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Dometic Portable Ice Maker Model HZB-12SA

voltagedrop

Apr 22, 2021
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Hello, first post, just joined, and hoping to exploit some knowledge as I teach myself electronics for the fun of it.

My family uses a portable ice maker in our holiday trailer. As it makes ice, it harvests into a "bin" of its own. Near the top of the bin, is an Infrared LED, and across from it a photodiode. When the ice builds to this level, the ice blocks the IR beam, and the unit shuts down. As the ice melts, or is removed from the machine, and drops below the beam, the machine restarts.
The machine began not shutting off, and would keep producing ice, overflowing the machine, or until it ran out of water.

I disassembled and found rust coming out of the Photodiode, where the leads enter the epoxy. I acquired new a new LED and Photodiode from Digikey, a "matched pair". replaced them, ensuring the polarity was the same at the old pieces (using the flat on the epoxy for reference).

Now I have the opposite problem, and the ice making immediately goes into "ice full" mode, indicating the beam is "broken".

I measured current and voltage drop at the photodiode, 5 v across diode at 1.4 microamps. If I shine a bright flashlight into the photodiode, I can get the voltage drop to decrease and currant to increase. If I hold the new LED approx 1" or less, voltage drops to near 0, and current increases to 40-50 microamps, and unit functions as normal. As soon as I move beyond the approx 1" approx, "ice full". Fully assembled, the LED and photodiode sit approx. 7" apart.

Attached is a rough sketch of "ice full circuit".

Any ideas? Seems either the LED isn't "powerful" enough, or the Photodiode isn't sensitive enough. I played around with the LED's limiting resistor until I had it up to approx. 100mA (max per spec sheet), and the Photodiode changed very little (up to around 10 microamps). This also made the 5V linear regulator a touch warm (specs out to 100 mA max), however the regulator stays at 5V throughout.

Thanks for your thoughts.
 

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  • Ice Maker Schematic.pdf
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Harald Kapp

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Have you checked the correct mechanical alignment of transmitter and receiver? If the components are not well aligned, the beam from the LED may not hit the photodiode, thus simulating a full bucket.
 

voltagedrop

Apr 22, 2021
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I have checked alignment. The diodes slide into cylinders cast into the 2 plastic side housings, and the lip near the bottom of the lens sits on a ledge inside the cylinder. Then they are held with a dab of silicone on the back side. So no adjustment possible.

Having said that, it was a thought I had as well, so I have, with one side panel off, moved it around while watching the current of the photodiode. I get no significant change until I am grossly misaligned (the small amount of current drops), or I get within that 1" previously mentioned (current increases).
 

Harald Kapp

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The schematic shows a photodiode, but the circuit is unusually simpe for a photodiode. It migt be an oversimplified diagram. A phototransistor will deliver a much higher current (mA vs. µA)
 

voltagedrop

Apr 22, 2021
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A phototransistor would have 3 leads, so I'm certain it would be a photodiode. But, I am still learning here. Though I realize a phototransister would work with some redesign, this has been a functioning circuit as is for many years. And, as the photodiode used is a barrel design, to fit into the plastic housing previously mentioned, I don't think phototransistors are available in that package.

When I selected the replacement parts, I couldn't find any great differences between manufacturers, model #'s, etc besides effective angles.

Thanks for the ideas.
 

voltagedrop

Apr 22, 2021
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In playing around I found that about 40 microamps is about the magic # to trigger the input of the microcontroller. I am having a difficult time understanding the spec "manual" for the microcontroller to determined what "should" be required.
 

Bluejets

Oct 5, 2014
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Microcontroller digital inputs do not work on current.

If the numbers on the diagram are the voltages you are getting with the new parts installed, then that part is ok.
5V no light and 0.33v with light should be working as an input to the micro.
 
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voltagedrop

Apr 22, 2021
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Ah. OK. I didn't measure the voltage when it was triggering.
Microcontroller digital inputs do not work on current.

If the numbers on the diagram are the voltages you are getting with the new parts installed, then that part is ok.
5V no light and 0.33v with light should be working as an input to the micro.

The problem being I am only getting those numbers when I have the LED's 1" or closer. When they are properly mounted at the 7" apart, there is hardly any change at all. When the LED's are close (within the 1") the machine operates as it should.
 

voltagedrop

Apr 22, 2021
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Not necessarily. You can buy phototransistors without external base, check the link I gave.

Very interesting. I didn't notice the link in your previous post (that what happens reading on my phone). I will look in those. I just did some reading on them after reading your post, and the "third terminal" (base) comes from the photons on the chip. Thanks for the input. I'll look into some options.
 

bertus

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Nov 8, 2019
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Hello,

In the attached application note, the photo transistor does not have a base connection.

