If you’ve ever wondered how street lights know that it’s dark outside,
well there’s a good chance it’s thanks to one of these.
This is a photocell light control.
It has a light sensor on its side and a few electrical connections on the bottom,
and when wired appropriately into a light fixture, it will prevent that light from coming on when the sun’s up
(and turn it on when the sun sets).
But… how exactly is it doing that?
Simply saying “a light sensor turns the light off”
doesn’t really explain what’s actually going on inside of here, does it?
After all, what even is a sensor?
Perhaps that sounds like a silly question, but the thing inside of this that actually turns on and off the light it controls…
is a thermostat.
Yeah, this thing is a little more rube-goldbergy than it might seem at first glance.
The actual light sensing doodad is likely a familiar sight.
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.
The squiggly dark orange line through a shiny metal plate is the tell-tale of a photoresistor,
an electronic component also known as a light-dependent resistor or LDR.
As its name might suggest, the electrical resistance of this component changes based on the amount of light that hits it.
We can see this in action if I take this apart and hook up a multimeter.
Notice that as more light hits the photoresistor, the measured resistance decreases.
In other words, the more light that hits it, the more easily electric current can flow through this component.
You might have realized that this is backwards to the goal of a nightlight -
the nightlight should come on when it’s dark,
and the photoresistor on its own would make it harder to light the bulb at night and easier during the day.
Accordingly, the photoresistor is only part of the equation - hence the circuit board.
In addition to the photoresistor, there’s also an ordinary resistor and a thyristor.
This trio of istors is what actually makes the circuit function.
Power flows in from the live pin and up through this bit of copper which passes for a lamp holder these days.
Once it’s made it through the filament of the incandescent bulb,
it comes back down this other bit of copper and hits one side of the thyristor.
That thyristor is a semiconductor which serves both as a diode
(meaning it only allows current to flow through it in one direction, which, fun fact,
has the side-effect of only running the lamp at half-power which’ll greatly extend its life)
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.
This is pretty confusing to look at so I’ve drawn this as a circuit diagram.
What we really have here is an ordinary lamp circuit with the thyristor interrupting it in the middle.
As drawn, this would never light the bulb.
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,
then it will relax and allow current to flow from one side to the other, which will complete the circuit and light the lamp.
That activation current comes from the resistor here.
It provides a path for current to flow between this point and the thyristor’s gate.
With this in the circuit by itself, now the light will never go out (assuming the bulb isn’t burnt out).
The resistor will always provide enough current for the thyristor to activate and allow the light to turn on.
However, that resistor is a 2 megaohm resistor.
It lets a tiny, tiny amount of current through itself, in fact barely enough to activate the thyristor.
It would thus be very easy to disrupt that input, which is precisely what the photoresistor is for.
Notice that it’s connected between the neutral pin of the plug and this point here.
When it’s dark enough,
the resistance of the photoresistor is sufficiently high to where it doesn’t affect anything about the circuit and the lamp lights.
But when light hits it, its resistance falls.
Once it’s low enough, it provides an alternative path for that activation current coming from the resistor to take,
so that current bypasses the thyristor’s gate and the lamp doesn't light.
It's pretty simple.
Because of the way this circuit works, some current is always flowing - but it’s hardly any at all.
That 2 megaohm resistor, at 120V, dissipates a whole 0.0072 watts.
And that’s assuming it's wired directly to neutral.
Which it isn’t - it’s in series with the lamp filament, which adds another 400 ohms or so.
The thyristor needs absolutely miniscule amounts of current to become activated,
so while this circuit is technically drawing a little power during the day, it’s practically zero.
Also noteworthy, thyristors exhibit a sort-of latching behavior and, once activated with sufficient current flowing through the gate,
stay activated until the zero-crossing of the AC power cycle.
This is why the lamp will fade to life as it gets dark rather than just snap to full brightness.
When most but not all of the activation current bypasses through the photoresistor,
the thyristor will periodically kick the lamp to life at the tail end of each power cycle, behaving much like a dimmer switch does.
This circuit is really quite elegant and it works just fine for controlling a tiny little light bulb.
But it’s not enough to control a street light
which might have a lightbulb like this -
and this thing draws 1,000 watts.
For this, you’re gonna need something a lot more robust.
Like the thing I showed you at the beginning.
This light control can interrupt the full 15 amps of a standard US lighting circuit,
and in fact is even rated for up to 277 volts.
This style, with its twist-lock plug, is commonly used atop individual light fixtures and meant for easy replacement,
but they’re also available in little modules like this that you wire into whatever you want to control.
