Please note that the range of time periods is only approximate. With perfect components the maximum time period should be 4½ minutes, but this is typically extended to about 10 minutes because the 220µF timing capacitor slowly leaks charge. This is a problem with all electrolytic capacitors, but some leak more than others. In addition the actual value of electrolytic capacitors can vary by as much as ±30% of their rated value.
Adjustable 10 minutes timer schematic
![adjustable timer schematic](http://www.electroschematics.com/wp-content/uploads/2010/08/adjustable-timer.gif)
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