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If you’ve ever wondered how street lights know that it’s dark outside,

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well there’s a good chance it’s thanks to one of these.

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This is a photocell light control.

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It has a light sensor on its side and a few electrical connections on the bottom,

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and when wired appropriately into a light fixture,
it will prevent that light from coming on when the sun’s up

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(and turn it on when the sun sets).

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But… how exactly is it doing that?

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Simply saying “a light sensor turns the light off”

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doesn’t really explain what’s actually going on inside of here, does it?

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After all, what even is a sensor?

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Perhaps that sounds like a silly question,
but the thing inside of this that actually turns on and off the light it controls…

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is a thermostat.

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Yeah, this thing is a little more rube-goldbergy than it might seem at first glance.

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The actual light sensing doodad is likely a familiar sight.

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Take a look at a common night light and you’ll find a smaller 
but very similar looking component under this bit of clear plastic.

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The squiggly dark orange line through a shiny metal plate
is the tell-tale of a photoresistor,

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an electronic component also known as a light-dependent resistor or LDR.

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As its name might suggest, the electrical resistance of this component
changes based on the amount of light that hits it.

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We can see this in action if I take this  apart and hook up a multimeter.

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Notice that as more light hits the photoresistor, the measured resistance decreases.

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In other words, the more light that hits it,
the more easily electric current can flow through this component.

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You might have realized that 
this is backwards to the goal of a nightlight -

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the nightlight should come on when it’s dark,

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and the photoresistor on its own would make it harder to light the bulb at night
and easier during the day.

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Accordingly, the photoresistor is only part of the equation - hence the circuit board.

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In addition to the photoresistor,
there’s  also an ordinary resistor and a thyristor.  

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This trio of istors is what actually makes the circuit function.

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Power flows in from the live pin and up through this bit of copper
which passes for a lamp holder these days.

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Once it’s made it through the filament of the incandescent bulb,

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it comes back down this other bit of copper and hits one side of the thyristor.

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That thyristor is a semiconductor which serves both as a diode

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(meaning it only allows current to flow through it in one direction, which, fun fact,

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has the side-effect of only running the lamp at half-power
which’ll greatly extend its life)

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but also, and more importantly, it serves as a power switch, interrupting the circuit and preventing the lamp from lighting unless it sees a control input.

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This is pretty confusing to look at so I’ve drawn this as a circuit diagram.

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What we really have here is an ordinary lamp circuit with 
the thyristor interrupting it in the middle.  

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As drawn, this would never light the bulb.

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But if we provide an input by way of a bit of current flowing to the gate of the thyristor, that’s its middle pin,

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then it will relax and allow current to flow from one side to the other,
which will complete the circuit and light the lamp.

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That activation current comes from the resistor here.

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It provides a path for current to flow between this point and the thyristor’s gate.

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With this in the circuit by itself, now the light will never go out
(assuming the bulb isn’t burnt out).

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The resistor will always provide enough current
for the thyristor to activate and allow the light to turn on.

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However, that resistor is a 2 megaohm resistor.

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It lets a tiny, tiny amount of current through itself,
in fact barely enough to activate the thyristor.  

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It would thus be very easy to disrupt that input, 
which is precisely what the photoresistor is for.

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Notice that it’s connected between the neutral pin of the plug and this point here.

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When it’s dark enough,

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the resistance of the photoresistor is sufficiently high to where it doesn’t affect anything about the circuit and the lamp lights.

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But when light hits it, its resistance falls.

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Once it’s low enough, it provides an alternative path for that activation current coming from the resistor to take,

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so that current bypasses the thyristor’s gate and the lamp doesn't light.

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It's pretty simple.

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Because of the way this circuit works, some current is always flowing - 
but it’s hardly any at all.

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That 2 megaohm resistor, at 120V, dissipates a whole 0.0072 watts.

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And that’s assuming it's wired directly to neutral.

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Which it isn’t - it’s in series with the lamp filament,
which adds another 400 ohms or so.

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The thyristor needs absolutely miniscule amounts of current to become activated,

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so while this circuit is technically drawing a little power during the day,
it’s practically zero.

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Also noteworthy, thyristors exhibit a sort-of latching behavior and,
once activated with sufficient current flowing through the gate,

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stay activated until the zero-crossing of the AC power cycle.

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This is why the lamp will fade to life as it gets dark
rather than just snap to full brightness.

