Boe-Bot Antenna: How does it work?

Mrincognito

Is the antenna sensor just a normally open SPST switch? I don't have the kit's antenna and looking at them on Parallax's product page, I notice that they seem to be a pair of wires that will make contact (completing a ckt) when the robot runs into a wall.

On the circuit board, there are two inlaid rings onto which stands (to which the antenna are fixed) are mounted. I wonder how this is related to the circuit diagram (see attachment). Why does connecting the antenna connect both antennae to the ground? Before the antenna are connected, isn't the antenna connected to ground (the chassis already)?

Duane Degn

The whiskers are connected to ground (the holes in the corner of the board are electrically connected to ground). Yes the whiskers act as a SPST switch which is normally closed. When the whisker bumps something it pushes it away from a header pin inserted into the breadboard. This pin in the breadboard is connected electrically through a resistor to Vdd and directly to the an I/O pin (IIRC). I'm not sure your schematic is correct. I think your schematic could work as long as the resistor closest to the I/O pin is much less than the resistor connected to Vdd. I'd use 10K for the Vdd resistor and 330 220 ohm for the I/O pin resistor.

So the pin is normally connected to ground (possibly with a weak resistor, though I don't think you really need it as long as you don't make the pin an output) but when the circuit is broken it is pulled high. At least that's the way I remember it. There are lots of different configuration that could work but I'm pretty sure all of this is pretty well described in the Robotics with the BOE-Bot book so it should be too hard to find the exact circuit they use.

JLocke

This is from page 149 of "Robotics with the BOE-Bot v3.0":
Each whisker is both the mechanical extension and the ground electrical connection of a normally open, singlepole, single-throw switch. The reason the whiskers are connected to ground (Vss) is because the plated holes at the outer edge of the board are all connected to Vss. This is true for both the Board of Education and the BASIC Stamp HomeWork Board. The metal standoffs and screw provide the electrical connection to each whisker.

When the antenna (or whisker) hits something, it will bend back and contact the matching header on the breadboard, which is providing the other side of the switch circuit. Each input pin will be high until the whisker contacts the header, pulling the input low.

Page147 from Robotics with the BOE-Bot v3.0.pdf

The circuit in the manual has the two resistors on the input pins at 220 ohm, and the resistors to VDD at 10K.

Duane Degn

Here's the schematic from V2.2 of the BOE-Bot book (page 168).

Mrincognito

Ahh the header pins! I thought those were used as dummy-inserts.

The function of the whiskers and the circuit diagram make more sense now.