The circuit is simply three-components across the 24VAC supply: a
two-wire photocell and two 12V (1156 auto bulbs) in series. One would
expect that the resistance of the photocell is close to zero in the
dark, to apply the full voltage across the bulbs, and highest in
daylight to turn them off.
My actual measurements on the cell alone are opposite: about 10,000 ohms
in bright light and close to infinity in the dark. Thinking the cell was
bad, I ordered a replacement; same oddball measurements.
What am I missing? My ohmmeter uses a 1.5 volt battery.
It turns out that the problem was a bad crimp where one of the wires
going to the center contact in one socket was bad.
(Do not reply to the non-working email address next to From.)
On Wednesday, March 25, 2015 at 5:05:12 PM UTC-4, Rebel1 wrote:
A photocell by itself is not normally able to turn a light on and off.
A photocell is usually connected to a circuit with a transistor or two and a relay or Triac or something to go with it.
The photocell controls the relay which controls the light.
On 3/25/2015 5:39 PM, email@example.com wrote:
I can't visualize how that would work. There are only two wires into the
photo cell. Maybe with the higher 24 VAC applied, something changes to
lower the resistance across the cell, but then 24 VAC wouldn't be
applied anymore across the cell.
A couple hours ago, I sent the question to the company that makes the
cell. I'll post the reply.
Is this a circuit you made, or is this something that came already made?
If the latter, what all came already made? Were the lights included in
If the latter, does the thing have a brand name and model number, or is
there a website about it?
If you made the circuit, could you draw the circuit and post it on
something like flickr.
On anything other than a simple resistance, don't expect to get the
same resistance with a 1.5v battery as with 24 volts. The way to
measure is to measure the voltage drop across the photocell and the
current in one of the leads, while everything is in operation, and
calculate the resistance. At least in a low voltage item like this.
But Mako's post is right on.
So it's fixed? So you have no quesions? So what did you write to the
manufacturer about? I wouldn't expect much help from them except the
spec sheet, which might well be online.
What was the question you asked them.
The post was installed when the development was built in 1999. The
photocell I thought was bad has a date code of 2006. Here's the company:
I have the low-voltage version, but instead of today's LED lamps mine
are a pair of 1156 12-volt auto bulbs in series across 24VAC. The shells
of the sockets are mechanically (and electrically) connected to each
other. Each supply line goes to the center contact of one bulb socket.
The replacement cell didn't solve the problem, and the company has not
answered my request of how the cell works. It turns out that there was a
bad crimp that connects one of the supply wires to the center contact.
Couldn't fix it by soldering. I'm waiting for a replacement. Probably be
Odd thing about the transformer. Even with no load across the output, it
gets quite warm to the touch; my thermometer gun gives the reading at
85F. But it puts out 24 VAC.
On 3/27/2015 5:34 AM, micky wrote:
On Friday, March 27, 2015 at 5:29:43 PM UTC-4, Rebel1 wrote:
I'd suspect that the reason you're seeing the behavior with the
photocell and the VOM is because the photocell is an electronic
device designed for 24V AC and the VOM is giving it <1.5V DC. I think
it was Micky that pointed out that these on/off photocells are
typically some electronic circuit using SCRs and are going to have
chararcteristics reflecting that, ie they aren't just a resistor or
The module holding the photocell is roughly a cube, 1.1 inches on all
sides. Certainly big enough to contain other circuitry. It's just I
can't understand how the circuitry would work. My guess is that the
voltage drop across the cell module doesn't drop to zero when the lights
are on. It must maintain a small voltage to keep the internal circuitry
energized. After the replacement socket set arrives, I'll measure the
voltage across the module and report here. Doesn't look like the module
will arrive until Monday.
I USED to have failures of my pole lights photocell detector. 2 or3 times a year it would just quit working. expensive at 35 bucks each..
then one night I saw it happen. I was coming out of the house at dusk. The light controller must of turned on, the bulb went brilliant white then off.
Ahh bulb burned out.
I replaced bulb, but it wouldnt go on. after trying some other bulbs, and finally replacing the photo cell again.
I realized th white flash damaged the photocell:(
So I tried CFLs in it and perhaps 10 years later that same photocell is working fine.
Recently I upgraded to LED bulb, and thats been fantastic
The replacement socket set, including two 1156 frosted bulbs, arrived
today and everything is once again working.
In daylight, the voltage across the photocell is about 21 VAC. Covering
it with my hand, the voltage across it drops to 2 VAC, which apparently
is what keeps the internal circuitry energized. There is no DC in the
It takes about 1.5 seconds for the bulbs to reach full brightness. Nice
feature: it minimizes inrush current and should extend bulb life.
Thanks to all who commented.
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