There are two important things to note about these things:
First, the photoresistors on them are a lot larger than what we see in the nightlight.
And second, both of them have three electrical connections.
There’s the incoming power, the switched outgoing power, and neutral as well.
That tells us that the photocontrols operate independently of the loads they control, and likely consume a little bit of power themselves.
Now you might think that there’s some sort of logic circuitry in these things controlling a relay or something,
but their operation is in fact much simpler than that.
Let’s pop the cover off this one.
It’s pretty empty in here, isn’t it?
We just have the photoresistor on a little mounting plate, then some kind of component touching a small circuit board.
It’s clear that this component is the actual switch as it has fairly large wires connected to either side of it,
but what do you suppose is going on beneath it?
That little board has a lead going to the incoming power, and another going to the photoresistor.
Then the other side of the photoresistor is wired straight to the neutral pin.
To get a better idea of what’s going on, let’s remove that mystery component.
Betcha didn’t think we’d find thermal paste in this thing, did ya?
If you’ve ever done any sort of appliance repair, you might recognize this thing.
This is a thermostatic switch.
Normally it’s closed and will allow power to flow through.
But inside of it there’s a bimetallic disc which will deform as temperature rises.
[click]
Once it’s past a certain temperature, that disc will snap and open the switch contacts inside.
You often find these used as safety switches in appliances that produce a lot of heat, like clothes dryers or gas-fired furnaces.
In those applications they might be called limit switches, and they’re designed to shut the appliance down if things are getting too hot.
What’s it doing in here?
Well, before I removed it it was pressing against this little circuit board with a little thermal paste in between.
That circuit board has nothing on it but a couple of surface-mount resistors
(as well as a PTC thermistor to serve as a fuse, but that’s not important right now).
Those resistors are doing the simplest job a resistor can possibly do - produce heat.
That’s right, this circuit board is functioning as a heater.
Surface mount resistors are certainly an unconventional heating element,
but that’s what they’re doing and they’re doing a bang-up job, too, so don’t judge.
But notice how that heater circuit is wired.
It’s across live and neutral, but it makes a little diversion through our friend the photoresistor.
Knowing what we’ve learned about photoresistors, think about what that would do.
If I wire this up and we take a look with a thermal camera,
in a dark room we can see that the heater is on, but it’s just a little warm.
It’s trying to heat up, but the photoresistor is in a state of high resistance, preventing power from flowing.
When I shine a light onto the photoresistor, though, its resistance falls and the heater immediately gets warmer -
in fact it gets pretty hot.
So we have a heater circuit that gets hotter when more light hits this thing,
and we have a limit switch that opens a circuit when it gets too hot.
I wonder what would happen if we put them toget—
oh, right, that's how it was when I got it.
With it reassembled, here’s what we see in the thermal camera.
Again, when powered on in the dark, we can see the faintest little hot-spot.
But when I shine a light on the photoresistor, that hot spot gets bigger and...
hot.
The resistors on that board are dissipating about 2.5 watts,
and the thermal paste is transferring that heat energy into the face of the thermal switch.
The switch remains closed for about 2 minutes,
but eventually
[click]
it opens.
One thing to note is that the photoresistor is also a resistor in the circuit
so it, too, dissipates a bit of heat.
However, the big metal plate it’s mounted on doubles as a heat sink, so it’s barely visible to the thermal camera.
I’ve wired up this totally-legit streetlight in a definitely-not-sketchy fashion
to provide a demonstration of the photocontrol in action.
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.
But as we approach sundown, the heater slowly loses power as the photoresistor’s resistance increases,
and thus its heat output falls.
In this... simulated sunset, the heater instantly shuts off,
but it takes a while for the thermostatic switch to cool down.
Once it does, the switch will close, and the light turns on.
[click]
Ah, there we are.
The delay is actually quite useful, especially for lighting technologies like the high pressure sodium lamp you see here.
Ideally, you want to minimize the number of times these lamps start up,
and the varying lighting conditions as the sun rises and sets could result in some wild behavior at dawn and dusk.
The thermal mass of the thermostatic switch, along with its deadband between activation temperatures,
provides the perfect combination of delay factors to ensure the light it controls doesn’t behave erratically.
Some people would really like me to say the word “hysteresis.”
You’re welcome!
Now, earlier I said the resistors in here dissipate about 2 and a half watts.
That means that, ironically, this thing consumes power in order to save power.
Since the smallest lamp one of these might control is about 50 watts, it’s still definitely worth having one of these.
[click]
But putting one on top of every light fixture doesn’t always make sense.
