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I want to build this circuit but I do not have a 7555 low power timer so wondered if I can use a NE555? Also can I use a 2N2222 trasistor instead of a BC108? I don't want to try and build it without making sure I won't damage some components.

Here is the link to the project Light-sensitive Alarm Project and here is the schematic in question, note that while the schematic shows a 555 the parts list mentions a 7555 :

enter image description here

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  1. Yes, the NE555 will work fine in place of the 7555 CMOS version: There is nothing particularly low power being done in the circuit where the difference in performance between the two would matter. Your 9 Volt battery might last a bit less.
  2. Yes, the 2n2222 will work in place of the BC108. Basically any general purpose small signal NPN transistor will work. The BJT is used in very low current condition, well within the voltage range of either BJT, and the circuit overall does not demand any particularly high frequencies.
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    \$\begingroup\$ Note that the maximum value of the timing resistor (1M) is a bit high compared to the maximum input current required by a normal 555 (250uA, compared to almost zero for a C555). Hence the on-time might be somewhat longer than when using a C555. In this design this aspect isn't critical at all (and can be adjusted), so this won't be a problem. \$\endgroup\$
    – Wouter van Ooijen
    Sep 2, 2013 at 15:52

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