Reconfigure NyBoard V0_1 (for pro users)

Warning:

If you are going to stick with the metal servos included in Nybble kit, this reconfiguration will usually improve the servos' stability. If you are going to use plastic servos, DO NOT try it because the direct voltage from battery will burn your servos.

I've got some users who wish to make Nybble run faster and longer. In short, there’s an easy fix to unleash the full potential of Nybble. However, I'll discuss the pros/cons in design thinking before you make the decision.

Nybble is using “digital + metal gear + aluminum alloy case” servos. The manufacturer rates the lifespan as 500K cycles. It’s the best servo I could find at its price range to make Nybble affordable. You can check my previous study on servo comparison.

With the aluminum alloy case for heat dissipation, it can stand the batteries' voltage (could be 8.4V with fresh charged batteries) directly and provide maximal output. However, there will always be tradeoff for performance:

1. The servo’s motor brush, gear system and potentiometer will wear out much faster; It’s more likely to break the mechanics if a leg hits obstacles by accident.

2. Long time running with large current (3~4A) will overheat the step-down chip and powering cable.

3. Battery will run out faster if Nybble keeps running; Larger batteries will add burdan to the servos and won’t necessarily improve the overall user experience.

4. Also consider safety. The best USB power bank outputs 5V x 2A = 10W. But Nybble could consume 8V x 4A = 32W when running.

So I don’t recommend running Nybble as a racing car, but a platform for studying programming and motion planing.

NyBoard was designed as a general purpose driver board not only for Nybble, but also for other projects. Some people want to drive their cheaper plastic servos that cannot stand 8V. So the step-down circuit for servos is introduced. The chip is rated at 5A, but the momentary current fluctuation by many metal servos could accumulate and trigger the over-current protection in the chip or the batteries. In other cases poor quality batteries simply cannot sustain a high discharging rate.

Now talk about reconfiguring NyBoard to make Nybble run faster and longer. In condition that:

1. Your battery can supply large current and its over-current protection is not set too low.

2. You are using the servos included in Nybble kit, not cheaper plastic servos.

3. You understand the tradeoff between performance and hardware's lifespan, and have spare servos for possible replacement.

4. You need to upgrade the battery holder's wires with thicker ones to avoid too much heating up.

you can bypass the step down circuit as described below:

1. Desolder inductor L2 from pad A, B.

2. Then there are two paths you could take:

• Path 1 directly wires BATT+ and pad A. L2 is removed from the board.

• Path 2 wires BATT+ to one leg of L2, and the other leg of L2 is still soldered to pad A. Add insulation (could be a piece of paper) between pad B and L2.

• Theoretically Path 2 should supply more stable current. But I don't see a big difference on the servos’s end with oscilloscope. And L2 will generate moderate heat.

I think it's better to keep L2 on the board in case you need it for other projects. So below is my modified board for making a “racing Nybble”.

You can further improve the circuit by adding an additional capacitor in parallel to the servos' power line, which is quite common for servo driver boards. NyBoard already has a 1000uF capacitor in place.

On NyBoard V0_2, the similar reconfiguration will be more convenient with a jumper selector SW3, but less convenient than a slide switch. The limit to moderate Nybble‘s running time will be done by a cool-down counter in the program.