On the previous page, you saw a video of the Back And Forth Mark II robot. There was also discussion of the wheels, motors, and their mounts. What controls the motors and supplies them with power?
The motherboard follows the same circuit as the original Back And Forth robot. In that article, there are a number of pages that describe the schematic and how it operates. The overhead images of the Mark II robot (see below), shows the same central chip (IC1), power switch (SW1), and obstacle switch (starting with SW2 and SW3).
The primary differences are:
Back And Forth Mark II overhead
The motherboard features large holes near the edges to connect multimeter hooks during testing (see GND and +6VDC in the image above). Some cool images are included on the motherboard, but are covered by parts on the assembled robot. Below is an example image underneath the motor driver chip socket.
8 pin dip motor driver chip artwork
The board’s solder mask is a beautiful dark purple and the holes and pads are gold plated. This is standard for boards manufactured by OSH Park.
To see the motherboard circuit, click on the file and save it.
Download Copper Connection, the PCB layout software.
Free to display, edit, and etch at home.
One of the things that I like to experiment with on new robot designs is different batteries. The original robot had 1/3 lithium batteries. The Mark II has 1/2 height AA alkaline (4LR44 / 476A / PX28A / A544 / K28A / L1325). The battery is commonly used for invisible fence dog collars (Amazon $5 for 10 pack)
Even though the battery is half the height of a standard alkaline cell, it produces 6 V, not 1.5 V (or 0.75 V, I suppose). Why?
Alkaline 6V 4LR44 battery peeled open
Surprise! Inside the battery are four individual cells.
Surprise again! Each cell is an individually labeled stand-alone 1.5 V LR44, such as would be used in a digital caliper. So, that’s 1.5 V + 1.5 V + 1.5 V + 1.5 V = 6 V.
I guess it was easier to reuse cells from an existing product line than it was to create custom cells for this particular battery. They did need to add some washers and terminals.
Battery contains tabs, washers, and individual cells
In an attempt to standardize my future battery purchases, I tried putting four LR44 batteries into the battery holder without success.
Four LR44 cells are not quite long enough to replace half AA battery
This does raise an important concern: coin cells (like the LR44) cannot produce much current.
To compensate, this robot has a pair of batteries in parallel.
That means the voltage is the same (6 V, not 12 V) but the capacity and maximum current doubles because two batteries are doing the work of one.
If you scroll back to the top of the page, you’ll notice one battery on each side of the motherboard for mechanical balance. Furthermore, the batteries are directly over the motors. The weight of the batteries improve tire traction.
The battery holder is made by Keystone Electronics. It has the amazingly simple part number of “108”. You can get it from Mouser for $1.50.
Keystone Electronics Half AA 108
The holder is made of a decent quality plastic, and isn’t flimsy like most cheap ones. It has a nice feature, which is a nub or peg on the underside of only one side of the holder. This makes certain you insert the holder onto the board with the correct battery polarity.
Plastic nub for polarity on Keystone Electronic battery holder
One last thing worth mentioning is the way in which the switches are soldered. Instead of soldering them flat as is intended, each switch was tilted up or down slightly. Theoretically, this may provide slightly more obstacle coverage.
Back And Forth Mark II side view
The new version of the robot is definitely more capable of performing its functions compared to the original robot. It is attractive and reasonably rugged.
On the downside, it is expensive to produce due to the cost of four motors. It seems like overkill for a basic robot. Also, even the pairs of LR44 batteries are undercapacity for this amount of current drain. It isn’t going to win an electrical endurance contest.
If I were going to make a Mark III, I would switch back to a single motor. The single motor would be attached to all four wheels such that they would all spin at the same rate. In doing so, I theorize that the robot will drive fairly straight, which may be a benefit to certain contests and school projects.