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電容 C 通過 R1 和 R2 充電，當(dāng)電壓上升到 2/3 的電源電壓時，六腳檢測到此狀態(tài)，并且 7 腳 連到地。電容通過 R2 放電，直到它的電壓小于
1/3 的電源電壓，并且 2 腳檢測到該狀態(tài)，關(guān)閉 七腳，重復(fù)以上的循環(huán)。 上面的電阻是為了防止 7 腳被損壞，因為當(dāng)六 腳檢測到 2/3 的電源電壓時它短接到 0V。它的 阻值相對于 R2 來說很小，并且不會干擾到振蕩 周期的計算。
下圖適用于非穩(wěn)態(tài)電路：
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Using the graph:（計算部分，不敢隨便翻譯）
Suppose R1 = 1k, R2 = 10k and C = 0.1u (100n).
Using the formula on the graph, the total resistance = 1 + 10 + 10 = 21k
The scales on the graph are logarithmic so that 21k is approximately near the "1" on the 10k. Draw a line parallel to the lines on the graph and where it crosses the 0.1u line, is the answer. The result is approx 900Hz.
Suppose R1 = 10k, R2 = 100k and C = 1u
Using the formula on the graph, the total resistance = 10 + 100 + 100 = 210k The scales on the graph are logarithmic so that 210k is approximately near the first "0" on the 100k. Draw a line parallel to the lines on the graph and where it crosses the 1u line, is the answer. The result is approx 9Hz.
The frequency of an astable circuit can also be worked out from the following formula:
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文章摘自：50個555定時器電路分享(一個國外優(yōu)秀文檔的翻譯，個人覺得是對于掌握555定時器電路最有幫助的資料)每個電路都有詳細的解說，全部內(nèi)容請下載:http://www.torrancerestoration.com/bbs/dpj-80748-1.html
作者:MCU起航
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