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555 Timer Circuit Q's

Discussion in 'Electronic Basics' started by Jon Hoyle, Jan 10, 2004.

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  1. Jon Hoyle

    Jon Hoyle Guest

    Hi there,

    Basically I'm trying to drive a DC motor (3vdc 110ma) at varying speeds.
    There are loads of circuits out there, but I am interested in this one, for
    simplicity; http://www.eleinmec.com/figures/003_03.gif

    For 'C' I intend to use a 0.1uf cap, and R1 a 1k resistor. Could I replace
    R2 with a 100k pot in series with another 1k resistor to give a range of
    speeds? The range is not important, even if I get a duty cycle af 25% - 75%
    I'll be happy, but the more the better I suppose :eek:)

    Another thing, I need to drive 2 motors, and I expect I will need to
    duplicate this circuit to drive the second one?

    Many Thanks,

    Jon Hoyle
     
  2. That motor current (110 ma) is going to heat the 555 up pretty badly.
    Loads that large are normally switched by an additional transistor
    added between the 555 and the load. The transistor will produce a lot
    less heat than he 555, because it can be configured to switch on so
    hard that it drops only a fraction of a volt, while the 55 will drop
    something like 1.5 or 2 volts with a 110 ma load. This lower drop
    also allows a higher top speed. If you want the two motors to go at
    the same speed, you could use a bigger transistor and parallel them.
    You also need a diode across the motor(s) to carry their inductive
    current during the time between pulses. This would take a transistor
    rated for something like 1 ampere (to get good saturated turn on at
    220 ma). A TIP29 would be fine and a TIP31 even more rugged. You
    might find these at Radio shack for under a dollar.

    As to timing, I think you should try putting a pot resistance between
    two fixed resistors, with the wiper going to pin 7, and the other ends
    of the fixed resistors going to the cap and the positive supply. That
    way, both the on and off time can be varied, to get a very wide range
    of average voltage out. 1 .01 uf cap and a 100k pot in series with a
    pair of 10k resistors should give you about 1000 pulses per second.

    Tie the output (pin 3) to an NPN transistor base through a current
    limiting resistor (with a value of say, (supply voltage times 20)/
    motor current in amps). Connect the emitter to the negative rail, the
    collector to the motor/s and the other side of the motor/s to the
    positive rail. Diode across the motor/s with cathode to positive
    supply. It is also a good idea to connect a bypass capacitor (say,
    100 to 1000 uf rated at more than the supply voltage) across the rails
    at the point there the emitter and motor attach, to carry a lot of the
    pulsation that would otherwise have to pass along all the supply
    lines. That would be the 220 uf cap at the left side of your diagram.
     
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