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Motor circuit for science olympiad project

  1. Feb 15, 2010 #1
    I am just learning about this and don't know if I am doing it right.
    I need to turn my car on with a push button switch and have it turn off when it hits another switch on the axle and turn on a light when the motor turns off.
    I made the circuit shown in my drawing but I have to hold down the push button switch for a few seconds before the relay will stay seated. I need the car to go the second the switch is pushed down.
    My Dad thinks I need to add a capacitor in there but I had know idea until today what a capacitor was or how to add it to my electrical drawing.

    Were only allowed to use Wires Switches, Mechanical relays, Resistors, capacitors, Motors. battery's 4.8vdc max
    The motor and battery are from a Milwaukee, 2.4 Volt Two Speed Screwdriver
    The relay is 5 volts
    The car only needs to run for 1 minute at a time

    Attached Files:

  2. jcsd
  3. Feb 15, 2010 #2
    The relay has a little coil in it that stores energy. It looks like it's normally "on" and it divides the voltage so that the motor can't run. When you push the button and short the relay, it needs a way to discharge that energy and let the switch close. It looks like you want it to be connected to some kind of ground but the ground is not connected to the negative terminal of the battery. I think the problem is that your "ground" actually goes through a diode to return back to the negative terminal of the battery. It makes it take a long time to discharge the coil's energy in the relay. Try this:

    Motor control.JPG

    If I'm reading your design right, connecting the negative terminal to ground may cause the motor to start running before you push the start button.

    In that case, you could try option 2: Put a really big capacitor across the relay like this:

    Motor control1.JPG

    No guarantees but it may let the coil dump some of it's energy right away so that the switch closes quicker.

    Those are both just attempts at a quick fix.
  4. Feb 15, 2010 #3
    Thanks for the information, but I am now more confused.
    Here is another drawing, red shows how I think it should work when the momentary switch is pressed and the blue shows how it should work when the other switch is pressed.
    I am using a 5v relay when I should be using a 3v relay but I don't know where to get one.
    The relay does trip when I push the switch but turns right back off unless I hold the momentary switch for a few seconds.
    Someone told me the motor is taking in to much current when it starts that theres not enough yo hold the relay until the motor is going full speed, but I don't get that part.

    Attached Files:

  5. Feb 15, 2010 #4
    Take a look at this:

    Motor control2.JPG

    I'm not sure that you even have it hooked up like this diagram shows. However, if the relay is really hooked up to ground, it is going to be on before you press the push button. The only thing keeping the motor from running is some kind of voltage divider when the relay is on. The motor doesn't actually have to turn in order to let current pass through it and into the relay. When you push the button, you are shorting the relay and turning it off so that the motor gets enough voltage to start spinning. However, you have to hold it down long enough to let it discharge fully. This is the best I can tell from looking at your design.

    I'll help you out a bit. First, I don't know if you're in the U.S. but you can get a 3V relay at radio shack. Second, here's how I would do what you want.


    I purposefully left out the limit switch. I'll let you figure out where it should go.
  6. Feb 16, 2010 #5
    Thanks Okefenokee,

    I think I drew the symbol for the battery wrong, here is a new drawing showing how ground is hooked up.

    I did not know you could run the power through the coil of the relay to the motor, I will need to think about that some more and see what I can come up with.

    I really only need one LED so I can see when the axle has hit the limit switch, the axle of the car is a threaded rod with a wing nut on it. This way I can turn the wheels by hand and see when the limit switch has been pressed and then calculate how many times I need to turn the wheels backwards to make the car go what ever distance the judge says it needs to go. It is either 5, 10 or 15 meters.

    I'll have my dad bring me to Radio Shack and look for a 3V relay


    Attached Files:

  7. Feb 17, 2010 #6


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    Science Advisor

    This may be a better way of doing it:

    http://dl.dropbox.com/u/4222062/motor%20switch.PNG [Broken]

    LED1 should light when the batteries are put in. You can put a switch in series with the batteries if you like, or just omit LED1.

    Pushing the start switch will cause the relay to pull in and start the motor.

    The relay contact will keep power on the motor and the relay coil, and light the RHS LED. LED1 will turn off when the motor is running.

    Pressing the stop switch will remove power from the relay coil, stopping the motor.

    The resistors in series with the LEDs should be about 100 ohms.
    Last edited by a moderator: May 4, 2017
  8. Feb 18, 2010 #7
    He's right. Good catch.
  9. Mar 15, 2010 #8
    a capacitor charges slowly, holds a charge for some time then will discharge either quickly or slowly (if a resistor is used). They are regularly used in large motors requiring huge startup currents. I doubt you will need it to start a tiny low-voltage motor for this type of project.

    With that being said, you can use a setup with capacitors and resistors with a relay, as a time-delay circuit. A large enough electrolytic capacitor and small enough relay, (like a reed relay), will stay closed a fairly long time, by adding a variable resistance from the positive side of the relay to ground, you can adjust the time the relay stays latched. However, the variable resistance must not go to 0, or you will burn it out. You probably should put at least a 100 ohm resistor in series with the control so your adjustment goes from something like 100k ohms down to 100 ohms.

    To describe this a little better, put an appropriate 100 microfarad or larger electrolytic capacitor (rated at a higher voltage than you are using), in parallel with the relay. Put a 100 ohm 1/4 watt resistor in series with a potentiometer (variable resistor) at 1/4 watt. Put the resistor/potentiometer combination in parallel with the capacitor. Now you will be able to control the time the relay stays closed. If you go over 5 volts for the relay coil though, you will need to increase the 100 ohm resistor value or you will burn out the 1/4 watt resistor or potentiometer or both.

    It is probably easier to have the car hit a shut-off switch though.

    Your delay problem with the relay is odd. Perhpas your relay coil requires more current than your batteries supply. If your power sources is borderline, you may need to change it. Perhaps the voltage you supply across the relay is less than the specified voltage. This will also cause it to be slow in response. The delay for most relays are measured in miliseconds. Therefore there is something wrong with your relay or the power you are supplying to it.

    Oh, as a side note, our team used a reed relay at 5v. This worked quite well. However, due to a lack of knowledge, the judge ruled the reed relay to be non-mechanical and our team went from 2nd place to teir two, essentially not getting an award. Somehow everything needs to be clarified to the judges. Some of the judges themselves really do not have the expertise to evaluate the components used in competition.

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