Robotic Project

dmigadmiga Posts: 6
edited 2005-02-08 - 22:45:29 in Robotics
Hello all,

To briefly explain what I am trying to do;
Use the Stamp and Board of Education to drive an RC car DC drive motor and steering servo. The project goal is to simply drive the vehicle (autonomously) straight 10', turn right 90 degrees, drive another 6', stop and launch three raquetballs towards some (bowling) pins.

I think it would be best if I did not drive the steering servo·directly from the BOE and keep·a separate supply for the servo,etc.·The DC drive motor is powered by the RC rechargeable battery pack (tested at 7.8V and 19.5A no load). We also decided to use a spring mechanism for each ball each having a "trigger" that would be controlled by the microcontroller. The people responsible for this part of the design thought that they would use a spring loaded plunger mechanism inside a tube that will launch the ball when ready. They intended the trigger that would release the plunger assembly to be a cotter pin that would be pulled out by a small electromagnet. I purchased a ULN2308A Darlington Array to take the signal out of the microcontroller and hoped to use it to control a relay that would activate the electromagnet that would pull the trigger.

We could not get the ULN2308A to work. We tried different power supplies for the chip and never got a current out of it when the input to the ULN from the Stamp I/O was high (we used four AA batteries and then tried a 9V). We tested the outputs of the ULN and thre was no (measureable) current. We were hoping to drive a relay with it. I heard that the ULN2308A safely isolates the microcontroller signal from possibly damaging current and EMF. Also, the small trigger electromagnets that were supposed to activated by the relays were not strong enough to pull the trigger pin. We never got the relay to work with the ULN2308A but tested the mall electomagnets directly with a supply.

I realize that this is not a robotics message board but you all seem to have a lot of well rounded experience using microcontroller for projects.

I need to power the microcontroller, a steering servo, drive motor (RC battery), ULN2308 chip, and some kind of trigger or ejection mechanism for the balls.·I would like to know the best way to power everything else so that the Stamp is used to accomplish this seemingly simple task. Is it possible to tap into the RC battery supply to power the servo, ULN chip, and trigger mechanisms? Should I try to power the servo again using a 9V battery (I do not believe there is enough amperage). I believe that I need to separate the microcontroller supply from the servo supply.

As if this post is not long enough (sorry all) ... if there are any suggestions from people for a mechanism that will either pull the trigger pin for the launch mechanism or a completely different approach, I would be grateful. I thought of replacing the spring/trigger mechanism with a plunger/electromagnet system similar to pinball machines. Or maybe some self-contained device that will allow you to compress its spring and apply a voltage to release it. I thought of using servos to pull the pins too but, no matter what is decided, there is still the power issues to address.

I am obviously not and electronics guru so please be gentle with your suggestions, if you have any.



  • Jonathan AllisonJonathan Allison Posts: 96
    edited 2005-02-07 - 04:08:19

    Try using a solenoid for pulling the "trigger". They work from electromagnetism as well but have a higher torque rating.

  • allanlane5allanlane5 Posts: 3,815
    edited 2005-02-07 - 15:03:26
    1. How you measured the current from the batteries under "no-load" I can't figure out.

    2. The Darlington array 'sinks' current when switched on. You don't get current out of it.
    I believe you switch it on by providing a 'OUTPUT HIGH' to the base. The device
    then becomes a very-low-resistance path to ground. Please see for more info.

    3. A TIP120 3-terminal device may be more what you need, as it will sink 5-amps.
  • dmigadmiga Posts: 6
    edited 2005-02-08 - 21:34:01
    I was told of the voltage and amps on that battery. I believe he just used a voltmeter to measure the voltage of the contacts then measures the amperage on the battery contacts without anything else connected.

    I did get the UNL2803 chip to work and have not decided if it will be necessary as of tey because we do not know if we will be using any type of solenoid, etc. that will draw a large current. I got the Darlington to trigger a 5V relay that I got from Radio Shack (#275-232) that says it has a contact rating of .5A at 120V AC and nominal current of 20mA. I'm not sure what this reaaly means in our plan but I do not think I can drive a solenoid or such with it. With this setup, the microcontroller is putting out a small 20mA current to the Darlington which can sink 500mA. Unless the relay attached to the Darlington can handle large currents (>>500mA), what is the purpose of this setup that was suggested to me (by someone else)?

    Your suggestion of the TIP120 3-terminal device might just be the ticket if we need it. Thank you for hte suggestion.


  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-02-08 - 22:10:39
    While a battery can handle a "no load" for the period of time to take a measurment, this will undoubtedly place the battery in a state way beyond its specified range, trying to drive a battery in this manner for any appreciable length of time will cause the battery to significantly reduce its lifetime in the best scenario, leak casutic chemicals in a normal scenario and violently explode in the worst scenario.

    I nearly ruined my GPS in a cross country trip because the rechargeable batteries I was using I would recharge each evening and use again the following day. On the fourth day the GPS died after only 3 hours of use (was getting 8-9 hours per charge), I opened the battery container and acid and dissolved plastic came pouring out, requiring me to pull over at the nearest grocery store, buy some baking soda to neutralize the acid, gently cleaning the compartment and filing and retinning the quickly rusting battery contacts.
  • steve_bsteve_b Posts: 1,563
    edited 2005-02-08 - 22:42:12
    was the failure mode on your batteries just from overcharging? (boiling off?)


    "Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-02-08 - 22:45:29
    Nope they were clean and dry when I inserted them. The casing was slightly expanded and cracked when I removed them (just 1 of 4 did this, the GPS works just fine on other batteries, normal current draw), likely due to overheating from discharge.
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