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the heart of this circuit is a "light dependent resistor" (ldr) |
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the resistance value of this device is over 1 meg when in the dark, and less than 2 k when under bright lighting conditions |
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the 2.2 k resistor in series with the ldr is inserted to limit the current to a value that will prevent it from overheating |
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in actual applications, this high current may be experienced if the 555 encounters an internal fault, or when the 100 k resistor is somehow shortcircuited |
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light is provided from an external source and is assured that the ambient lighting conditions do not neutralize the effect of the light source |