Introduction: Garage Door Obstruction Sensor Repair
The Chamberlain and Liftmaster garage door obstruction sensors seem to have a failure problem. These sensors use an IR (Infrared) beam, between a transmitter and receiver, to ensure the area under the door is clear before lowering the garage door. These sensors have generated a lot of web based commentary about their failures, all of which revolves around realigning the sensors or checking the wiring. In several instances various schemes for bypassing the sensors are presented, which is a safety hazard and can result in injury or damage. None of the articles, blogs, or YouTube videos, that I reviewed, do a failure analysis to determine why the sensors fail or how to repair a failed sensor.
Recently, I experienced a second failure of the sensor and decided to determine the specific cause of the failure and how to fixit.
1/8” drill bit
1/8” pin punch
47 ohm, 0805 SMD Resistor
Step 1: Sensor Testing and Measurements:
Figure 2 is the wiring diagram for the sensor’s connection to the controller. The white terminal is ground and the gray terminal is a 6.3 Volt supply.
Figures 3 and 4 are the waveforms between the gray and white terminals for a working sensor. The period of the sensor pulse is about 6.4 milliseconds, while the pulse width is about 440 microseconds. The pulse width was relatively stable with only a few microseconds of jitter
Figures 5 and 6 are the waveforms between the gray and white terminals for a failed sensor. The period of the sensor pulse is about 6.4 milliseconds, same as the working sensor, but the pulse width averages about 375 microseconds with a lot of jitter. The pulse width jumped around from a low of about 280 microseconds to a high of about 400 microseconds, or about 120 microseconds of jitter.
My interpretation of the high jitter value of the failed sensor was a signal strength problem.
Step 2: Sensor Testing and Measurements Continued:
The failed sensor is used with a double wide garage door, with 16’4” between the transmitter and receiver. The first test was to move the transmitter closer to the receiver. Figure 7 shows the test setup with the transmitter positioned about 8ft from the receiver. Figures 8 and 9 are the are the waveforms for the failed sensor with 8ft separation, which are very close to the waveforms for the working sensor with 16 ft separation.
This test reinforces the signal strength theory.
Step 3: Sensor Testing and Measurements Continued:
With the failed transmitter replaced to the 16 ft separation, the waveform was measured with a slower time base to observe more pulses. Figure 10 shows the intermittent missing pulse from the sensor. A missing pulse, while the garage door is closing, will trigger the controller’s obstruction response. A logic analyzer was used to capture a long stream of sensor pulses, which is shown in Figure 11. The three segments of the pulse stream shows the randomly distributed missing pulses.
The question now is: Why is there a signal strength problem?
Step 4: Reverse Engineering:
The transmitter and receiver are wired in parallel and connected to the 6.3 volt supply and ground terminals. While the sensor is functioning the 6 Volt terminal is pulled to near zero volts, which indicates an impedance in series with the 6 Volt supply voltage.
The source impedance of the 6 Volt power supply is calculated by dividing the open circuit voltage between the two terminals, 6.31V, by the short circuit current flow of 0.0623A between the two terminals. The source impedance Rs = 6.31V/0.0623A = 101.3 Ohms. Figure 12 is a simplified schematic of the controller and sensor circuit.
Step 5: Opening the Transmitter and Receiver Cases:
The cases are challenging to open. Figure 13 is the top of the case showing the latches, one on each side, that holds the case together.
One method is to use a Dremel tool to cut through the sides of the case and cut the latches off.
A second method is to drill a small hole, on both sides of the case, as shown in Figure 14. Using a pin punch to push the latch clear of the latching tab, while using a small screw driver, inserted into the case around the wire feed through grommet, see Figure 15, to apply upward pressure on the top of the case. Repeat this process for the latching tab on the other side of the case.
Step 6: Reverse Engineering Continued:
Figures 17 and 18 are of the receiver’s PCB. Figure 19 is the schematic I developed by tracing the PCB connections. The TSOP31238 is an IR receiver for a 38KHz carrier frequency using OOK, On Off Keying, modulation. The output of the TSOP31238 has a high to low transition when a 38KHz IR signal is detected. The circuitry connected to the VOUT pin is a pulse forming network that uses transistor Q2 to short circuit the 6 volt supply to ground, from the controller, for about 440 microseconds. This is the signal seen by the controller to determine if there is an obstruction.
Step 7: Reverse Engineering Continued:
Figure’s 20 and 21 are of the transmitter’s PCB. Figure 22 is the schematic I developed by tracing the PCB connections. In this circuit the LM393 implements two relaxation oscillators. IC1A generates a 250 microsecond pulse every 6.4 milliseconds. This pulse enables IC1B, which oscillates at about 38kHz for about 250 microseconds. The 38kHz signal drives the transistor Q1, which drives the IR Emitter. The resistor R17, a 47 ohm resistor, limits the current through the IR emitter. Figure 23 is the voltage across R17 displaying 10 current pulses of a 38 kHz carrier signal. Figure 24 is the timing between the 38kHz bursts.
The current pulse through the IR emitter is 2.28V/47 ohm = 0.049 amps. This current controls the signal strength of the IR beam, but the emission power of the IR emitter, for a given current, does degrade over time.
Step 8: Looking at Component Specifications:
Referring to the TSOP31238 datasheet, an IR Emitter referred to in testing it is the TSAL6200. A similar device, the TSAL6100 is also available with a small ½ angle emission pattern. The IR emitter used in the transmitter circuit does not have a part number visible, but its physical characteristics match the TSAL6xxx devices.
The TSAL6xxx IR emitters use a GaAlAs material, which impacts the IR output degradation rate. Vishay has published application note “Aging of Infrared Emitter Components” (publication number 8-12190.pdf) that documents the degradation of various IR emitter materials. The second page has the relevant data. GaAlAs material has the highest degradation rate with its output decreasing 15% over 4000 hours of operational time.
The datasheet for the TSAL6xxx emitter specifies a maximum continuous current of 100mA and a peak current of 200mA. The current drive for the IR emitter, in the transmitter, can be increase substantially.
Step 9: Implementing a Fix and Testing the Fix
I doubled the current drive for the emitter by soldering a 47 ohm, 0805 size, resistor in parallel with R17, which reduced the emitter resistance to 23.5 ohms.
Figures 25 and 26 are the IR current waveforms with R17 equal to 23.5 ohm. The peak current is
2.20V/23.5ohms = 0.0936 amps. The current is a little less the twice the original value due to additional voltage drops in the circuit due to the higher current level.
Figure 27 shows the location of R17.
The fixed obstruction sensor was reinstalled and is functional.
This fix may not be applicable to all of the obstruction sensor failures, but base on the failure mode descriptions by a number of blog sites, it may work for a significant number of them.
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