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555 timer not working

  1. Jul 16, 2009 #1
    I have been having a great amount of difficulty with the 555 timer (Texas Instruments NE555P). I am going through a book published by McGraw Hill titled Understanding Electricity and Electronics. While I was waiting for the parts to finish my power supply, I started to work with the 555 timer. I found a tutorial on the 555 which showed a schematic of “the most basic” form of a timing circuit using the 555 timer.

    I went to radio shack and purchased a solderless breadboard, the 555 timer, 12 volt LED and all the other various components that I thought I would need. This timer has not worked since I put it together. My first thought was that maybe the timer was bad so I went to radio shack and purchased another 555 timer with the same result. I have come to this forum to find out just what the heck I am doing wrong. Below I have a photo of my breadboard with the 555 timer and all the connections. My power supply is a 12-volt transformer with a bridge rectifier. The LED that I am using is a straight panel mount 12 volt LED. I am using a N.O. momentary switch for the trigger. I am using a 470-microfarad capacitor and a 10 K-ohm resistor. The resistor capacitor combination (if I figured right) should give me a pulse of 5.17 seconds not counting for capacitor leakage or so I have read. When I first power up the circuit there is 4.4 volts at pin 3. If I understand what I have read in the tutorial, shorting pin 2 to ground briefly should start the timing cycle. I am yet to see any cycle with my current configuration. When I push the momentary switch the voltage at pin 3 only rises when the switch is depressed. I have changed resistors, capacitors, timers, looked and pondered and I obviously don’t have a clue why it doesn’t work. Can any one please tell me where I am going wrong? I did change over to a 9-volt battery for a while with no change in the way the circuit worked (or rather didn’t work).

    I am starting school here in Ohio in the fall at Zane State in their Electrical/Electronics Engineering program (after I take the classes I should have taken in High School of course). I am beginning to think that I am way to dense to bother going back to school at the age of 41 if I can’t understand this.

    Dave Roderick

    Attached Files:

  2. jcsd
  3. Jul 16, 2009 #2
    So it looks like you have the 555 set up in monostable mode, where a negative-going edge on the input should produce a positive-going pulse on the output. From the picture you took, the device appears to be wired up properly.

    How are you delivering the trigger signal? You mentioned a momentary switch, but what it is switching? According to the NE555P datasheet at http://pdf1.alldatasheet.com/datasheet-pdf/view/161277/TI/NE555P.html, on page 10, the input must be brought low to trigger the one shot.

    I recommend you use a pullup resistor to hold pin 2 high, and use the momentary switch to bring the pin down to ground when the switch is pressed. If this is what you're already doing, then read on.

    Here's some things that you could diagnose:

    Check the output of your power supply and ensure it is a nice, steady DC value. It should be between 4.5V and 16V.

    Check the continuity of your circuit. Make sure each wire is truly a conductor. Breadboards are nefarious for bad connections if the wire isn't inserted properly.

    Check to make sure you are supplying the ground properly, and the power supply negative terminal is connected to ground.

    Good luck!
  4. Jul 16, 2009 #3
    I am using the momentary switch to short pin 2 to ground. I did not know what a pullup resistor was when I read your reply. After I read up on the definition it sounds like a handy component. I could get a pullup resistor when I place my next order. My first thought is that in all of the pages that I have read on the 555 timer and all of the schematics I have looked at, I see no mention of using this resistor. According to the tutorials that I have read this should work. My hope was that I had just connected things together improperly due to my lack of experience. After all, this was the first time I had ever put a circuit down on a breadboard before. Now that it appears I have made the correct connections I will start trying to back track.

    Some of the things that I have already checked: I have a steady 11.37 DC volts coming out of my bridge rectifier. I have continuity between pin 1 and the negative side of the bridge rectifier. I also have continuity between pin 2 and the negative side of the bridge rectifier when the momentary switch is depressed. Pin 2 is putting out .83 volts. When the momentary switch is pressed pin 2 goes to 0. Pins 6 and 7 have .02 volts. When the momentary switch is pressed pins 6 and 7 read 4.97 volts. Pin 8 shows 11.35 volts.
    Dave Roderick

    Attached Files:

  5. Jul 16, 2009 #4
    You'll basically need any resistor, above 10k should do it, and connect one end to your Vcc, the other to the input pin. As you may have read, this resistor holds the pin high until you press the button. When the button is pressed, the pin is shorted to ground, creating the negative-going edge that you're after for the 555.

    I've found that there's always something new to learn, and the best way to do it is to keep trying new things and asking questions when you're stuck.

    Good luck on your project.
  6. Jul 17, 2009 #5


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    That resistor should fix it. Have a look at the circuit on this page under monostables.

    Incidentally, it seems you don't have a filter capacitor on the output of your bridge rectifier or you would be reading about 17 volts with a 12 volt supply.

    This is a bit high for a 555 but the filter capacitor is very necessary for smooth DC.
    Maybe you could just use the battery until you arrange a lower supply voltage.

    Something like 1000uF at 25 volts would be OK for the filter capacitor.
  7. Jul 17, 2009 #6
    Let me make sure I understand what I need to do with this resistor. I would connect it to the positive voltage on my breadboard and run it to the input trigger (Pin 2). Is that correct? Also, I did not have a filter capacitor on my power supply. Could that have damaged my 555 timer?

    Dave Roderick
  8. Jul 17, 2009 #7


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    Yes, that is right about the resistor. See the diagram for a monostable in
    (Click on the line above)
    They use a 10 K resistor with a switch on the left. Ignore the reset switch.

    The lack of a capacitor wouldn't damage the 555 but it would stop it working as a monostable because the supply would be switching off every 8mS or so due to the power supply pulses. This would reset the monostable so you wouldn't see any output pulse.

    The peak voltage from your bridge rectifier would still be about 17 volts which could have damaged the 555 chip. So you should just use a 9 V battery or a wall wart with a lower voltage instead.
    You can get a voltage regulator and produce a regulated 12 volts from the 17 volts out of the bridge rectifier with filter capacitor if you like. Look up the 7812 chip.

    If the 555 chip is damaged, you might have to get another one, of course.
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