Operating a DPDT relay with a 555 Timer?

Discussion in 'The Projects Forum' started by raj_C, Feb 7, 2014.

  1. raj_C

    Thread Starter New Member

    Feb 7, 2014
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    I wish to operate a 5 volt DPDT relay with a 555 timer. Based on suggestions on the internet, I figured that the output of the 555 timer is connected to one pin of the coil of the DPDT relay through a diode and the other pin of the relay coil goes to ground. Is this correct?

    But when I am physically connecting the output of the timer to the relay (whether through a diode or not), something strange happens. The relay switches on as soon as the output of the timer goes high but after that it remains switched on and the output of the timer also remains constantly high. I have checked the relay and the timer separately and they both seem to work fine. The timer produces the output pulse (goes high and low) at specific intervals as expected when not connected to the relay... I am even able to drive a flashing LED setup with it, but when I connect the output of the timer to the relay it doesn't work as expected.

    The relay is a non-latching DPDT type, connecting and disconnecting the coil pins directly to the power rail causes it to close and open as expected.... its only when I am connecting it to the output of the timer it behaves this way.

    I am using a 9V power supply. The voltage at the end of the diode as it reaches the coil becomes constantly +6.5 V when connected to the relay and shifts between +16V and +4.5V when not connected to the relay. The timer output when the diode and relay are disconnected is a constant pulse of +7.6V and 0V. The timer is running in astable mode with the time interval of 4 seconds with 50% duty cycle.

    What am I doing wrong? Can someone please help me figure this out?
     
    Last edited: Feb 7, 2014
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    For clarification, please post ur diagram.

    Need relay voltage and current rating too

    If only the relay does not exceed 555 rated output current. Even then u might need a resistor in series with the relay.
    U should place a diode across the relay too in reverse polarity or you will destroy the 555
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Sounds like you are trying to draw to much current from the IC. Try using a driver transistor like a 2222.
     
  4. TANDBERGEREN

    Member

    Jan 20, 2014
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    The 555 is capable of sourcing/sinking 200mA.
    +5V to the one pin for the coil, the other pin to the 555 output.
    Don't forget to place a protective diode over the coil. (Cathode to +5V)
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    It's also possible that the 9V power supply cannot furnish enough current to operate the relay. That would explain the voltage drop, and 4.5V is too close to the 555 minimum required voltage. In order to know, you must look at the power supply current rating and the relay coil resistance.
     
  6. raj_C

    Thread Starter New Member

    Feb 7, 2014
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    Thank you all for your valuable suggestions.

    I am trying to make a 555 timer based bidirectional motor controller.

    I have modified the circuit design as suggested by TANDBERGEREN and it works now to some extent, the relay closes and opens at the set time interval of the timer but only when there is no load on the relay. If the relay is connected to a motor then the motor just vibrates and the relay closes and opens very rapidly (almost at the frequency of the vibration of the motor) or sometimes the motor rotates only in one direction intermittently stopping with a quick jerk and then continuing in the same direction again, when it should be rotating for 2.2 seconds in one direction and 2.4 seconds in another direction.

    I am attaching the the circuit diagram below, please have a look.

    The power supply I am using is 9V 1A SMPS wall adapater. The motor is a 6V-9V DC motor that takes a max of 100mA at full speed but the stall current is 700mA.
     
  7. raj_C

    Thread Starter New Member

    Feb 7, 2014
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    By "motor is vibrating" I mean it rattles by trying to rotate in two directions in successions very rapidly.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    When using that small of a supply on motor load, (9V 1A) you will need large capacitor directly at the relay contacts input terminals.

    Also add a small one on the input to the 555.
     
    Last edited: Feb 7, 2014
  9. John P

    AAC Fanatic!

    Oct 14, 2008
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    A little 9V transistor battery? Those are made to operate low-current loads, and anything involving relays or motors would be very dubious; you'd see the voltage dip when the current increases, and nothing will work right. People do it all the time though, because that kind of battery is so easy to use (no need to wire a bunch of cells in series) but it often leads to problems. A wall wart that delivers the voltage you want would be much better, but there the problem might be that they usually deliver a voltage much higher than they're rated for, unless they're loaded down with the rated current. So you'd most likely need a voltage regulator too.
     