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When most but not all of the activation current bypasses through the photoresistor,

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the thyristor will periodically kick the lamp to life at the tail end of each power cycle, 
behaving much like a dimmer switch does.

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This circuit is really quite elegant
and it works just fine for controlling a tiny little light bulb.  

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But it’s not enough to control a street light

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which might have a lightbulb like this -

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and this thing draws 1,000 watts.

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For this, you’re gonna need something a lot more robust.

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Like the thing I showed you at the beginning.

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This light control can interrupt the full 15 amps of a standard US lighting circuit,

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and in fact is even rated for up to 277 volts.

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This style, with its twist-lock plug, is commonly used atop individual light fixtures
and meant for easy replacement,

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but they’re also available in little modules like this that 
you wire into whatever you want to control.

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There are two important things to note about these things:

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First, the photoresistors on them are a lot larger than what we see in the nightlight.

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And second, both of them have three electrical connections.

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There’s the incoming power, the switched outgoing power,
and neutral as well.

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That tells us that the photocontrols operate independently of the loads they control, and likely consume a little bit of power themselves.

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Now you might think that there’s some sort of logic circuitry in these things controlling a relay or something,

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but their operation is in fact much simpler than that.

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Let’s pop the cover off this one.

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It’s pretty empty in here, isn’t it?

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We just have the photoresistor on a little mounting plate,
then some kind of component touching a small circuit board.

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It’s clear that this component is the actual switch
as it has fairly large wires connected to either side of it,

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but what do you suppose is going on beneath it?

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That little board has a lead going to the incoming power,
and another going to the photoresistor.

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Then the other side of the photoresistor is wired straight to the neutral pin.

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To get a better idea of what’s going on, let’s remove that mystery component.

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Betcha didn’t think we’d find thermal paste in this thing, did ya?

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If you’ve ever done any sort of appliance repair, you might recognize this thing.

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This is a thermostatic switch.

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Normally it’s closed and will allow power to flow through.

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But inside of it there’s a bimetallic disc which will deform as temperature rises.

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[click]

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Once it’s past a certain temperature,
that disc will snap and open the switch contacts inside.

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You often find these used as safety switches in appliances 
that produce a lot of heat, like clothes dryers or gas-fired furnaces.

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In those applications they might be called limit switches,
and they’re designed to shut the appliance down if things are getting too hot.

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What’s it doing in here?

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Well, before I removed it it was pressing against this little circuit board
with a little thermal paste in between.

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That circuit board has nothing on it but a couple of surface-mount resistors

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(as well as a PTC thermistor to serve as a fuse, but that’s not important right now).

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Those resistors are doing the simplest job a resistor can possibly do - produce heat.

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That’s right, this circuit board is functioning as a heater.

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Surface mount resistors are certainly an unconventional heating element,  

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but that’s what they’re doing and they’re 
doing a bang-up job, too, so don’t judge.

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But notice how that heater circuit is wired.

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It’s across live and neutral,
but it makes a little diversion through our friend the photoresistor.

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Knowing what we’ve learned about photoresistors,
think about what that would do.

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If I wire this up and we take a look with a thermal camera,

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in a dark room we can see that the heater is on, 
but it’s just a little warm.

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It’s trying to heat up, but the photoresistor is in a state of high resistance,
preventing power from flowing.

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When I shine a light onto the photoresistor, though,
its resistance falls and the heater immediately gets warmer -

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in fact it gets pretty hot.

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So we have a heater circuit that gets hotter when more light hits this thing,

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and we have a limit switch that opens a circuit when it gets too hot.

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I wonder what would happen if we put them toget—

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oh, right, that's how it was when I got it.

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With it reassembled, here’s what we see in the thermal camera.

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Again, when powered on in the dark, we can see the faintest little hot-spot.

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But when I shine a light on the photoresistor,
that hot spot gets bigger and...

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hot.

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The resistors on that board are dissipating about 2.5 watts,

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and the thermal paste is transferring that heat energy
into the face of the thermal switch.

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The switch remains closed for about 2 minutes,

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but eventually

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[click]

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it opens.

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One thing to note is that the photoresistor is also a resistor in the circuit

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so it, too, dissipates a bit of heat.

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However, the big metal plate it’s mounted on doubles as a heat sink,
so it’s barely visible to the thermal camera.

