help to control a relay/solenoid

Discussion in 'The Projects Forum' started by Franco_oz, Nov 16, 2009.

  1. Franco_oz

    Thread Starter Active Member

    Jan 12, 2009
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    i need to energize a little solenoid for about 10sec after a motor stops. I only have +12, GND and the +Vmotor available. I can't touch the motor controller due to warranty problems and i can't tap anywhere exept at the junction where this 3 wires are. I need to empty a solution tank when the motor (recicle pump) stops. The pump stops when the solution need to be changed and would be nice to automatically empty the tank once the pump stops :)
    My problem is i don't know where to start. How can i control a relay only when Vmotor goes to 0V? +12 is always available and my only way would be to sense Vmotor and energize the relay for about 10sec. How can i do that?
     
  2. BMorse

    Senior Member

    Sep 26, 2009
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    You could try using a 555 timer circuit, have the VMotor voltage control the reset on the 555, when VMotor goes to 0, 555 could trigger as a one shot, that can turn on the relay for the specified amount of time (10 secs)....... you can use the constant +12 for powering the 555 circuit and solenoid...... Once the pump starts up again, this could reset the 555 for the next cycle...

    My .02
     
  3. Franco_oz

    Thread Starter Active Member

    Jan 12, 2009
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    BMorse, that is what i tried but i can't figure out how to connect the trigger pin and the rst pin.
     
  4. BMorse

    Senior Member

    Sep 26, 2009
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    Try this monostable setup of a 555 timer..... maybe someone could improve on it if needed but this is a basic setup for a monostable (one shot).... the only thing about this circuit is the reset pin is tied high to VCC, I am not sure if the 555 will trigger again on the next incoming low signal if Reset was not taken low first...... I will have to put it on a breadboard and let you know....

    On the output of the 555 you will have to use a transistor / Mosfet to switch the Relay/Solenoid load......

    The circuit schematic has the different values on it for the Resistor and Capacitor combination you would use to get approximate time delays...

    Let me know if this helps, you can always check out Bill Marsden's 555 circuits..... check out his blogs....

    My .02
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
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    Is the motor AC or DC, @ what voltage?? If DC, use an optical coupler & reaistor, if AC , a cap. diode, resistor.
     
  6. Franco_oz

    Thread Starter Active Member

    Jan 12, 2009
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    Motor is a small dc pump running @ 12V.
    BMorse, i made a small timer with a 555 but i can't figure out how to hook it up. I use the steady 12v to power it with rst pin to vcc and connected the trigger to Vmotor via 1k res in series but it doesn't work. I am thinking to add a cap in series and pull up the trigger?
     
  7. Duane P Wetick

    Active Member

    Apr 23, 2009
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    It looks like Monostable circuit will work, but you will need some additional output drive for a 12vdc., 1.2 watt solenoid; 3.2A inrush, 0.34A hold. The inrush will interval will be short; .072 sec., ASCO data. Use a power Mosfet transistor; (G to pin 3) to run the solenoid valve; (+12 to D, S to coil). Don't forget to add a free-wheeling diode across the solenoid coil.
    Also, a small 12v relay paralleled w/ pump will give you the start signal; C & NC contacts pull pin 3 to gnd.

    Cheers, DPW
     
    Last edited: Nov 19, 2009
  8. Franco_oz

    Thread Starter Active Member

    Jan 12, 2009
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    I think the main problem i have is that once Vmotor is 0V it keeps tied down the trigger and the timer never stop? I built a monostable with RST pin tied high and i can get it working with trigger on a push button but when i wire it up it never energize the solenoid.
    I used a BC337 at the output of 555 to drive a small relay and the relay drives the solenoid. Flywheel diode was added after blowing 2 BJT :(
    Any help?
     
  9. BMorse

    Senior Member

    Sep 26, 2009
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    let me run a few tests on a breadboard and I will get back to you in a little while, I will try to build it as close to what you got to see what you need to do, I will just simulate the 0V / 12V signal.......
     
  10. Franco_oz

    Thread Starter Active Member

    Jan 12, 2009
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    Thank you, forgot to say that the trigger pin is pulled high via a 4k7 resistor and i have the push button to ground. In this way it works fine but doesn't when i use Vmotor. Geez do i make any sense?
     
  11. BMorse

    Senior Member

    Sep 26, 2009
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    Ok, played around with it a bit, and I can see what you are saying that if the input stays low, the solenoid would run the whole time instead of the 10 secs.... so I played with it a bit and come to this, if you add this circuit to the input of the 555 (No other connections to pin 2, just this) this should make it to where you get a brief low pulse on your input regardless of how long the trigger signal stays low...then the whole cycle will reset when the trigger goes high again then back to low, but the 555 only fires for the set duration (10Secs (RA=1M, CA = 10uF))
     
  12. Franco_oz

    Thread Starter Active Member

    Jan 12, 2009
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    Thanks BMorse, i'll try it. Is this a sort of "debouncer" with a very long lag?
     
  13. BMorse

    Senior Member

    Sep 26, 2009
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    It is a type of "debounce" circuit, if the trigger is low, the cap (.01) and resistor(1M) will only "hold" the trigger pin low for a few ms, then it will pull it back high via 22K resistor... I also forgot to mention to keep the reset pin (pin 4) high, just tie it to VCC.
     
  14. Franco_oz

    Thread Starter Active Member

    Jan 12, 2009
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    Thanks BMorse, it's all working fine now and it doesnt miss a beat!
    Thanks to all the guys who took time to help me.
     
  15. BMorse

    Senior Member

    Sep 26, 2009
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    No problem..... anytime....
     
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