Modifying servos for continuous rotation

My son was thrilled by the “Super Ant Weight” (250g) Sumo Robot that he helped build and fight at the Robot Wars stand at the Model Engineering Exhibition at Alexandra Palace earlier this year – he was beaming from ear to ear when his robot won first place overall out of 4 robots, 3 battles each round to determine the winner.

I had a look at what was involved in making one and it looked pretty straightforward – a battery pack, a receiver and 2 servos converted to continuous rotation.

He has been pestering me ever since to buy the items so he can make one, so with my last Hobby King order, I finally ordered the parts.

First off a 4 x AA battery holder  at $1.44,

then a couple of HK15138 standard analog servos at $3.25 each

I already had a few OrangeRx R615 receivers for which I paid $6.99 each

I actually bought these from the EU Warehouse because the UK Warehouse is almost permanently out of stock and the EU warehouse happened to have some.
Next up the servos need modifying for continuous rotation.
First unscrew the 4 screws and pull/push off the top cover.
Remove the final drive gear

Close-up of drive gear with tab still attached.

Close-up of drive gear with tab removed.

I used a sharp knife to remove the tab, the two cuts took just a few seconds.

Close-up of the rear of the drive gear showing where the potentiometer attaches.

Close-up of the same gear once this area has been drilled out with a 4mm drill.

I used a battery powered hand drill and just held the gear in my other hand with some tissue paper to stop it slipping or collecting any dirt.

Gears re-installed and checking the central position with a servo tester ($5.82 from Hobby King).

This step is not essential and can be done by eye, or simply left at the factory setting.

With this tester minimum rotation is 70 degrees, maximum is 230 degrees, so the central point should be 150 degrees, however the transmitter still moves the servos with this as a mid point, so I set it at 149 degrees, which is also how the servo comes straight from the factory.

All the parts connected together for an “Super Ant Weight” Sumo robot, total weight including batteries is 198g, so 52g still available for cardboard body and decoration.
You drive the robot like a tank, so each servo is on a different stick. Both up for forwards, both down for reverse, left up and right down for right, right up and left down for left, both in the middle to stop.
All that is left now is to mount the items on a board and make some cardboard wheels to attach to the servo horns and we are ready to go.
In all the cost of parts is $14.93 ($1.44 + 2 * $3.25 + $6.99) or just under £9.00, well worth it in my opinion for hours of fun for children of all ages.
The most expensive part will be the transmitter, although you can pick up a new Devo 7E for under £40.00 on ebay from China, that can then be flashed with the deviationtx firmware.
Just create a new model on the receiver set the transmitter type to DSM2 6 channel, disable the channels you are not using (I am just using Throttle for the left stick and Elevator for the right stick), bind to the OrangeRx R615 receiver and you are good to go.

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