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Need a countdown timer that I can breadboard

  1. Nov 11, 2008 #1

    I need a simple timer that will count down from 1-20 seconds and then have a continuous output of high to run a electric motor. And time can easily be adjusted (potentiometer?)

    Ive been playing with 555 timers and timer circuits from diagrams of "555 timer" through google, but all I can make it a pulse generator/Astable oscilator out of a 555. None of the timer cicuits work.

    The goal is to turn the circuit on, wait 2 seconds, run the motor, and turn off after 15 seconds, (but those times need to be adjustable)(and Ill run both the timers through an XOR gate to get the motor to shut off (because both will be high)) So can the 555 chip and a 7400 chip take 9 volts?

    Has to be breadboardable to

    any help will be much appreciated.
  2. jcsd
  3. Nov 11, 2008 #2
    CD 4060.. Oscillator/divider is an alternative. Goggle circuits using this chip.

    You could use a watch crystal and maybe a second 4060 and get 1, 2, 4, 8, and 16 sec pulses.
  4. Nov 11, 2008 #3
    To do this with 555/556, pots, and caps is do-able, but will take some work to implement latching and reseting. For this type of activity I use microcontrollers that are programmable via a serial interface. Most microcontrollers have internal oscillators requiring you only have a buffer/optoisolator to interface to higher power devices.

    I know you said 'simple'. The microcontroller can allow for a smaller part count, a simpler circuit, and can come in DIP form for your breadboard. It really turns into more of a software project rather than a hardware one. Not sure what your limits are.
  5. Nov 11, 2008 #4
    dont you need drivers for the motors as well?
  6. Nov 11, 2008 #5
    Sure you do. But depending on how much control you want over the motor's speed, direction, and torque, you will need to use different solutions. You can use a simple 2N2222 transistor and completely saturate it to turn the motor on. You can also send a PWM pulse to same and vary the the motor's speed.

    Regardless, you can place an optoisolator with a pullup resistor and let the microcontroller pull that to ground. On the other end of the optoisolator you can use the 2N2222 with either a pull-up or pull-down (depending on if you want active-on or active-off) to power your motor.

    The opto is a nice method to prevent damage to the microcontroller and the PC that may be talking to it.
  7. Nov 11, 2008 #6
    well, I might be in over my head.

    My only experience with this stuff is a 4 credit college course in digital electronics, I thought it was in depth stuff.

    I dont know what a optoisolator is, or a pullup resistor is. And we never played with microcontrollers. I think someone recommended me this, but are they big and expensive?

    Did think it would be so hard to have a cicuit wait for so long and jsut turn on a motor. Not sure what a motor driver is or I would would need one because i can just take a standard motor and hook it right up to a 9v battery and it works just fine
  8. Nov 11, 2008 #7


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  9. Nov 11, 2008 #8
    It works if your intent is to just get the motor running. If you want to control the motor, you might need a couple of extra components depending on how precise you want to be able to control it.

    A motor driver basically amplifies the signal coming out of a microcontroller to control a motor. Depending on your microcontroller and motor, sometimes the output signal might not be big enough to turn the motor. Also, you could protect your precious microcontroller because most likely the driver is the first to go in case something goes wrong.

    Looks like a good place to start if you want to learn more, or you could take a few classes on robotics or something like that.
  10. Nov 12, 2008 #9
    Actually I think I was recommended a PLC type of thing, do you guys mean like a PIC microcontroller?

    Theirs got to be a simpler way to get a 15 second count down timer. What about a digital timer, something like these, and split it part to find the beeper or what ever and hook that to a motor (well a relay, or driver cause surely it wouldnt have enough power to run a motor)
  11. Nov 12, 2008 #10
    It really depends on where your design strengths are. If you are trying to do this with digital components only, avoiding the programmable types of devices, it will be a more complex circuit. The PIC or Microcontroller eliminate complicated circuits by allowing you to use software in place of complex timing, latching, and resetting circuits to accomplish your task.

    You can use your 555 with a counter IC like a 74LS163. The 163 could accumulate pulses from the 555. Once you hit your target value, the 163 could enable a power transistor to turn on. As the counting continued, the 163 would eventually rollover like an odometer turning the motor off. If you want different on and off times, you would need to have two 74LS163 devices that would reset each other when required. This will make timing adjustments difficult. Again, if you are not very experienced with digital design this may be more complex than you want, but it avoids the programmable devices.
  12. Nov 12, 2008 #11
    mcmaster.com appears to have a bunch of off-the-shelf time delay relays if you just need something that will work now.
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