555 timer

Discussion in 'The Projects Forum' started by avinash1976, Oct 23, 2015.

  1. avinash1976

    Thread Starter New Member

    Oct 23, 2015
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    I am working on project which includes 555 timer in monostable state to drive relay . once I trigger the 555 timer the relay will activate for predefined period . I want circuit so that after time period of timer relay should be on for few mili seconds . please help .
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    That takes two "timers"; either two 555s, or one 555 and some sort of RC delay where a capacitor discharges through a resistance, which could be the coil of the relay.

    To help you, I need to know the supply voltage, the delay time, the time before the relay resets, how many poles on the relay, the relay coil voltage and resistance, etc????
     
  3. avinash1976

    Thread Starter New Member

    Oct 23, 2015
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    I am using 12v power supply , ineed relay should be swithed on for 30 milliseconds after time delay is over. I am using DPDT relay with coil voltage of 12v and coil resistance is 240 ohm . to activate the relay I am using transistor BC 547 ,
    Thanks .
     
    Last edited: Oct 23, 2015
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You could let the 555 switch a fet transistor, and put a resistor/capacitor across the gate, source to give you an " off delay".
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    13.gif
     
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  6. avinash1976

    Thread Starter New Member

    Oct 23, 2015
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    Hi mike,
    thanks for circuit diagram . I want to use NPN transistor . and load is connected to collector . Please give me a circuit .

    Thanks
     
  7. MikeML

    AAC Fanatic!

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    Dont be so cheap; go buy a PNP!

    Two NPNs: one wired as a logic inverter, the second wired to drive the relay
     
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  8. Bernard

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    Aug 7, 2008
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    I think this is what Mike said in post #7: Second transistor would need to be a darlington or FET for lack of drive from R2-- or maybe hit cancel. Relay Pulser 00000.jpg
     
  9. avinash1976

    Thread Starter New Member

    Oct 23, 2015
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    hi , mike & Bernard ,
    Thanks For circuit diagrams . I have used PNP Transistor and it worked .
    I have another 555 timer circuit where I have problem with trigger input please see the circuit diagram . I want when switch Sw1 is on then only SW2 is active . if sw1 is open trigger should not be applied . I am using following ckt . but problem is as soon as I switch on the sw1 switch ( not pushed the sw2 ) the 555 timer is giving the out put at pin 3 . Please help me .

    Thanks .

    555timer problem_800x526.jpg
     
  10. Dodgydave

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    Jun 22, 2012
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    Remove the transistor and 1K resistor, and put the switches in place of the collector and emmiter.
     
  11. MikeML

    AAC Fanatic!

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    You need a low-going trigger pulse at pin 2, so the inverter may be superflous. Use the two switches in series like I used the single switch in post #5. If sw1 has to be in the high side, then use the inverter as you drew it, but increase the 1K resistor to 10K and add a 10K resistor from base to emitter of the NPN.
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    The files size that you attached were too too big, I was compressed them to the normal size, they are from about 2MB compressed to about 29KB, next time before you upload the files, please compress the photos to 800x600 or 640x480, if necessary then you can compress to 1024x768, almost the photos using 800x600 image is enough, thanks.
     
  13. Marcus2012

    Member

    Feb 22, 2015
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    Can I ask what type of relay you are using, e.g. mechanical or reed relay etc? I ask because mechanical relays have a switching speed of about 100ms and reed relays are slightly lower. So if you want it on for a few milliseconds it might have an unacceptable delay. You might want to consider using a Power MOSFET with an optocoulped driver if you need galvanic isolation if you're switching DC or a solid state relay if its a mains supply.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    A 556 dual timer would save a couple of pins.
     
  15. avinash1976

    Thread Starter New Member

    Oct 23, 2015
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    I am using 555 timer as monostable timer for time delay. As soon as power is on timer will start the delay without pressing reset switch . How to avoid power up reset for monostable . Please help.

    Thanks
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Put a RC circuit on the reset pin, just a first thought.
     
  17. ian field

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    Oct 27, 2012
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    That's so much standard practice, we all thought it had already been done.
     
  18. avinash1976

    Thread Starter New Member

    Oct 23, 2015
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    how to connect a 555 timer monostable output both to drive relay and activate buzzer ( using 555 timer in astable mode and want to connect pin 4 to output of monostable ) . can i use two outputs from single 555 timer . one to relay driver and another to other 555 timer in astable mode . Please Help .
     
  19. ian field

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    Oct 27, 2012
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    The 555 can handle about 200mA output current - the CMOS version rather less.

    Usually with relays, you need a clamp diode to catch the back emf, the totem-pole output is less vulnerable than an open collector, but I wouldn't chance leaving out the diode. If you want to really make sure - put 2x 1N4001 in series across the supply so neither are conducting, with the output to the tap between the 2. Anything that tries to force the output outside the rails, has to get past those 2 diodes first.
     
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