Advanced component testing
Most of these test will require at least a multimeter. A basic multimeter should be able to measure voltage (DC) and resistance. A nice feature is a continuity setting where the multimeter beeps when the resistance measure is below a few Ohms. Resistance and continuity tests must be done with power disconnected. Many multimeters can also measure current flow which can occasionally be useful. More advanced multimeters can measure frequency and period of a pulsed DC signal; these are also visible on an oscilloscope.
If your multimeter has common probes that are about 2mm diameter (3/32") with a fairly blunt point, you will not be able to make good contact with the wires in many of the connectors (which are only about 2.5mm apart from each other). These can be upgraded if you wish; I have some "aftermarket" multimeter probes that are so finely pointed I've drawn blood more than once! (Blood sacrifice not required nor recommended.) More immediately, if you have some 24-28AWG solid wire, you could remove the insulation from a couple inches and wrap it around the multimeter probe to make a more precise extension. In a "pinch" you could hold a sewing pin or needle against the side of the multimeter probe and use it for basic continuity checks. If you need to do this on both sides, a second person can help as it tends to slip as soon as you look at the other one.
Slip ring and controls cables
These cables are somewhat delicate and might break. If any of the laser, servo, tape sensor, or external controls are misbehaving a good first step is to make sure that each conductor in these cables has continuity.
The most common mode of failure is a break in the wire at the crimp connector that plugs into the board. Often this will be visible as the wire will come loose and lift out of the connector housing, but sometimes the insulation holds it in place despite the broken conductor. Checking continuity can help here.
You can test the connection in place, without removing the slip ring or "knobs" connector if your probe (or extension wire/pin) is thin enough to slip in beside the wire from the top. Otherwise, carefully remove the connector by pulling up on the plug housing, not on the wires. On one side you will find the openings into which each crimp terminal locks into the housing. These should provide a large enough access point to test.
The slip ring is fairly easy as the color coding is the same on the circuit board and in the rotating arm. Unplug the failing device and check for continuity on the wires that go to it: blue and green for the laser; brown, red, orange for the servo; yellow, purple, and black for the tape sensor.
The controls use (most of) an standard Ethernet cable which follows the EIA/TIA 568B standard. The white-orange wire is #1 and connects to the uppermost wire (nearest the sliders) that goes across the top of the jack on the controls board. Both #1 (white-orange, 3.3V) and #2 (orange, ground) are used by all functions of the controls. #3 (green-white) is the signal from the left slider (rotation); #4 (blue) is for the middle slider (top angle), #5 (blue-white) is for the right slider (bottom angle), #6 (green) carries about 1.65V when the controls switch is pressed, and #7 (brown-white) is for the controls piezo buzzer. We do not use #8 (brown) but that position in the jack is also connected to #1 on the board.
If one or more of the wires on these 7-8-position plugs does break, please let us know (david_h_brown@uri.edu). We expect these can be repaired by flattening the locking tab of the crimp terminal and removing it. This will then be replaced by a pre-crimped length of wire and joined to the original wire (cut off just enough to see clean wire in the insulation) using a telephone-style splice connector. We have obtained a small supply of these parts and hope no one needs them, but can send them as needed while the supply lasts.
If you prefer to repair the cable yourself, these are JST PH-style plugs.
Scarecrow power general notes and test points
This section is primarily for background understanding of the scarecrow design. If the LED on the microcontroller illuminates, that means that all three voltages (12V, 5V, 3.3V) are acceptable.
The laser scarecrow should require less than 400mA at 12VDC (see Operating Manual section 3.3). While it can function with as little as 8VDC, know that you'll damage lead-acid batteries if run down below about 11.6V. (Do not test with consumer 9V batteries -- alkaline batteries cannot deliver the required 0.53A and while Lithium batteries can, they drop below 8V almost immediately at that discharge rate.) The scarecrow can also function with as much as 30VDC as an absolute maximum (the piezo buzzer and its driver MOSFET on the circuit board are likely going to be damaged first), so if you find yourself with a 24V solar system, for example, it should not be a problem to hook up the scarecrow directly. (The buzzer will be noticeably louder, but still not much compared to some common farm equipment.)
The "+" wire of the power input goes first to a fuse, then the key switch, then the components it powers: the stepper motor, the piezo buzzer on the bucket, and a 5V converter for the rest of the electronics. The "–" wire of the power input goes to a diode (to protect against reverse hookups) and then to the ground plane of the circuit board. Because of the diode's voltage drop, measuring the voltage between the "12V" test point (to the right of the key switch connector) and a ground test point (one is to the right of the power plug) should give a value approximately 0.3VDC lower than if you measure voltage at the input screw terminals.
5VDC power for the microcontroller, laser, and servo is generated from the input power using a buck converter at the front right of the board. There is a test point on the left edge of the board near the microcontroller.
The microcontroller operates with 3.3V logic and includes a power regulator on its own small board to generate this level. This voltage is shared with other components including the tape sensor, magnet sensor, light sensor, tilt sensor, the external control sliders (and buzzer), and control signals for the servo and stepper driver. This test point is at the back short side of the microcontroller, near the light sensor connector.
There are three self-resetting polymer fuses (PTC) on the circuit board. The first (F1) is between the 12V jack and the key switch connector. It is designed to carry 500mA indefinitely and trip at 1A. The second protects the output of the 5V regulator (carries 1.1A, trips at 2.2A) and is in the front right area of the board. The third protects the 3.3V output (carries 200mA; trip at 400mA) and is next to the 3.3V test point. You should be able to measure <1Ω across each.
Laser module
⚠ Warning: block the direct emission of the laser when testing the laser module. If the cap is too effective, try putting it underneath a rag or covering the aperture with a piece of black Gorilla / duck tape in which you've pricked a tiny hole.
The laser module requires +5VDC (on red wire; black wire is "ground") and draws between 200mA and 350mA when operating. If its current draw at 5V is very low (under 50mA) it has failed.
Servo
With the scarecrow off, you can check for continuity between the brown wire and either ground test point on the circuit board.
If you already checked continuity in the slip ring cable (brown/orange/red) and your probes are fine enough, you can test from the circuit board slip ring plug while the arm rotates (and the servo can remain connected).
To test the signals at the servo connector in the arm, temporarily disconnect the stepper motor so the arm won't rotate (Assembly Guide 1.6.3).
With the tape jumper set for self-test mode (Assembly Guide 5.1), turn it on and after the pre-laser warning has sounded, you should be able to measure +5VDC between the red and brown wires.
If you have an oscilloscope, connect the probe to orange and the ground to a ground test point or the brown wire. You should see a pulse (square) waveform at 3.3V, 50Hz with pulses varying from 750ns to 2250ns (0.750ms to 2.250ms) width.
If your multimeter can measure frequency and duty cycle, you should be able to measure 50Hz pulses between orange and brown. The duty cycle will vary between 3.75% and 11.25%.
If your multimeter does not include frequency/duty cycle, it may be able to hint at the duty cycle of the pulses by measuring the voltage between orange and brown. This should be roughly the duty cycle multiplied by the 3.3V logic level. So somewhere from 0.12VDC up to 0.37VDC.
If all these signals seem good, the servo has probably failed. (At present writing [July 3, 2022], that will be a first for the Futaba servos excepting one I deliberately damaged.)