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Electronics Components: Control the Time Intervals in an Astable 555 Circuit

An astable 555 timer circuit in an electronic project works like a metronome: It keeps running until you turn it off. This mode is also called oscillator mode, because it uses the 555 as an oscillator, which creates a square wave signal. There are three important time measurements for a square wave:

  • T: The total duration of the wave, measure from the start of one high pulse to the start of the next high pulse.

  • Thigh: The length of the high portion of the cycle.

  • Tlow: The length of the low portion of the cycle.

Naturally, the total time T is the sum of Thigh and Tlow.

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The values of these time constants depend on the values for the two resistors (R1 and R2) and the C1.

Here are the formulas for calculating each of these time constants:

T = 0.7 (R1 + 2R2) C1
Thigh = 0.7 (R1 + R2) C1
Tlow = 0.7 R2 C1

There’s an interesting fact buried in these calculations that you need to be aware of: C1 charges through both R1 and R2, but it discharges only through R2. That’s why you must add the two resistor values for the Thigh calculation, but you use only R2 for the Tlow calculation. It's also why you must double R2 but not R1 for the total time (T) calculation.

Now, plug in some real numbers to see how the equations work out. Suppose both resistors are 100 kΩ and the capacitor is 10 μF. Then, the total length of the cycle is calculated like this:

T = 0.7 (100,000 Ω + 2 100,000 Ω) 0.00001 F
T = 2.1 s
Thigh = 0.7 (100,000 Ω + 100,000 Ω) 0.00001 F
Thigh = 1.4 s
Tlow = 0.7 100,000 Ω 0.00001 Ω
Tlow = 0.7 s

Thus, the total cycle time will be 2.1 s, with the output high for 1.4 s and low for 0.7 s.

If you want, you can also calculate the frequency of the output signal by dividing the total cycle time into 1. So, for the above calculations, the frequency is 0.47619 Hz.

If you use smaller resistor and capacitor values, you’ll get shorter pulses and higher output frequencies. For example, if you use 1 kΩ resistors and a 0.1 μF capacitor, the output signal will be 48 kHz, and each cycle will last just a few millionths of a second.

You may have also noticed that if the two resistors are the same value, the signal will be high for twice as long as it's off. By using different resistor values, you can vary the difference between the high and low portions of the signal.

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