How to Design a 555 Circuit
The 555 timer IC is an inexpensive, versatile pulse source in a compact 8-pin package. It covers a timing range from microseconds to hours, and can be configured in astable, one-shot or bistable mode. Its outboard timing network determines pulse duration and duty cycle and consists of two resistors and a capacitor. Designing a 555 circuit is a matter of reading the datasheet, determining the configuration for your application, and calculating values for the timing network. You may need to add other components at the input or output side of the IC.
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Instructions
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1
Look over the 555 datasheet. Note the voltage range for the power supply, the IC’s maximum drive current and the kind of pulses it takes for one-shot and bistable circuits. The datasheet lists a few different formulas for the timing network component values, resistors Ra and Rb, and capacitor C.
The sheet lists the following formula for period:
T = .693 x (Ra + 2Rb) x C
Using T for time in seconds, Ra and Rb for resistance in ohms, and C for capacitance in farads. Duty cycle is calculated by this formula:
D = Rb / (Ra + 2Rb)
Having D is the duty cycle where the on-time is a fraction of the total cycle, and Ra and Rb are values of the two resistors in ohms. -
2
Sketch the 8-pin 555, drawing positive power to pin 8 and ground at pin 1. Also draw a line coming from pin 3, the output, though you’ll leave this dangling for now.
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3
Choose one of the 555’s modes for your application, astable, monostable or bistable. Astable is a free-running pulse oscillator having a cycle period and duty cycle given in the formulas above.
The monostable mode accepts a pulse as an input and produces its own pulse at the output. The 555's pulse duration uses the following formula:
T = 1.1 x Ra x C
Where the terms are similar to those used above. Note that only one resistor, Ra, is used for timing.Bistable mode latches high or low when you trigger the 555's set or reset inputs. No formula is used for timing a bistable circuit, since it depends on the incoming triggers.
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4
Sketch in remaining connections to the 555 based on your choice of mode. Draw the timing network, if needed, and label the resistors and capacitors with values you calculated in earlier steps.
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5
Add external input (pins 2 and 5) and output (pin 3) circuits to your schematic as your application dictates. For example, the incoming trigger pulse might need conditioning. The trigger might come from an electronic pulse or it might be a switch. The 555’s output might be driving an LED, an amplifier or some other external circuit. Continue to refine the schematic until it meets your application’s needs.
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Tips & Warnings
You can purchase inexpensive computer software that will automatically guide you through the 555 design process.
References
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