555 servo controller

Discussion in 'The Projects Forum' started by jayadmt, Oct 18, 2009.

  1. jayadmt

    Thread Starter New Member

    Oct 4, 2009
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    hi guys,
    i tried to make a simple servo controller from the circuit given below. i made the circuit and its not working. i connected a sub micro servo with operating vooltage 4.8-6. & 1.5kg/cm torque. its not even twitching. can you please give some suggestions please??
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Tony van Roon's website has moved.
    Links to his servo drivers are on this page:
    http://www.sentex.ca/~mec1995/gadgets/gadgets.htm

    Unfortunately, the link for servo driver #1 (which I think is what you're going by) is not currently working.

    I believe that the "68K" for R3 is a typographical error. Even 6.8k would be a bit high, but it may work. To be assured of proper saturation of the transistor T1 when Vcc is 4.8v, R3 should be reduced to 4.7k Ohms. For operation at 6v, R3 should be 5.6k Ohms.

    Make certain that you have T1 connected properly. It's all too easy to get the base/collector/emitter leads swapped around. With the flat face of the transistor with the leads down, the left lead should be the emitter, the middle the base, collector on the right.

    Check for shorts.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. jayadmt

    Thread Starter New Member

    Oct 4, 2009
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    0
    hey man, thankx. it worked. the mistake was the transistor pin connection. but i left r3 be 68k but even its working. a discontinuous humming noise with a coinsiderable vibration is coming from the rc even in no load. its a sub micro servo. 4.8-6v i/p, 1.5kg/cm torque one. but when it is at middle position, its calm. is there any problem? will it run continuously without any damage?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    OK, good find.

    I would still reduce it. 68k is more than 10x too large for the accepted standard "rule of thumb"; to ensure that a transistor will be saturated when used as a switch, base current should be 1/10 the expected collector current.

    Let's say your Vcc is 4.8v.
    Transistorized (standard) 555 timers have a Darlington follower in the output (pin 3) circuit. Even under light loading, there is a 1.3v "drop" from Vcc; so 4.8v - 1.3v = 3.5v.
    Then there is T1's Vbe to consider. Under light base current, Vbe will be around 0.7v. 3.5v - 0.7v = 2.8v.

    R2 is a 680 Ohm resistor. In order to sink enough current to get the collector of T1 close to 0v with a Vcc of 4.8v, 4.8v/680 Ohms = 7mA (approximate).
    1/10 of 7mA = 0.7mA. So, 2.8v / 0.7mA = 4k Ohms. (See, even my initial recommendation of 4.7k was a bit high).

    Well, humming and vibration isn't good.
    There are a couple of items that are missing from Tony Van Roon's schematic, which are quite important.
    There should be two additional capacitors in the circuit across the 555's Vcc and GND pins; one 220uF and one 0.1uF (100nF). The 220uF cap can be located nearby (try to keep the wires short) but the 0.1uF capacitor should have VERY short leads.

    The 555 timer momentarily creates a dead short across the power supply when it's output changes states. The capacitors are necessary to take care of these momentary transients. Without the caps, you will have very large spikes on your Vcc, and your circuit will not be very stable.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    If the frequency of the signal to the servo is too low then they will also hum like that. There should be less than 20mS between each high pulse.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Thanks, Roman.

    Just ran it through simulation again with P1 and P2 set to max and min values. Looks like the maximum PRT (using ideal components) is about 9.3mS; that's leading edge to leading edge.

    If our original poster has used other values (particularly for R1 or C1) then that would help explain things.
     
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