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I’ve wired up this totally-legit streetlight in a definitely-not-sketchy fashion

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to provide a demonstration of the photocontrol in action.

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Right now, the sun is up, so the photoresistor is letting enough current through to keep the heater powered and the thermostatic switch nice and toasty.  

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But as we approach sundown, the heater slowly loses power
as the photoresistor’s resistance increases,

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and thus its heat output falls.

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In this... simulated sunset, the heater instantly shuts off,

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but it takes a while for the thermostatic switch to cool down.

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Once it does, the switch will close, and the light turns on.

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[click]

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Ah, there we are.

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The delay is actually quite useful, especially for lighting technologies
like the high pressure sodium lamp you see here.

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Ideally, you want to minimize the number of times these lamps start up,

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and the varying lighting conditions as the sun rises and sets 
could result in some wild behavior at dawn and dusk.

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The thermal mass of the thermostatic switch,
along with its deadband between activation temperatures,

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provides the perfect combination of delay factors
to ensure the light it controls doesn’t behave erratically.

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Some people would really like me to say the word “hysteresis.”

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You’re welcome!

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Now, earlier I said the resistors in here dissipate about 2 and a half watts.

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That means that, ironically, this thing consumes power
in order to save power.

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Since the smallest lamp one of these might control is about 50 watts,
it’s still definitely worth having one of these.

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[click]

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But putting one on top of every light fixture doesn’t always make sense.

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Not only does that make each light fixture more expensive,

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but it also means every single one is consuming a little bit of power all the time

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and that adds up.

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So in many cases, banks of streetlights are wired to a central location,

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and a single one of these things is used to control a contactor
at the distribution point.

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The fact that these need power to turn things off also adds a fun little side-effect.

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The switch inside closes because the heater stopped heating,
which usually happens because the sun set,

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but that’s not the only reason it can happen!

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If, for example, there's a power outage during the day,

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the switches inside every one of these things will cool down and close
within just a few minutes despite the sun shining brightly.  

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So when power comes back on,

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all those streetlights (and anything else controlled by one of these)

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will also come on for a minute or two
before the internal heater warms up enough to shut them off again.

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I mean, it’s pretty inconsequential, really, but it’s still fun.

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Something notable about this photocontrol is that its design
is actually quite a bit different from designs of the past,

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which you can perhaps tell by the massive lack of stuff inside of it.

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This is an old, standardized form-factor so the casing didn’t change

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but older versions often had something similar to what 
we find inside this wire-in module.  

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In practice, this is the same exact thing,

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with a photoresistor wired in-series with a resistor that acts as a heater,
only allowing that heater to function during the day.

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But instead of heating an off-the-shelf limit switch,

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it heats this bimetallic bar.

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As it warms up, the bar starts bending downward and applying force
to a spring-loaded switch contact through this screw.

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Once it’s warm enough (and thus bent enough),
 that switch contact snaps open.

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And on this particular module, it barely opens.  

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Which is actually pretty disconcerting.

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But whatever, once the sun sets and the photoresistor
restricts flow through the heater,

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the bar starts to cool back down, move back to its original shape,
and eventually the switch contacts snap together again.

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Older versions of this puck-style twist-lock thingamajig 
often had a very similar mechanism to this inside,

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perhaps with more robust contacts for handling 
the inductive load of a big magnetic ballast.

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It’s not much of a surprise that this new one uses a limit switch, honestly,

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because that’s an off-the-shelf part used in gajillions of applications.

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The bimetallic bendy switch mechanism with a resistor taped to the bar 
needs purpose-built parts.

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It also seems rather fiddly to put together,
and it looks as though the screw tension needed calibration in the factory.  

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This design, on the other hand, is a three-component circuit board
which probably cost a few pennies,

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a dab of thermal paste, an off-the-shelf limit switch, and some screws.

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Wire in the photoresistor and you’re done.

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Is this as robust as the old designs?

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Who knows.

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But this surely isn’t the only way to handle this, is it?

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I mean, with microprocessor chips cheap as chips these days,

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wouldn’t a little code running on a microcontroller powering a relay make more sense than a heater running all day just to keep a thermostatic switch open?

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Well, take a look at this little fella.

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It’s like a baby version of the other one!

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And its photoresistor looks pretty small, 
not much bigger than the night light’s.

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Let’s take a look inside.

