A relay is an electrically controlled switch. If the coil (c) is powered then the switch state goes from its default state to the alternate. So a default on swi

A relay is an electrically controlled switch. If the coil (c) is powered then the switch state goes from its default state to the alternate. So a default on switch passes on the value of in to its output if c = 0, but when c = 1 (the coil is powered) the switch is opened, or toggled off. For a default off switch it is the reverse. c = coil, if it is powered it will toggle the switch. in = the value (1 or 0 here) that will pass through if the switch is closed. In code: def default_on(c, in): if c == 1: return 0 if c == 0: return in def default_off(c, in): if c == 1: return in if c == 0: return 0 The separate power source allows you to obtain a 1 value (since this is around digital logic I'm not going to break it down further) regardless of what the two input values are. Consider a situation like this: How would you output 1 every time both inputs are 0? You need some high value to draw from, that's what the power source is for. So the input signals themselves may not be enough.