Editing 555 timer IC (section)

==== Shorter duty cycle ====
To create an output high time shorter than the low time (i.e., a [[duty cycle]] less than 50%) a fast diode (i.e. [[1N4148 signal diode]]) can be placed in parallel with R<sub>2</sub>, with the cathode on the capacitor side.<ref name="Signetics_1973_555-556_Databook"/> This bypasses R<sub>2</sub> during the high part of the cycle, so that the high interval depends only on R<sub>1</sub> and C, with an adjustment based on the voltage drop across the diode. The low time is unaffected by the diode and so remains <math display="inline">\ln(2) \, R_2 \, C \, .</math> But the diode's forward [[voltage drop]] ''V''<sub>diode</sub> slows charging on the capacitor, so the high time is longer than the often-cited <math display="inline">\ln(2) \, R_1 \, C</math> to become:

: <math>t_\text{high} = \ln\left(\frac{2 \, V_\text{CC} - 3 \, V_\text{diode}}{V_\text{CC} - 3 \, V_\text{diode}}\right) \cdot R_1 \cdot C,</math>

where ''V''<sub>diode</sub> is when the diode's "on" current is {{Frac|1|2}} of ''V''<sub>CC</sub>/R<sub>1</sub> (which [[Diode forward voltage drop|depends on the type of diode]] and can be found in datasheets or measured). When V<sub>diode</sub> is small relative to ''V''<sub>cc</sub>, this charging is faster and approaches <math display="inline">\ln(2) \, R_1 \, C</math> but is slower the closer V<sub>diode</sub> is to ''V''<sub>cc</sub>:

<blockquote>As an extreme example, when ''V''<sub>CC</sub> = 5&nbsp;V, and V<sub>diode</sub> = 0.7&nbsp;V, high time is 1.00&nbsp;R<sub>1</sub>C, which is 45% longer than the "expected" 0.693&nbsp;R<sub>1</sub>C. At the other extreme, when ''V''<sub>cc</sub> = 15&nbsp;V, and V<sub>diode</sub> = 0.3&nbsp;V, the high time is 0.725&nbsp;R<sub>1</sub>C, which is closer to the expected 0.693&nbsp;R<sub>1</sub>C. The equation approaches 0.693&nbsp;R<sub>1</sub>C as ''V''<sub>diode</sub> approaches 0&nbsp;V.</blockquote>