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Liftmaster CPS-EI Interface

Ziedar

New Member
Does anyone know which resistor or other electronic component goes bad on a Liftmaster CPS-EI Interface module?
I am trying to pin point which component to replace and cant seem to locate the faulty resistor
 

KMoffett

Well-Known Member
Symptoms? and a photo of interior (lighted and focused!)
 
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gophert

Well-Known Member
Most Helpful Member
Liftmaster CPS-EI
I would say, for $100, a commercial door controller with life-saving anti-entrapment functions should be replaced with a new one - not one with internet forum suggestions
as a repair. I don't think we should be giving advice for this, elevators, airbags, glucose monitors or other such devices.
 

Ziedar

New Member
The faulty CPS-EI unit causes door to reverse and go all the way up. With the unit disconnected, all the functions of the door work perfectly except for there is no sensor reversing function. I purchased and installed new interface and everything works perfectly. Being that this is the 2nd time around for replacing this in a few years, it would be nice to know what is actually causing these to fail, and maybe try to address the issue.
 

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KeepItSimpleStupid

Well-Known Member
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Often what's missing in commercial products is adequate surge suppression. e.g. transorbs

The first few things I see, is no heatsink on the regulator. The second is the HC https://assets.nexperia.com/documents/data-sheet/74HC_HCT123.pdf part has a maximum voltage of 6V and a 12V regulator. I'd check what the supply pins are seeing.
one possibility is that the designer thought that HC logic family uses the higher voltages of the CD4000 family.

The heatsink is still important because it allows the regulator to shut down.

So, that's my guess as the bad part. (the 74HC123)

You can use Target3001 to help reverse engineer the PCB. With target, you can operate in reverse. e.g. photo of top of board and mirror image with transparency of the bottom. When you connect the components, you get a schematic.

Once you have a schematic, "we can look at it for obvious problems.

I'd look for the presence or lack of bypass capacitors on the regulator.

The large filter capacitor can be an issue.

Look at the voltage on the 74HC power pins? it must not be greater than 6.

EDIT: Bad part unclear.
 
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Diver300

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Most Helpful Member
The faulty CPS-EI unit causes door to reverse and go all the way up. With the unit disconnected, all the functions of the door work perfectly except for there is no sensor reversing function. I purchased and installed new interface and everything works perfectly. Being that this is the 2nd time around for replacing this in a few years, it would be nice to know what is actually causing these to fail, and maybe try to address the issue.
A photo of the back of the board would be useful. None of the components on that are complicated or expensive, so repair should be possible.

If the fault is time-related it could be the capacitor, but that is just based on the fact that electrolytic capacitors degrade with age.
 

KMoffett

Well-Known Member
I've had a bunch of HVAC temperature controller modules go bad because they had filter capacitors next to heat generating components, like that regulator. Couldn't move them so installed new caps wrapped in shiny aluminum foil, and never had a return. Check that electrolytic.
 
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Les Jones

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Most Helpful Member
I think Driver300 is thinking along the right lines. I suspect the IR beam unit sends a string of pulses to the board when the beam is NOT broken. (This is the way my Chamberlain door works.) As the IC is a dual retriggerable monostabe I think it is configured as a missing pulse detector. If you trace out the schematic of the board we will probably be able to suggest how to trace the fault on the board. It is just possible that the IR beam unit is sending out pulses at a frequency that is just on the threshold of the missing pulse detector. (Slightly too low a frequency.) Do you have an oscilloscope to check the pulses though the board ?

Les.
 

Ziedar

New Member
I think Driver300 is thinking along the right lines. I suspect the IR beam unit sends a string of pulses to the board when the beam is NOT broken. (This is the way my Chamberlain door works.) As the IC is a dual retriggerable monostabe I think it is configured as a missing pulse detector. If you trace out the schematic of the board we will probably be able to suggest how to trace the fault on the board. It is just possible that the IR beam unit is sending out pulses at a frequency that is just on the threshold of the missing pulse detector. (Slightly too low a frequency.) Do you have an oscilloscope to check the pulses though the board ?

