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Consumer Product Design of DC motor at Illustrative Designs LLC

Project requirements:

  • Use USB to charge batteries that provide power to a motor that requires more than 5 Volts to run.
  • Have an LED indicator to reflect the charge state of the batteries.
  • Must be able to control the motor speed.
  • Do all this without a microprocessor.
  1. First, we looked to the internet to find a solution for this one. We needed 7.4 volts to run the motor so we chose DFRobot Gear Motor at 15 RPM 6V as Digi-Key part number 1738-1281-ND. However, it was discovered that you can’t charge 7.4 volts with a 5 volt USB.

Basically, you switch two 3.7V batteries to a series connection when charged to get your 7.4 volts and isolate them from each other when charging using transistors as switches.

We chose the “LIPO CHARGER CIRCUIT” circuit that looked pretty solid.

Since we have no uProcessor we felt like we could switch the FET inputs high and low as detailed by the Engineer for that circuits’ detailed notes with a dip-switch.  That didn’t work.  Maybe a timing issue?  On the next go-around we think we can try the first circuit.  Very similar to something we came up with on our own.

  1. Our customer wanted something like the LED Circuit Board Indicator from “Lite-On Inc.” to show the state of the batteries purchased off the shelf from Digi-Key part number 160-1067-ND shown here:

From what we can understand, batteries only drop slightly below their rated voltage just before they go kaput.  So we think the only way to give the customer the LED light bar display they desire is to have a uProcessor programmed to estimate current usage while running and correlate that with the rated mA Hour value of the batteries.

This ratio can then be applied to the Light Bar.  But since we have no uProcessor, we used a couple Op-Amps as comparators with the input of one being a 3.6V Zener and the other a 50% divider from the input voltage.  Each Op-Amp controlling a multi-colored LED (Red/Green) to indicate the charge status. Thus when the voltage drops slightly below “rated” the LED will turn red.

  1. So we needed a low profile high torque motor that would operate on 7.4V. Here is what we found from from the Manufacter DFRobot with a product description of “Gear Motor 15 RPM 6V Metal” Digi-Key part number 1738-1281-ND:


We had never worked with motors before and assumed you could just change the voltage with a potentiometer to adjust the speed.  Wrong!  You need to give the motor the voltage it wants and adjust the speed with Pulse-Width-Modulation (PWM).  So, we are pretty sure you can accomplish this with a 555 Timer, a resistor and a POT,  and an NPN transistor.  So, you adjust a POT to change the pulse width output of the 555 Timer.  The duration of the high time of the pulse determines the amount of time voltage is delivered to the motor; controlling the speed.  The transistor provides the needed current that the 555 can’t provide.  We took the easy way out and chose this:

So with the “KNACRO motor controller with digital disaply” you can hook up the standard 7.4 volts to power and the motor as indicated, with the ability to adjust the speed with the included POT shown here:

Lesson learned. Use a uProcessor! This makes things much easier as long as you know how to program the things and hopefully this Blog is helpful to some of you electronics hobbyists out there!

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