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Help on the right parts for a beginner

Hi, I want to create a prop for a Halloween costume that relies on a small, battery-operated servo that rotates in one direction, pauses, rotates back, and repeats this loop indefinitely. Ideally, I want it to be powered by a small but powerful battery to last multiple hours, and to aid in power consumption I want to be able to turn it on and off when not in use with a simple button/switch. (I don’t know the exact strength of servo I need, so I am going to buy the Sub-Micro Servo 3.7g, Power HD HD-1440A, and HD-1600A to test out. I plan to control it with a Micro Maestro 6-Channel Controller.)

I noticed that Pololu only sells NiMH batteries. I was thinking of using a 4.8V, 2200mAH battery. However, I noticed another site has a 3.7V, 2500mAH Lipo battery that is much thinner and lighter (I also already have a Lipo charger so would not have to purchase a new one just to recharge the NiMH for this project). But the Maestro controller needs 5V as a minimum. Could I use the 3.7V Lipo in tandem with a 5V Step-Up Voltage regulator? Would a 3.7 V battery even be sufficient, and would the torque of the servos be reduced significantly? And if I get the partial Maestro without the pins to conserve space, is it correct that I would need to solder and resolder the battery every single time I need to recharge it?

Would a Rocker Switch work for on/off functionality, and would I wire it between the battery and the controller? I noticed Pololu sells “Power Switches”, but I don’t quite understand their specific use, and I want a small switch that is not on a PCB. I also saw the Mini Pushbutton Switch, but I assume this cannot be a “global” on/off and would only work (if I program it via the controller) to command the servo to begin/end its movement, meaning the controller would be drawing power constantly, even when the servo is not in use (would the power-draw be significant, or negligible?)

Thank you in advance for your help.

Hello.

First, thank you for linking to all the products you mentioned in your post. It makes it much easier to help.

You could use a step-up voltage regulator to supply a more appropriate voltage to the Maestro. I recommend using a 6V regulator so you are not operating right at the edge of the Maestro’s limit. The 6V version of the U3V50F5 regulator should be fine.

All of the servos that you mentioned list their operating voltage as 4.8V to 6.0V, so a 3.7V battery is not an appropriate power supply for the servos. If you still want to use a 3.7V battery, you can use the step-up regulator to power the servos as well as your Maestro. Servo performance is directly proportional to the supply voltage, so you should see better speeds and more power with the higher supply voltage.

I do not recommend soldering a rechargeable battery directly to the Maestro. If you do not want to install the header pins, I recommend soldering a connector that corresponds to the battery you are using, so you can disconnect the battery to recharge it. Alternatively, if your application allows it, maybe you could use right angle headers to reduce the profile maestro.

The mini pushbutton switches are momentary tactile buttons, so they would not make sense as a power switch. A rocker switch would work for controlling power, and yes, you would place it between your battery and maestro (or between your battery and regulator if you use one). Our Pololu Power Switches are intended as alternatives to mechanical switches like that rocker switch. You can read more about the differences between a Pololu Power Switch and a mechanical switch in the “Benefits and limitations” section under the description on the product page for the Pololu Power Switch.

By they way, we would love to see your Halloween costume when it is done. Feel free to share your costume in the Share Your Projects section of our forum.

Grant

Thank you so much for you response!

Regarding the battery situation:

  • Can I use one of your 4.8V batteries with the Maestro, even though it is rated at 5V? I assume a 4.8V battery is at a higher voltage when charged. If do I use 4.8V, do you have any idea what percentage of the battery I will actually be able to use before the voltage drops too low for the Maestro?

  • For the 6V regulator, am I correct in looking at the graphs that as long as my servo and controller are not drawing more than 500 total mA, a 3.7V battery will last approximately 90% as long as if it was operating without the voltage regulator?

For the power switch:

  • If I did get the Rocker Switch, how exactly would I wire it to the battery? Let’s assume I use one of your rechargeable batteries that is terminated by a “JR”-style connector. Would I need to cut off the connector and strip the battery wires, and then attach terminals to the ends to be able to hook up to the rocker? I do not see terminals on your site, but know a local store that has them.

  • Would I then need to attach Female Crimp Pins to the two wires that will be connecting to the Maestro in order to simulate the normal battery connector?

Thanks again!

I do not recommend powering a Maestro directly from a 4.8V battery. While it will be over 5 V for some time while fully charged, it will leave you almost no margin for dips on the supply line, which can happen from noise or when actuators draw large bursts of current.

That calculation is not correct for battery life. The graphs on the product page for the regulator are showing efficiency of the regulator. To calculate the difference in power usage, you can use the following equation:

power in × efficiency = power out

Power is current times voltage, so if the regulator has 90% efficiency with 3.7 V in, the input current will be approximately 1.8 times bigger than the output, which means the battery will drain approximately 1.8 times faster than if it were supplying that output current directly.

The switch should be in-line with the power going from the battery to the regulator, so that you can use it to break the connection between the positive terminal of the battery and VIN of the regulator. If you have access to the corresponding connector for your battery, it would probably be easier to install that into your system than changing your battery contact. You should have a battery connector in your system, but it doesn’t have to be right at the Maestro. The connector should be right before the battery.

If you are still unsure about what connections to make, you can post a purposed wiring diagram, and I would be glad to take a look at it.

Grant

Thank you so much for your response. Below is a crude diagram – does it look OK? I ended up buying the 3.7V, 2500mAh battery, and this tactile on/off switch which I liked a little better than the rocker. (I also bought the U3V70F6 Regulator because the Note on the U3V50F5 recommended it for less than 20V.)

I also have a question regarding current and battery capacity. Let’s assume I am using the HD-1440A Servo, which has a stall current of 320 mA. The Maestro says its supply current is 30 mA, so the total current used is 350 mA assuming worst-case of constant stall during operation.

Using your calculation above of the battery draining 1.8 times faster with the voltage regulator, it seems my 2500 mAh battery is akin to a 1388 mAh battery in this scenario. So using the 350 mA figure, should I expect the battery to last around 4 hours (1388 mAh/350 mA)? Or will I need some sort of resistor to limit the actual current? I am looking to maximize battery life. Thanks again.

Your proposed wiring looks fine. Your calculations seem fine too, but as you note, your calculation are for a worst case scenario, so you should get better performance than that because the average current draw of the servo should be significantly less than the stall current.

Can you just do some tests with this setup and see what kind of battery life you get? (By the way, be careful not to over-discharge your battery!)

Grant