Les.
Unfortunately I do not have an oscilloscope.
 

KMoffett

Well-Known Member
I thought this was about the module for "Edge Sensors" not optical beams.
 

Ziedar

New Member
A photo of the back of the board would be useful. None of the components on that are complicated or expensive, so repair should be possible.

If the fault is time-related it could be the capacitor, but that is just based on the fact that electrolytic capacitors degrade with age.
Disregard the left-over soldier globs from removing the screw terminal connectors in order to access the board.
 

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KeepItSimpleStupid

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Most Helpful Member
Measure the voltage between pin 8 (Gnd) and 16 (Vdd). The diagonal across from the DOT in the corner. It could be on your working board.
The datasheet has the wrong package, but the pin #'s would be the same. DIP packages are getting hard to come by.

Pin #1 has some sort of marking, usually a dot and you count counter-clockwise around the IC. Pin 16 is opposite of pin #1 Pin #1 often has a square pad.

It looks like 5V is coming from off the board.

You can check to see if Q and notQ are opposites of each other. e.g. 1Q and 1~Q. Not Q has the bar over the Q.
 
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Les Jones

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I originally thought that the TO220 device was a voltage regulator (LM78xx series or an LM317) But the way it is connected does not fit. What is the part number of the TO220 device ?.
160920.jpg
I have uploaded a rotated and mirrored picture of the etch side to make tracing the schematic easier. Can you describe how the edge detector works or post pictures of it so we can try to understand how it works ?

Les.
 

Diver300

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Most Helpful Member
The common connection of the 7812 is connected via D2 to ground. I assume that D2 is a 5 V zener, as Vcc (pin16) of the IC is also connected to that.

That makes the output of the 7812 to be about 17 V, which powers the big resistor as a current source to the sensor.

The 5 V relay is supplied via D5, so I guess that is about an 11 V zener to drop the voltage to the relay to give about 6 V. The IC drives the relay via Q2 and Q3 connected as a darlington pair, so there'll be about 1 V drop there, leaving 5 V to run the relay.
 

Diver300

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Most Helpful Member
R1 provides around 30 mA to the sensor. D3 limits the voltage on the sensor, and C4 will reduce the effect of noise.

When the sensor activates, the voltage on the sensor wire goes low. That is connected via R2 to pin 1 of the IC. Pin 1 is the not-A input, and when that goes low, the output, pin 12, goes low for a short time. Pin 12 is connected via R4, Q2 and Q3 so when the pin 12 is high, the relay is on.

I think that in normal running, there are continuous pulses from the sensor, so pin 12 is always low. When the pulses stop, the pin 12 of the IC goes high and the relay operates.

When the relay operates, it connects T5 to T6, the ones labelled "sensing edge input". I guess that reverses the motor. I don't think that the relay can operate for long times, because D5 would get quite hot if it were run for a long time.
 

Les Jones

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Most Helpful Member
Hi Diver300, I agree with your conclusion. I had assumed at first that T5 and T6 were inputs to the board but they are outputs, The two wire IR beam brake circuit seems to be the standard way that the transmitter and receiver are connected. (The TX and RX units are connected in parallel.) I don't understand why they used a relay with a 5 volt coil which required D5 (Which must be a zener diode.) to drop the voltage from 17 volts to 5 volts. I think if the OP does not have an oscilloscope and pulse generator the best he can do is DC voltage checks and check for collector emitter shorts on Q2 and Q3.. (They seem to be in a darlington configuration..)

Les.
 

Diver300

Well-Known Member
Most Helpful Member
The design could be driving the relay at a higher voltage than 5 V if it doesn't need to be on for a long time. That would make its activation time shorter.

I think that the time that the relay is on has to be limited, or the zener in series with it would overheat.
 

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