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Circuit help

  1. Aug 11, 2010 #1
    Basically what i'm trying to do is have two LDRs connected to a differential op-amp. The configuration has it high when LDR1 is low and LDR 2 is high with the rest of the combinations outputting low. Theres two op amps connected so that there are two distinct output channels so i have two high signals coming from the op-amps for both the low and high outcomes of the LDRs respectively, which represents the forward and reverse highs (for a motor).

    Now thats all well and good.

    But heres my problem. I want say for a high signal for forward, the motor to run for 5 seconds. So i've set up a monostable 555 timer circuit to do this. The problem being that the trigger of the 555 timer must be a momentary high, but the way my op-amps are they create a constant high.

    I don't know how to make it so that when I have a high signal for 'forward' from my op-amps the 555 timer to run just once and concurrently when i have a high signal for 'reverse' then the same thing (right now i'm using 2 555 timers, however thats if there is a simpler way to use one 555 timer to time both forward and reverse for my motor which uses a H bridge).

    Sorry if this sounds confusing. Ill try get a semi circuit up here if it is unclear. Thanks.
  2. jcsd
  3. Aug 11, 2010 #2


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    Does LDR stand for Light Dependent Resistor in your circuit? And yes, a schematic would be a big help.
  4. Aug 11, 2010 #3
    Are you trying to build a line follower robot or something similar?

    As berkman said, a schematic would be very helpful.

    http://www.doctronics.co.uk/555.htm" [Broken] site has very good documentation about 555 timers. In this case I would try to use the 555 in an astable mode.
    Last edited by a moderator: May 4, 2017
  5. Aug 11, 2010 #4


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    Well a schematic would definately help but from what you are describing I could envision you coupling through a capacitor and resistor to generate a pulse when the output of the opamp goes high. You will need a diode to protect the input of what the cap drives (anode to ground, cathode to cap output) when the signal from the opamp goes back low. Oh BTW, the trigger for a 555 is a LOW.
  6. Aug 11, 2010 #5

    Yeah, ill probably just use the output of the opamp to drive a transistor.

    [PLAIN]http://img440.imageshack.us/img440/6956/circuitht.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  7. Aug 11, 2010 #6


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    The way the schematic is LDR2 really won't change anything.
  8. Aug 11, 2010 #7
    Well, sure, you can do just about anything with a 555 timer, but are you really doing what you want to be doing? Maybe you could say a little bit more about the project you're working on, and we could help you with your current circuit or lead you to some different ideas if needed.
  9. Aug 12, 2010 #8
    I think i forgot a resistor going to ground at the positive terminal there. But anyway the problem is not with the op amps. The op amps are outputting fine.

    Yeah I figure as much. Basically I want a motor to operate for a set period of time and in a certain direction depending on the inputs of the LDRs. The logic table in the pic represents what I want to happen, so forward is only high for one set, and concurrently it is reverse for all else.

    I suppose there is something I can do with the comparators to output a momentary high and then switch off, only turning on when a state is changed.

    Can you explain further? I'm thinking since a cap blocks dc then it would discharge the cap whenever the outputs change from high to low or low to high which is really what I need to happen.
    Last edited: Aug 12, 2010
  10. Aug 12, 2010 #9
    Oh yeah, if all you want is to have the motor operate for a set period of time based on light input you just set a comparator to trigger when your resistor hits a certain resistance, and then you can do lots of stuff for your timing. You could discharge a capacitor and have the comparator turn off after a certain amount of RC constants. A schmitt trigger would would great here: use the comparator to charge the cap and then have it turn off at a lower threshold determined by the schmitt trigger, very simple. You could use the 555 timer. You could also set up something with switching transistors, that gets even more complicated.