Bertus
 

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Bluejets

Oct 5, 2014
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Ah. OK. I didn't measure the voltage when it was triggering.


The problem being I am only getting those numbers when I have the LED's 1" or closer. When they are properly mounted at the 7" apart, there is hardly any change at all. When the LED's are close (within the 1") the machine operates as it should.

Perhaps your choice of tx rx are not compatible.
Usual to buy them as a pair, saves problems at times especially if the spec sheet does not display all the info or you don't understand it.
 

voltagedrop

Apr 22, 2021
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They weren't a pair per-sea but were "recommended" from one to the other on Digikey. Same manufacturer, and same wavelength.
 

Bluejets

Oct 5, 2014
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What about power output?
Did they "recommend" that also for the distance you require?
 

Harald Kapp

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Wavelengths match. The current through the photodiode is way to small. That is why I recommended a phottransistor.
 

voltagedrop

Apr 22, 2021
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Thanks for all of the reply's everyone. I am waiting on some phototransistors and matching IRLed Emitters (I ordered extras) from Digikey at 940nm wavelength.

In an effort to better understand the circuit, can anyone explain the the point of the capacitor (C1) on the schematic. If I am understanding the circuit properly, Resistor R3 is a limiting resistor for the phototransistor/photodiode (D2). R2 is a pull up resistor for the CMOS (pin16). I don't understand why a capacitor is on the logic pin, to ground. My thought is it would create a delay in the input at pin 16, possibly so the unit does not immediately go into "ice full" mode if something momentarily breaks the IR beam (someone reaches hand in, or even just the ice falling from the freeze tray to the hopper will cross the beam).

It should be noted that not included in the schematic, is a capacitor between U3's pin 5 (+5v) and 11 (gnd), which I understand would be a decoupling capacitor.

(it should also be noted I screwed up in labeling U3 and what looks like 1 in the lower right is 11, and it and 16 should be swapped, Therefore top left = +5v is pin 5, bottom right -gnd is pin 16, and upper right is input pin 11.) (it is a 20 pin IC).

Thanks.
 

Harald Kapp

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R2 + C1 make a low pass filter to reduce "chatter" e.g. under different lighting conditions or when the ice box is almost full so the ice maker won't turn on and off in rapid succession. As you imagined.
 

voltagedrop

Apr 22, 2021
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FIXED. Finally got parts (photo transistor and matching 940nm IR Diode) and time to put it back together and she works like new. Thanks for all the help.
 

skilliknow

Aug 12, 2022
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Hello, first post, just joined, and hoping to exploit some knowledge as I teach myself electronics for the fun of it.

My family uses a portable ice maker in our holiday trailer. As it makes ice, it harvests into a "bin" of its own. Near the top of the bin, is an Infrared LED, and across from it a photodiode. When the ice builds to this level, the ice blocks the IR beam, and the unit shuts down. As the ice melts, or is removed from the machine, and drops below the beam, the machine restarts.
The machine began not shutting off, and would keep producing ice, overflowing the machine, or until it ran out of water.

I disassembled and found rust coming out of the Photodiode, where the leads enter the epoxy. I acquired new a new LED and Photodiode from Digikey, a "matched pair". replaced them, ensuring the polarity was the same at the old pieces (using the flat on the epoxy for reference).

Now I have the opposite problem, and the ice making immediately goes into "ice full" mode, indicating the beam is "broken".

I measured current and voltage drop at the photodiode, 5 v across diode at 1.4 microamps. If I shine a bright flashlight into the photodiode, I can get the voltage drop to decrease and currant to increase. If I hold the new LED approx 1" or less, voltage drops to near 0, and current increases to 40-50 microamps, and unit functions as normal. As soon as I move beyond the approx 1" approx, "ice full". Fully assembled, the LED and photodiode sit approx. 7" apart.

Attached is a rough sketch of "ice full circuit".

Any ideas? Seems either the LED isn't "powerful" enough, or the Photodiode isn't sensitive enough. I played around with the LED's limiting resistor until I had it up to approx get more ice makers guide at icedetective. 100mA (max per spec sheet), and the Photodiode changed very little (up to around 10 microamps). This also made the 5V linear regulator a touch warm (specs out to 100 mA max), however the regulator stays at 5V throughout.

Thanks for your thoughts.
Had you verified that the transceiver are mechanically aligned properly? The photosensitive may not be hit by the LED beam, imitating a pony keg, if the element alignment is poor.
 
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