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Delayed timer switch...

Discussion in 'Electronic Design' started by Jeff Leininger, Jul 6, 2004.

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  1. I am trying to create a toy/novelty circuit that should turn a 3V DC motor
    on for a preset time period (about 5 min). I used a monostable 555 timer
    circuit to create an output that goes high for a period defined by the R*C
    delay of the circuit. Then, I tied this output to the base of an NPN
    transistor, which connected the motor from the high to its collector and the
    low to its emitter. So, whenever the input goes high the transistor should
    "switch" on and allow current through the motor. Unfortunately, this design
    has failed me. It works perfectly when I substitute an LED and resistor for
    the motor. However, when a motor is in place, the circuit fails to work
    correctly.

    Some changes that I have tried...

    (1) inserted a diode in parallel w/ the motor to reduce transients

    (2) inserted a resistor between 555 timer outout and the transistor to
    reduce the current in the base

    (3) replaced the NPN transistor w/ a Darlington transistor (TIP120)

    At this point, I am at a loss and looking for some help. As I said,
    everything works perfectly with an LED in place of the motor. So, I know
    the timer circuit is correctly wired and the idea of tying this to a
    transistor is feasible. But, something about the motor (higher current,
    inductance, noise, ???) is messing things up. Please help if you can...

    Schematic of first circuit design:
    http://www.theLeiningers.com/Portals/57ad7180-c5e7-49f5-b282-c6475cdb7ee7/MotorControl.gif

    Schematic w/ some modifications:
    http://www.theLeiningers.com/Portals/57ad7180-c5e7-49f5-b282-c6475cdb7ee7/MotorControl2.gif

    Description of circuit at my website:
    http://www.theleiningers.com/DesktopDefault.aspx?tabid=75
     
  2. Start by putting something like 220uF/10V across the supply rails.

    Best regards,
    Spehro Pefhany
     
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