for precise timing like simple counter based appliances, as signal source in the buzzer or blinker etc. Output signal is square impulses with 50% duty cycle.
Work principle is based on perpetual gate stage change. As the capacitor charges up, voltage at the junction between the resistor between gates and the capacitor, which is also connected to the input of the left gate decreases until the LOW state voltage threshold is reached. At this point left gate changes state and the output state become HIGH. This causes right gate to also change state as its input has now changed from LOW to HIGH that change gate output state to LOW. Capacitor is now reverse biased and discharges itself through the input of left gate. Capacitor charges up again in the opposite direction determined by the time constant of both resistor and capacitor until it reaches the logic HIGH threshold value of left gate input. This make left gate to change state again and the cycle repeats.
Time constant in logic gate based multivibrator is T = 2.2RC in seconds with the output frequency given as f = 1/T. For circuit in example with R as 45.5 kOm and C as 10 nF (values that are easier to put together from available components IRL) output signal frequency is 1÷(2,2×(45,5×10^3)×(10×10^−9)) = 999.001 Hz ≈ 1 kHz.
FYI: Frequency in signal graph above is not precise due to EveryCircuit shortcomings. Prototype with real components (tried SN74HC04 and CD4011B) generate approx. 1 kHz output signal as expected. Warning! In circuits where output signal is passed to input with low electrical resistance, add another inverter gate or MOSFET based inverter to output (junction between right gate and capacitor). Otherwise multivibrator may change timing or will not work at all.
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