A common failure mode for the Kravitz Lab FED3 is a broken/burned out “beam break” or photointerrupter module. The can happen either because the lenses are marred via scratching or chewing, or the module itself has outlived its stated “50% degradation over 5
years” (see the SHARP GP1A57HRJ00F Datasheet or purchase them on DigiKey).
Note: These modules are not easy to replace; they have small leads and are placed in small PCB through holes (high aspect ratio), so it can be difficult to reflow solder enough to get all 5 pins free. We have tried this with manual desoldering (e.g., solder wick) methods but it almost necessitates a dedicated desoldering machine like the JBC Tools CS Precision Desoldering Station.
Testing the Photointerrupter Module
The photointerrupter has two pins for power and ground on each side to power the transmitter and receiver, respectively. The receiver side has an additional pin for the signal: as seen below, this is the middle pin.
To test the module for proper operation:
- Power the FED board either using a regulated 3.3V (as shown) or USB power. Since you are probing a pin, I recommend a current-limited supply in case you accidentally bridge pins during testing.
- Touch an oscilloscope (or multimeter) lead to the “middle” signal pin; ensure the other side of the scope/meter is grounded to your power supply or board for proper reference.
- Use your finger or a tool to break the photointerrupter on the bottom side and observe the signal (these modules should go from HIGH to LOW when an object breaks the beam).
Additional Testing
Even if the photointerrupters work a their main solder point, it’s possible that corrosion is prevent that signal from reaching the microcontroller itself. If you suspect this is the issue you may try to probe the microcontroller pins themselves according to the pinouts below.
Desoldering the Photointerrupter
By utilizing some solder flux and gentle pressure in a circular motion, along with a desoldering tool with ample heat and suction, the solder can be reflowed and removed from each pin.
The small pads on these boards have a tendency to lift/peel/scratch off, so be very gentle with the tip of the desoldering iron and do not apply heat for longer than required to reflow the solder.
When you have completed the desolder/resolder process, follow An Unofficial FED Cleaning Protocol to properly clean the boards before performing a final functional test.
Troubleshooting
If you can not get all the solder out of a hole you have a few options.
- If only a subset of the pins are stuck, clamp either the photointerrupter or the PCB, apply heat to the stuck pin with a soldering iron and pull the parts away from each other. Use may want to add more solder (and flux) to the pad to aid in the reflow process.
- If multiple pins are stuck, you may want to try to introduce fresh solder to them and retry desoldering. Solder will itself aid in the heat transfer. Unless you have experience with low-melt solder, I do not suggest trying to use it here.