  10. burger2227

    Member

    Feb 3, 2014
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    What is the purpose of the relay if everything is wired to the same power supply? A 9 volt battery won't last long with a relay and a motor on it.

    Why not use a transistor on the 555 output?
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    1A SMPS cannot run your motor.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Sounds like interference from, or loading by, the motor is resetting the timer.
    Try connecting a large value (at least 100uF) cap directly across the power supply terminals of the IC. If that alone doesn't work, try in addition a forward-biased series diode between the 9V supply line and the IC supply (pins 4 and 8), with the relay powered directly from the 9V line. A snubber (0.1uF cap in series with 100 Ohm resistor) connected directly between the motor terminals may also help.
     
  13. TANDBERGEREN

    Member

    Jan 20, 2014
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    I would also add the following:
    A diode in series with the powersupply for the 555.
    After the diode, add a capasitor, 470uF 16V.
    This will ensure smooth operation of the timer.

    A 1000uF capasitor before the relay is also a good thing.
    Not too big capasitor directly to the motor.
    100nF is quite enough there. A snubber network is prefereable as las Alec_t says.
    (Because of the directionchange wich will produce quite a lot of backwards EMF. )

    Use a low-drop diode, such as a schotcky diode to prevent too large voltagedrop as the diode in series with the 9V supply.
     
  14. bertz

    Member

    Nov 11, 2013
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    This is how I would build the circuit within the context that you have given us. However, you are going to wear out that poor relay. Why not use a microcontroller and a L293D motor driver?

    Good luck

    Al
     
  15. raj_C

    Thread Starter New Member

    Feb 7, 2014
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    I guess using a single power supply was the problem. As already suggested by Alec_t, it seems like the interference from the motor was resetting the timer. Even a 2 amps supply could not handle this setup when used to power up both the motor and the timer simultaneously so I used two separate power supplies. I hooked up the timer and the coil of the relay to one power supply and the rest of the pins of the relay to another power supply and it works perfectly fine.

    I haven't tried the designs with single power supply and capacitors as suggested by TANDBERGEREN , Alec_t and bertz yet. I will try them out this weekend and reply later.

    @bertz, thanks for the circuit design. I will try it out this weekend. As for L293D, I had already tried that but it didn't seem to handle the current that was drawn by the motor when it was reversing the direction of rotation. L293D is rated for 500mA only so I settled for the circuit with DPDT relay.

    In the circuit with L293D I connected the timer output to a CD4017 using it as a flip-flop (counting only to two then resetting) and the two outputs of CD4017 to the two input pins of L293D driver. The motor did rotate for a moment then stopped when it was reversing the direction. Or maybe it wasn't L293D and the problem here too was due to the same power supply used for both the timer and the motor... I am not sure.
     
    Last edited: Feb 8, 2014
  16. bertz

    Member

    Nov 11, 2013
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    The L293D will handle 1.2 amps if you double up on the channels; else use a L298 driver. The 555 will output a HIGH/LOW square wave so it can be used to input the driver. (See the truth table for the L293D) As I said, switching the relay every 2 seconds is crazy!

    Good luck

    Al
     
  17. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U know I never had any luck with L293's. I got plenty but each time how I try the bloody thing ends up in smoke.
     
  18. raj_C

    Thread Starter New Member

    Feb 7, 2014
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    How do you double up the channels? You mean enabling both channels and using the outputs from both sides of the chip for a single motor?

    Are you suggesting using the NOT gate transistor setup with the output of the 555 timer to produce 0,1 and 1,0 inputs for the L293D?
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You may also improve things with a large electrolytic de-coupling capacitor across the supply.
    If the motor is under load at all time, the longer it is going to take for the stall current to decay to a reasonable value.
    There is an advantage to control accelerate the motor for this reason.
    Max.
     
  20. sheldons

    Well-Known Member

    Oct 26, 2011
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    looking at your 555 circuit-when your 555 output goes low your relay should energise and release when the output goes high-add a transistor to drive your relay as shown here.....your 555 is probably latching up on the output due to being over driven-or you have a noisy supply and inadequate decoupling or a wiring error......
     
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