Changes

Based on what you’ve just seen, the three quantities of electricity which we introduced earlier - voltage, current, and resistance - are clearly tied together somehow. It turns out that they’re actually mathematically related - and that relation is very easy! The equation, called '''Ohm’s Law''', will be one of the most important tools in your toolkit:

'''V''' is the variable for voltage, '''I''' is for current, and '''R''' is for resistance. If you know two of the variables, you can solve for the third! Let’s do a simple calculation. We actually know the total ''V'' for the system: 9V. The LDRs you are using have a minimum resistance of 3kΩ, so we also know ''R''! All we need to do is solve for ''I''.  

Great! The LDRs have a ''maximum'' resistance of 200kΩ. Find the current when resistance is maximized.

The PCB also has a layer on top of the copper called the '''solder mask''', which protects the internal traces from damage. On this board, it is purple. Usually on top of the solder mask, the PCB will also have a '''silk screen''', which contains text and images helpful to anyone looking at the board. This is the circuit you’ll be soldering together:

You’ll notice that it’s just a combination of two of the circuits you’ve already built, with one addition: a '''switch'''. This switch has a single '''pole''' which can only connect to one other pole; it is said to only have one way to be '''thrown'''. Therefore, this is a '''S'''ingle '''P'''ole, '''S'''ingle '''T'''hrow switch; '''SPST'''.

Congratulations; you have just made a permanent prototype of your first circuit! Test it out to make sure everything works.