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Oh.

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It literally is just a baby version of the other one.

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That actually surprised me quite a lot,
but I guess it proves that if it ain’t broke…

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Perhaps there are more power-efficient ways to handle this task,

204
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in fact I’m certain there are.

205
00:16:42,078 --> 00:16:44,600
But, they all have trade-offs.

206
00:16:44,600 --> 00:16:51,675
Any solution involving a microcontroller
is gonna need its own power supply to function which adds 
costs.

207
00:16:51,675 --> 00:16:57,541
And then you’re gonna be sticking that thing on top of a light pole
with the sun beating down on it day after day.

208
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Capacitors tend to not like that.

209
00:17:00,556 --> 00:17:05,942
Plus, then you’re gonna need to figure out a way to introduce hysteresis in software,

210
00:17:05,942 --> 00:17:08,548
and if there’s one thing I know about software, it’s

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[sound locks up]

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00:17:09,982 --> 00:17:10,482
pre

213
00:17:10,842 --> 00:17:11,489
ee, and oft

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00:17:11,827 --> 00:17:13,208
half baked.

215
00:17:13,560 --> 00:17:18,239
There is definitely something to be said for a simple solution like this,

216
00:17:18,239 --> 00:17:25,380
and besides, the power they consume is minimal - 
especially when you use one to control 100 light poles.

217
00:17:25,980 --> 00:17:28,894
What I think is most interesting about these things

218
00:17:28,894 --> 00:17:35,894
is they are a perfect demonstration of how to turn an electronic component into something that’s actually useful.

219
00:17:35,894 --> 00:17:41,015
It’s one thing to learn what a photoresistor does,

220
00:17:41,015 --> 00:17:43,680
and quite another to make something happen with it.

221
00:17:44,220 --> 00:17:47,142
And there’s more than one way to go about it!

222
00:17:47,142 --> 00:17:52,923
The nightlight shows an elegant
semiconductor-based solution for low-power applications.

223
00:17:52,923 --> 00:17:58,781
And while turning a heater on and off with the sun
might seem pretty off-the-wall,

224
00:17:58,781 --> 00:18:02,820
when you know that bimetallic strips get all bendy when they heat up,

225
00:18:03,420 --> 00:18:06,060
that presents an opportunity for actuating a switch.  

226
00:18:06,960 --> 00:18:13,334
All that it takes to turn that into a useful product
is for someone to make the connection.

227
00:18:14,235 --> 00:18:16,889
♫ painfully smooth jazz ♫

228
00:18:18,240 --> 00:18:21,114
The thing inside of here… hooh

229
00:18:21,114 --> 00:18:23,339
You’re gonna need something a lot more robust.

230
00:18:23,339 --> 00:18:25,800
Like the thing I showed you [clunk] at the begin--- crap!

231
00:18:27,480 --> 00:18:28,633
[frustration noises]

232
00:18:28,633 --> 00:18:30,139
...and the lamp lights.

233
00:18:30,139 --> 00:18:34,320
But when light falls on it, its resistance falls. When

234
00:18:36,180 --> 00:18:40,140
Surface-mount resistors are certainly an 
unconventional heating element but they're, but 

235
00:18:41,160 --> 00:18:44,649
With a photoresistor wired in series with a res --

236
00:18:44,649 --> 00:18:45,403
crap!

237
00:18:45,403 --> 00:18:47,294
However, that resist...

238
00:18:47,294 --> 00:18:50,263
However, that resistor is a two…

239
00:18:50,263 --> 00:18:51,140
however,

240
00:18:51,140 --> 00:18:52,404
maaahhhh

241
00:18:53,372 --> 00:18:55,069
What I think is most….

242
00:18:55,069 --> 00:18:56,396
What I… [clears throat]

243
00:18:56,396 --> 00:18:57,855
What I think is ppppbbblb

244
00:18:59,026 --> 00:19:02,197
All it takes is to turn that… haaaaaaa

245
00:19:03,728 --> 00:19:07,111
What's going on inside these things is weirder than you thought, huh?

246
00:19:07,111 --> 00:19:09,672
Just like the title said.

247
00:19:09,672 --> 00:19:14,671
I didn't really know how they work, but when I realized the light would turn on the heater, it clicked.

248
00:19:15,617 --> 00:19:19,391
Both conceptually, and because the switch opened.