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Final Assembly

We’re almost there. Now all we have to do is attach the power and the motor wires to the control circuit and then the control circuit assembly to the top of our walker. It goes something like this:

Measure out enough of the positive (red) wire coming out of the back of the servo motor so that there won’t be too much slack in it when the controller gets glued to the top of the motor. In other words, you’ll want to solder the wires on before you glue the control circuit in place, but you don’t want to end up with a big loop of excess wire when the circuit is connected and glued in place. The positive wire will be connecting to pin 9 on the IC socket.

After you’ve measured and cut the positive motor wire, measure and cut the negative motor wire to the same length. It will connect to pin 12 on the socket.


If you really want to be a fancy-schmancy bot builder, you can put heat-shrink tubing over the wire join connecting the battery packs. To do this, slide a piece of heat-shrink tubing onto one of the wires before you wrap them together. After they’re wrapped and soldered, slide the tubing over the join and heat with your micro-torch or heat gun.

Measure the unconnected red wire from the switch so that it will comfortably reach pin 20 on the IC socket when it’s glued to the top of the servo. Cut it.

Measure the top negative wire from the battery pack so that it will comfortably reach pin 10 on the IC socket when it’s glued to the top of the servo. Cut it.

Now solder all of these wires that you’ve cut to the appropriate pins on the IC socket: positive wire from motor to pin 9, negative wire from motor to pin 12, positive wire from switch to pin 20, and negative wire from battery pack to pin 10.

You can see the end result of each of these first five steps in Figure 7.32.

Figure 7.32. The power and motor wires connected to the appropriate pins on the IC socket.

Now that all the wires are connected to the IC socket, you’re ready to plug in the 74HCT240 chip (if you haven’t already). Gently press the chip into the socket holes. Be careful not to damage any of the components and solder joins on the other side. Test fit the control chip assembly to make sure that it can sit on top of the motor. You might have to gently bend the pins so that the socket sits relatively level. It is going to be slightly funky and it will be resting on the leads you soldered on. That’s okay.

Before you glue on the control circuit, toss in some batteries, flip the switch, and make sure everything works (with the control circuit just hangin’ around on the ends of the motor and power wires). If it doesn’t work, check to make sure all of your connections were done properly (go back over the preceding steps and check each connection).

Now you’re ready for the last step: fastening the Bicore circuit to the top of the walker using epoxy. Test fit the control circuit first. You might have to bend some of the parts on the circuit, especially the caps, resistor, and motor and control connections to make sure the chip lies as flat as possible and pins/wires don’t touch each other. If wires touch that aren’t supposed to (such as a cap and a resistor lead), the circuit will not work. When the chip is prepped, mix up some epoxy, pile it on top of your motor casing, and squish the IC socket into it. Hold it until it stays. If you think you might want to monkey with the circuit some more, you don’t have to glue it down, you could just use some poster putty to temporarily hold it in place.



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