I want to suggest how it could be wired up. When there is nothing "touching," all rods remain in electrical contact with all balls: so there is an electrical path from one probe wire through the rods and balls to the other probe wire.

Imagine one of those wires connected to the same ground as the electronics in the controller (voltage = zero). As long as contact is maintained (nothing touching), the voltage anywhere and everywhere along that unbroken path remains the same (0V).

Imagine +5V connected to one end of a resistor (few Kohms) and the other end of that resistor connected to the remaining probe wire. The voltage at this end of junction, the side of the resistor away from the +5V, is 0 volts AS LONG AS there is an electrical path CONNECTING it to the ground at the other wire, but otherwise it would be indeterminate, or would "float." It doesn't float in this case, and IS determinate, because of the presence of the +5V. (The voltage is said to be "pulled up" to 5V when the short is removed, but returns to 0V when the short is present; this behavior is the source of the term "pullup resistor." The resistor is there for when the short IS present: without the resistor, there would be a dead short across a power supply, and either the supply or the short will lose, usually violently. With a resistor in between, current is limited to a reasonable value.)

A wire from the bottom of the resistor (where it connects to the probe wire, on the opposite side of the 5V) is the source of a "signal-voltage-in" "ACTIVE-HIGH" in this case because the thing of interest causes the voltage to go from low-to-high. (The very same voltage, "ACTIVE-LOW", signals when touching has been lost.)

Although I've mentioned three connections (+5V, ground, signal voltage), only two wires run from the controller to the probe: ground, and the ACTIVE-HIGH signal voltage to the signal-in pin, typ. on a breakout board, BECAUSE at or near that pin, inside the case, you'll connect the pullup resistor between that wire and +5V. (Inside the controller is the natural place as +5V is typically already present.)

BTW, I think the difficulty of understanding pullup and pulldown resistors is that people tend to think of resistors as "dropping" the voltage (Ohm's Law: Voltage drop across R, V=I*R), but they NEVER think about what that means when the current is ZERO.

According to Ohm's Law the voltage drop across a resistor when there's no current through it is EXACTLY 0 volts. BUT, connect the dots here, that means the voltage on ONE side of the resistor is EXACTLY THE SAME as on the other side. This is I think what's unnatural. If a powerline falls touching one end of a car, the voltage on that end of the car is certainly thousands of volts, and we certainly believe that the voltage on "the other end(s)" of the car is the same--but typically we think that because we're thinking of the metal being a conductor with R=0, and (mis)applying Ohm's Law about how we think about it: the voltage drop in this case is actually 0 V because it's 0 Amps X 0 Ohms.

But if a tree branch fell and hung suspended from the line where you could reach up from the ground and pull it off, would you? (NO) Should you? (NO) Your sense is (mis)telling you, "Well, it's not a dead short like an aluminum ladder, wood is an insulator!" (WRONG, and LIKELY DEAD WRONG).

The key is that--as there is 0 current "flowing" in the wood's non-zero resistance, no voltage will dropped: the voltage at the end of the branch within your reach (DON'T EVEN THINK ABOUT IT...) is still at thousands of volts.

The clever will note that, "Yes, UNTIL I touch it, after which current WILL flow through the branch [and THROUGH HIMSELF too] and the voltage WILL drop from thousands of volts [to lesser thousands of volts BUT STILL BIG ENOUGH TO KILL HIMSELF]." True, at the moment you [WHO WILL NOT EVER] touch the branch, you complete a circuit: ground at the substation through the power source, through the high-voltage line, throough the branch, through you, to the ground, back through the ground to the substation, your body will also DROP SOME VOLTAGE, in exactly the proportion that your body's resistance (salty water) is to the total resistance of you+branch--AND CURRENT WILL FLOW. How much voltage? Let me pose it differently: "Given that ALL IT TAKES to defibrillate your heart so it won't move blood is 16 MILLIAMPS, do you think you'd have to get ALL of the thousands of volts to do the job?" (If it does happen to you, for whatever reason, for heaven's sake, stay calm, and at least try to do that heart-pumping thing with your fists to give yourself CPR before you black out.)


I digress.

The point is "tree branches are pullup resistors too". That's what I meant to say... [DON'T USE THEM. WHATEVER YOU'RE THINKING, THERE ARE BETTER WAYS.]