'burst timer'

Discussion in 'The Projects Forum' started by aspland, Jan 22, 2009.

  1. aspland

    Thread Starter New Member

    Jan 22, 2009
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    Hi everyone

    I'm new to the site, I'm not that technical but I'll have a go. Appologies in advance for my lack of knowledge but is offset up by my enthusiasm to learn :).
    I stumbled on the following link http://forum.allaboutcircuits.com/showthread.php?t=16593 while trying to find a 'burst' timer. Can anyone tell me if it's possible to modify the circuit so that it self triggers approx every 15 minutes with the output variable from approx 5-30 seconds.
    I would like to use it to control a greenhouse mister. The setup will be controlled by humidistat to switch on when humidity drops too low and the output will be to a 24v solenoid to actuate the mister. I need to keep the greenhouse in tropical conditions!

    Thank you

    Steve
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Here is a circuit:

    http://www.uoguelph.ca/~antoon/circ/pwm555.html

    Actually, this is a PWM circuit but it will work. Once you apply power to it, it will output a 5-30 sec pulse, then go low and output another pulse after 15 mins and so on.
    To make it work, disconnect the 50K pot from D2 and connect it to P1. Then take a 1M resistor and place it between D2 and P1. Also, change C1 to 1000uF. The output is pin 3.
     
  3. aspland

    Thread Starter New Member

    Jan 22, 2009
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    0
    Hi mik3, thank you for the quick response and circuit diagram, just what I need. I will attempt it but I may have to draw on your superior knowledge if I run into any problems :D.

    Once again many thanks.
    Steve
     
  4. aspland

    Thread Starter New Member

    Jan 22, 2009
    6
    0
    Good morning, I have a few questions after looking at the circuit diagram as I'm a bit confused :confused:.

    1. Can I use 24v on the whole circuit or use 12v (as indicated in the diagram) then use a relay to switch the 24v feed?
    2. I'm a bit confused with the motor (MOT1), am I to use just the left part of the diagram (leaving out MOT1, C3,Q1,D3 & R2) suppling power to the 555 on pins 1+8 and the pulsed output will be from pin 3?

    Please help

    Steve
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You can't use 24V to power a 555, use 12V.
    You can remove the parts you said and connect only the relay to the output. However, take care that the 555 output can deliver only 200mA and if the relay coil needs more then the 555 will burn. If the relay coil needs more than 200mA use the circuit as it is and put the relay in the lace of the motor (you can use a BJT transistor instead of the MOS (Q1) shown). Also, if you don't use a transistor you have to put a diode in parallel with the relay coil to absorb the back emf with its cathode facing the positive side of the relay coil.
     
  6. aspland

    Thread Starter New Member

    Jan 22, 2009
    6
    0
    Hi mik3

    Thanks again for the quick response and valuable information. The mini relay I have only uses 35ma so I will use the relay option. I have just ordered the parts from Maplin, hopefully start the project next week.

    Steve
     
  7. mik3

    Senior Member

    Feb 4, 2008
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    Good luck then. If you have any problems come back here ;)
     
  8. aspland

    Thread Starter New Member

    Jan 22, 2009
    6
    0
    Hi Mik3

    I have the timer up & running but the duration is only 2secs & the delay is variable between approx 5secs to 1 1/2 mins. For C1 I used a Radial Electrolytic Capacitor 1000uF 16V (it's all I could find for the 1000uF value) and the diode used across the relay contacts was a 3A SchottkyBarrier Rectifier (for some reason I thought I needed a higher amperage diode & this was all I could find at Maplin). Ideally I would like the duration about 5secs and the delay about 1 to 10 mins. No doubt the problem is with the above components that I have used. I have no idea what to do next and would appreciate your comments & suggestions on the above components and where I could have gone wrong. The pressure was on once I had finished the timer, my work mates looked on eagerly as they had taken bets as to whether it was going to go up in smoke when I switched it on :rolleyes:!

    Many thanks
     
  9. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The delay shouldn't vary too much just a few seconds. Post your circuit to see what is going on.
     
  10. aspland

    Thread Starter New Member

    Jan 22, 2009
    6
    0
    Hi Mik3

    Found the problem, I had both the signal diodes round the wrong way (got confused with anodes & cathodes!).
    Sorted, now works exactly as you said it would, perfect!
    As I mentioned in the beggining I have very little experience with electronics and would not have been able to attempt this project without your help. So I am extremely gratefull for your guidance and pleased with myself for completing the project :).

    Many thanks
     
    Last edited: Feb 5, 2009
  11. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You are welcome :)

    Every one in this forum updates his knowledge, none knows everything.
     
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