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A more advanced version than my last, which consists of a PNP in addition to an NPN. I believe this is technically known as a put. For those who are interested, especially beginners: when your making a circuit like this that incorporates positive feedback, keep in mind what region of operation the transistor is in (such as: Saturation, Active, Reverse Active, or Cut-Off)! You may try to use only NPN transistors at first, simply because it may be what your more familiar with; since nearly all textbooks introduce circuits using just NPN type transistors at first... But, you'll most likely try to use the transistor in a way where its really acting as a voltage follower instead of as a switch or amplifier, which can mess up your intentions if you don't pay close attention to that. Here's why; In this circuit, its initial state is off since its being partially reverse-biased by a resistor, so to latch it on, you have to forward-bias it for a split second to get it to bias itself afterwards (that's the concept of positive feedback here). BUT, if the transistor is being used as voltage follower and not a switch or amplifier, then instead of latching and staying on, it'll come on at first, then quickly decay to 0, so be careful to pay attention to what region of operation the transistor is in to skip a mistake like this... but of course, mistakes are great if you learn from them and it's not too costly.
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