Timer IC 555 Tool
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You can calculate time for monostable mode, frequency and duty cycle for astable mode. The advantage of this app is you can decide if you start with capacitor, resistor or frequency. You can learn about block diagram, functions, operations mode and a wiring option on protoboard.
Also, the tool has a amazing feature to plot the behavior between a range selected.
Additional that is possible to share or save your calculations in your device.