Eric Ernst

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10 years, 5 months ago.

common ground problem billy bass?

Hi,

stepping further into my bmbb project I have come to a problem that stuns me. I think I know the issue but not the solution.

The billy bass is a fish powered with two motors. The motors run fine directly connected but not when hooked up to the PWNOUT.

I have made a LED to see if the PWNOUT switch at the time given to rule out a software error. That works fine. I connect the LED to GND and let's say p23 and the light blinks when it should. Measuring 3.6 volts that is also enough for the motor. As soon as I hook up the motor (+) to p23 and (-) to common ground on the breadboard. Nothing happens. Even if I hook up the LED in series the light hardly comes on an I measure a 0.15 volts. It's like there is not enough amps to run the motor and the voltage collapses. I for sure have enough juice. The entire setup is powered by a TTi external Power supply using 6 volts and 2 amps.

Driving me insane. I am so close to get this working. It must be something pretty basic.

Any help greatly appreciated.

2 Answers

Wim Huiskamp
10 years, 5 months ago.

What exactly is the input circuitry for the motor.

  • Are you trying to drive the motor directly from the mbed outputpin? No chance that this will work: the motor needs way too much current for an outputpin. No matter how gigantic your mbed powersupply is..
  • Is the motor a servo that needs a PWM input? This should work unless the input circuit load is too high. You will need an amplifier transistor or FET.
  • Is the motor input a simple transistor? This will need a series resistor or the threshold voltage of the transistor will more or less shortcircuit the mbed output.
  • The LED should also only be connected to mbed using a series resistor. You may get lucky and not damage mbed output pins because of some internal current limit and because the LED has a threshold of several volts ie close to mbed output voltage.

Accepted Answer
Eric Ernst
poster
10 years, 5 months ago.

thanks wim,

that makes sense now. looks like my pcb is different then in the cookbook. must be an evolution. I have two capacitators 10V 100uf and one transistor ss8050 on the old pcb. Anything I can solder inline with the motors to get this running. It's more like a proof of concept it doesnt have to be pretty. I can also get more elements here if there is anything you can recommend. Thank you.

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You will have to reverse engineer the driver circuitry to figure out how to connect the pcb to mbed. Some good pics (top, bottomside) may help us to take a look also. I noticed you mentioned 2 motors in your BMBB whereas the cookbook version seems to have 3 motors. Anyhow, the single ss8050 transistor (NPN type) may be driving one of your motors, but not both. Are you saying that in your BMBB the single transistor and the capacitors are ALL components except for the motors? In that case you may as well rip out the pcb and use your own motor driver. Some examples of circuitry are here and here.

posted by Wim Huiskamp 15 Nov 2013