Not only does that make each light fixture more expensive,
but it also means every single one is consuming a little bit of power all the time
and that adds up.
So in many cases, banks of streetlights are wired to a central location,
and a single one of these things is used to control a contactor at the distribution point.
The fact that these need power to turn things off also adds a fun little side-effect.
The switch inside closes because the heater stopped heating, which usually happens because the sun set,
but that’s not the only reason it can happen!
If, for example, there's a power outage during the day,
the switches inside every one of these things will cool down and close within just a few minutes despite the sun shining brightly.
So when power comes back on,
all those streetlights (and anything else controlled by one of these)
will also come on for a minute or two before the internal heater warms up enough to shut them off again.
I mean, it’s pretty inconsequential, really, but it’s still fun.
Something notable about this photocontrol is that its design is actually quite a bit different from designs of the past,
which you can perhaps tell by the massive lack of stuff inside of it.
This is an old, standardized form-factor so the casing didn’t change
but older versions often had something similar to what we find inside this wire-in module.
In practice, this is the same exact thing,
with a photoresistor wired in-series with a resistor that acts as a heater, only allowing that heater to function during the day.
But instead of heating an off-the-shelf limit switch,
it heats this bimetallic bar.
As it warms up, the bar starts bending downward and applying force to a spring-loaded switch contact through this screw.
Once it’s warm enough (and thus bent enough), that switch contact snaps open.
And on this particular module, it barely opens.
Which is actually pretty disconcerting.
But whatever, once the sun sets and the photoresistor restricts flow through the heater,
the bar starts to cool back down, move back to its original shape, and eventually the switch contacts snap together again.
Older versions of this puck-style twist-lock thingamajig often had a very similar mechanism to this inside,
perhaps with more robust contacts for handling the inductive load of a big magnetic ballast.
It’s not much of a surprise that this new one uses a limit switch, honestly,
because that’s an off-the-shelf part used in gajillions of applications.
The bimetallic bendy switch mechanism with a resistor taped to the bar needs purpose-built parts.
It also seems rather fiddly to put together, and it looks as though the screw tension needed calibration in the factory.
This design, on the other hand, is a three-component circuit board which probably cost a few pennies,
a dab of thermal paste, an off-the-shelf limit switch, and some screws.
Wire in the photoresistor and you’re done.
Is this as robust as the old designs?
Who knows.
But this surely isn’t the only way to handle this, is it?
I mean, with microprocessor chips cheap as chips these days,
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?
Well, take a look at this little fella.
It’s like a baby version of the other one!
And its photoresistor looks pretty small, not much bigger than the night light’s.
Let’s take a look inside.
Oh.
It literally is just a baby version of the other one.
That actually surprised me quite a lot, but I guess it proves that if it ain’t broke…
Perhaps there are more power-efficient ways to handle this task,
in fact I’m certain there are.
But, they all have trade-offs.
Any solution involving a microcontroller is gonna need its own power supply to function which adds costs.
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.
Capacitors tend to not like that.
Plus, then you’re gonna need to figure out a way to introduce hysteresis in software,
and if there’s one thing I know about software, it’s
[sound locks up]
pre
ee, and oft
half baked.
There is definitely something to be said for a simple solution like this,
and besides, the power they consume is minimal - especially when you use one to control 100 light poles.
What I think is most interesting about these things
is they are a perfect demonstration of how to turn an electronic component into something that’s actually useful.
It’s one thing to learn what a photoresistor does,
and quite another to make something happen with it.
And there’s more than one way to go about it!
The nightlight shows an elegant semiconductor-based solution for low-power applications.
And while turning a heater on and off with the sun might seem pretty off-the-wall,
when you know that bimetallic strips get all bendy when they heat up,
that presents an opportunity for actuating a switch.
All that it takes to turn that into a useful product is for someone to make the connection.
♫ painfully smooth jazz ♫
The thing inside of here… hooh
You’re gonna need something a lot more robust.
Like the thing I showed you [clunk] at the begin--- crap!
[frustration noises]
...and the lamp lights.
But when light falls on it, its resistance falls. When
Surface-mount resistors are certainly an unconventional heating element but they're, but
With a photoresistor wired in series with a res --
crap!
However, that resist...
However, that resistor is a two…
however,
maaahhhh
What I think is most….
What I… [clears throat]
What I think is ppppbbblb
All it takes is to turn that… haaaaaaa
What's going on inside these things is weirder than you thought, huh?
Just like the title said.
I didn't really know how they work, but when I realized the light would turn on the heater, it clicked.
Both conceptually, and because the switch opened.