It would be trivial to change to an normally open button, simply tie the source for the mosfet the button is connected to, through a resistor, to +V, then use a normally open button to short the source down to ground. Single extra resistor (which you should have a few of already, and cheap) vs. going out to find a not all that common button (yes I know that they do exist.)
-kizzap
I gave it some thought before posting, how to use a NO switch, and came up with the same idea. But the math was more involved and he said he was just learning so I didn't want to complicate it, but it can be done if it's hard to find a NC switch like that. I figured that there are many SPDT push buttons or DPDT so it should be not that hard.
If you do use a single NO switch, just make sure the resistor pulling the source of Q2 up to 5V is an order of magnitude lower than the dark resistance of the LDR, like maybe 20-30 ohms, 1W or so. For home use, or prototype, or just playing and learning, then a 1/2 W will be ok.It's a pulsed load on the reset switch. Just don't hold it down too long, the resistor will be over its rated wattage, and don't use 1/2W in any production release of something like this.
The dark resistance of the LDR is pretty low, like 250 ohms, and may go as low as 100 ohms in a really dark room.
so the source resistance plus the dark resistance of the LDR forms a voltage divider that has to have a voltage higher than the Vgs(on) of Q1. This means that the top resister of the divider (the source of Q2) has to be pretty low. And making it low enough to overcome the LDR dark resistance means that it needs to be high-wattage to handle the connection to ground without smoking..
In fact, this simple switch covers the basics of ohms law, voltage dividers and MOSFETS pretty well.
There is lots to learn from making this work, and making it safe.
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