A friend of mine took me to visit the company he works for, which builds small radio-controlled boats used for fishing. They are called "bait boats." He showed me some of the boats they make as well as the electronics inside. The boats had a speed controller, one battery meter, and a light switch.
When I saw the huge rat's nest of wires, I thought to myself, "Holy moly, what a mess in there with so many wires and connections." I asked my friend to ask his manager whether they would be interested in having me design a single board with a speed controller, battery meter, and light switch. The manager told him that I could start working on the design.
I immediately started learning as much as I could about speed controllers with reverse for brushed motors. In the beginning, I started to learn about mosfets and about driving mosfet gates. I first made my tests driving mosfet gates using push-pull made of MMBT2222 and MMBT2907, following this tutorial. The first test was OK as seen in the video I posted on youtube:
The only problem was that when the power consumption rose to a certain point, the mosfets were overheating. Unfortunately the push-pull was great, but the ringing on the mosfet gate was even greater, as shown by my ol' trusty Tektronix 2213A. Then I started reading about integrated mosfet drivers and found the TC4427 IC, which has two drivers in one SOIC8 package. I thought that using this driver wouldn't increase the price much, so I started drawing a schematic and designing the board in Eagle.
I used one cheap ATtiny24 as the brain, one L7805AC2T for 5V regulation, one TC4427 mosfet driver, two IRFZ44S N-MOS, and two IRF5305S P-MOS. This is the first testing of the board to check how well the TC4427 was driving the mosfet gates:
The software was made in WINAVR, and I was using USBtiny AVR to program the ATtiny24A. After that test I knew it was the way to go! After spending a few days thinkering with the algorithm used to control the mosfets, I managed to find the best and easiest one, which was providing nice soft break when changing rotation direction or decreasing the speed, and a user button for throttle interval programming in an easy manner. The soft break is a must for a longer life of the transmission mechanics of the boat.
In the beginning the speed controller does not have the throttle interval programmed, and you can see how easy is to program it! Now this controller is used in my little foam boat.
After the speed controller algorithm checked out, the next step was designing the board for the little bait boats. In a few days the result was a speed controller with light swich, buzzer for battery level alarm, and button for programming throttle interval. It consisted of two boards: the control board and the power board. I used an IRF4905S P-mosfet and IRF3205S N-mosfet for greater power and the same TC4427 mosfet driver.
The guys have been pleased with how the speed controller works, and now they use it in their boats. The price to manufacture is a lot less than other commercially available speed controllers, and the soft brake makes boat owners happy. Currently, I'm working on another version with four mosfets for forward and two for backward, because the bait boats need full throttle forward and half throttle backward to protect the propeller from the algae.
This story was submitted by George Bogdan for our Frankenstein's Fix contest. The winner has a chance to win a Tektronix MSO2024B scope! Send in your story of repairing or fixing something to enter. Deadline is October 26, 2013.
Nedelcu Bogdan Sebastian is 30 years old and studied computer science in high school and finished Politehnica University in Romania. He loves microcontrollers and embedded programming. He has just finished a successfull Kickstarter for the Matchbox ARM development board.
The Frankenstein's Fix has just come to an end. Stay tuned to read the submissions and see what kind of difficult job of judging we have ahead of us! Submission details and full contest rules here.