    There's lots of ways to do this, but designs always have to go according to fixed end goals. For example, do you need a fast response time? Do you want your motor to have breaks? What is the load that you need to drive, how much current does it need, how much inductance does it have? These are all important questions, and these are just the basics I can think of off the top of my head.
  11. Aug 12, 2010 #10


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    Yes, you want to block the DC but pass the 'edge'. However, you want to configure a diode to prevent voltage spikes from going in the direction you don't want them to.
  12. Aug 13, 2010 #11
    Thank you very much! An edge trigger setup did exactly what I wanted, triggering the 555 only when it swings from low to high.
  13. Aug 13, 2010 #12
    Just as a funny sidenote, i'm not sure exactly what ive done to my 555 timer but its so incredibly sensitive that it actually triggers just from me putting my hand within 5 centimeters of the trigger wire (no doubt causing slight earthing or so). I'm almost certain that i've wired it up as it is in the diagram above.

    Its a very interesting thing, a touch switch that requires no touching!

    [PLAIN]http://www.kpsec.freeuk.com/images/555trig.gif[/PLAIN] [Broken]

    Another problem, I'm using the edge trigger shown above but with a diode in parallel with the righthand side resistor. It works fine when I manually close the switch there grounding the left side, however it doesn't seem to work replacing that switch with a transistor. The base is connected to the signal from my op amp with the collector at the edge circuit and the emitter to ground. so I'm thinking that putting a current to the base would achieve the same thing as closing the connection manually.

    Am I maybe wrong to try to make the transistor do this?
    Last edited by a moderator: May 4, 2017
  14. Aug 13, 2010 #13


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    I would say you don't have it configured how your last posted schematic shows if you get that type of behavior.
  15. Aug 14, 2010 #14
    Sorry, it wasn't connected incorrectly, just my op-amps were outputting ~0.8v for low which meant the transistor was always on. I learn I learn!

    With that done does anyone know how I was able to make the 555 trigger without touching it? Its obviously sensing the capacitence from my hand at quite a distance away, but from my googlings I haven't come up with any circuit like it using a 555.
  16. Aug 20, 2010 #15
    Its me again!

    Same project different problem.

    [PLAIN]http://img180.imageshack.us/img180/7609/circuit.jpg [Broken]

    This setup here works perfectly, flip the switch and the 555 timer runs once until switch is reset. The input to the switch is actually a signal generated by an op amp which outputs 2 signals which are opposite i.e. one is high the other is low and vice versa.

    I'd rather not have to use 2 555 timers for each independent signal from the op amps so trying for what is ages now I cant seem to produce a workable product that utilises the same 555 timer.

    Can anyone spare any hints here? Ideally i'd like the op amp to fire every time the switch is flipped. Making two of the edge trigger circuits (the section with the diode in it) has not worked.
    Last edited by a moderator: May 4, 2017
  17. Aug 21, 2010 #16
    [PLAIN]http://forum.allaboutcircuits.com/cache.php?url=http://That.Homepage.dk/Img/Radio_On_Off_SCH.png [Broken]

    I found this circuit which SHOULD do what I want, when IGN is high the 555 triggers and when it goes low it triggers again.

    But its not working for some reason, both physically and in Multisim and I can't see why.
    Last edited by a moderator: May 4, 2017
  18. Aug 21, 2010 #17


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    C1 and C2 should have some sort of bleeder resistor across them. BTW, I don't think a 555 will drive an automotive relay directly. Isn't the coil current large compared to what 555 will drive?
  19. Aug 21, 2010 #18
    Automotive relay draws about 150mA @ 12V

    *checks 555 datasheet*

    Can source or sink 200mA, so you're cuttin it close. Micro relay takes less power, so you might want to use one of those. They can generally still handle 30A.
    Last edited: Aug 21, 2010
  20. Aug 22, 2010 #19
    That was just an example I found, my circuit doesn't use a relay. I just want it to run a small motor for a few seconds when power is turned on and when power is turned off.

    I threw 10k resistors parallel to c1 and c2 with no effect.
  21. Aug 22, 2010 #20


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    10K is to low. You need a resistor that will not cause the voltage on pin 2 of the 555 to go less than 1/3 of B+ except on edges.
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