Problem with 555 timer

Discussion in 'General Electronics Chat' started by Dawud Beale, Jan 6, 2013.

  1. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    Hi does anyone know how I can test this 555 timer

    http://www.ohmslawcalculator.com/555_astable.php

    I've constructed the circuit but the pint 3 output is just measuring a constant 8.8volts and I checked the current by with an ammeter from pin 3 to ground and it was hovering at 1.7.

    How can I test if this circuit is working correctly?

    I tried an LED through it and nothing happened. Do I need to use a limiting resistor even though the circuit alread has a high resistance? is it possible i blew the LED?
     
  2. Sam__

    New Member

    Dec 21, 2012
    10
    1
    You're measured 1.7Amps coming out of pin 3? Maximum output current of a CMOS 555 is 100mA I think so you have a borked chip or you're measuring something wrong.
     
  3. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    I took out the limiting resistor and connected an LED directly to pin 3 and ground and it was bright without a limiting resistor but is constantly on, it doesnt appear to blink which I would expect from a 555 timer output. Any ideas?
     
  4. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    Yes you need to put a resistor in series with the LED. If you connect the series combination of the resistor(470 to 1kΩ) from pin 3 to ground the LED should blink on for about a second and be off a little less than a second.
     
  5. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
    7
    Oh I measured the current again and it's 150mA although its constantly decreasing by about .1mA per second or slightly faster. Im guessing this decline is due to the potential difference between positive and negative terminal of the battery decreasing as the battery runs out?
     
  6. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    Thats what is should be doing but with a limiting resistor its off and without a limiting resistor it stays constantly on. it doesnt blink.

    what checks can i do to find out the problem?
     
  7. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    And if it was working correctly, what would I expect to see happen between pin 3 and ground in terms of voltage and current? would voltage alternate between 1V and 5V?
     
  8. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    I looked at your values again. The 47uF capacitor may have enough leakage current that the ciruit doesnt function properly. Try changing the resistors from 10KΩ to 100KΩ and changing the capacitor to 4.7uF.
    If you don't have a selection of resistors, try changing the capacitor to a lower value the flash rate will increase. In addition you should have a capacitor between pins 8 and 1. A 47uF electrolytic will be fine. + to pin 8 and - to pin 1.
     
    Last edited: Jan 6, 2013
  9. Wendy

    Moderator

    Mar 24, 2008
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  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
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    The circuit functions as it is drawn in the link. It does have a 10μF cap from pin 5 to ground, which is incorrect, but does not keep the circuit from functioning. This cap can be completely left out or replaced with a .01μF ceramic cap.

    The most likely construction errors are:
    1. Installing the LED incorrectly; the LED's anode should go toward pin 3 and its cathode should go to ground, or its cathode should go toward pin 3 and its anode to +Vcc.
    2. Leaving out the current limiting resistor for the LED. A 1k resistor is adequate for most voltages from 5 to 12 and should be in series with the LED.
    3. Installing the electrolytic capacitor backwards; the positive side should go to pins 6/2 and the negative side to ground.

    Pin 3 of the 555 should be alternating between +Vcc and ground every 1/2 second. Even without an LED, you should be able to see this on a DMM.
     
  11. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    If the 555 broke would it break the circuit or would current just pass straight through?

    Is there some voltage and current tests I can do on different parts of the circuit to find out where the problem is? Like expected behaviour vs actual behaviour. Surely there would be a predicted voltage on the capacitors, resistors etc?
     
  12. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    1+2 The LED just lights up, its not blinking. So the anode and cathode and limiting resistance cant be a problem.

    3 The larger capacitor has its negative face towards ground. If it was the other way, wouldn't the timer cease to function?

    WHats happening at the moment is 150mA is constantly coming out of pin 3.

    Any more idea's?
     
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Your power supply should be DC between 4.5 and 16 volts. If that is correct, proceed as follows.

    1. Check the polarity of the caps.
    2. Check the wiring against the schematic.
    3. Repeat steps 1 and 2.
    4. Check the voltage on pins 4 and 8; it should be at Vcc.
    5. Check the voltage between pin 4 and pin 1; it shoud be at Vcc.
    6. Check the voltage between pin 3 and ground; it should be alternating between Vcc and 0.
    7. Put a .1μF capacitor between pins 1 and 8 of the 555.

    If the LED has been installed without a current limiting resistor, it's probably shot. Test it by connecting it with a current limiting resistor between Vcc and ground: anode toward Vcc and cathode toward ground.

    If the IC has had Vcc connected to the wrong pins, it may be shot. If it isn't operating and you have correctly done the steps above, it's shot. Replace it.

    Before you apply power to the new components, follow steps 1 and 2 above. I have no idea how many times I have built 555 circuits, and I still make wiring and polarity errors.

    Here's a photo of the circuit built on a solderless breadboard. Except for the deletion of the cap from pin 5 to ground, it's exactly as drawn in the link you posted, and it's happily working away.

    The red wires are all at Vcc. The black wires are at ground. The yellow wire is to connect pin 2 to pin 6. The LED's current limiting resistor is hidden under the body of the LED.
     
    Last edited: Jan 6, 2013
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  14. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
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    I've checked 1,2,3,4 and 5 on your list and they are all ok. Still no alternating voltage on pin 3. Ive replaced the 555 and it hasnt improved the situation. Is there a way I can check the behaviour of each component and find out exactly where something isnt behaving as it should?
     
  15. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    What is your power source and voltage?
    Can you post a photo of your assembly?

    Except for the LED and the IC, there is essentially nothing that could be damaged about the components. I suppose the electrolytic cap could be bad, but it's not likely; have you replaced it?
     
  16. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    Power source is a 9V battery that goes through a voltage regulator and takes off about 2volts from the power source.

    Ill post up some photos...
     
  17. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    Have attached a pic. Let me know if you need any more specific pics
     
  18. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    There is no need for a voltage regulator; all it does is run the battery down faster. In addition, I don't see any components around the regulator; what kind is it? If it is an LM78xx, it needs caps on the inputs and outputs. The first thing I would do is bypass the regulator, and connect the battery straight to the 555. That might solve your problem.

    As to the photo, I can't tell much from it since the connections are hidden under the perfboard. Where is the LED and current limiting resistor?
     
  19. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    I have a pin connected to ground and pin 3 that I connect to a breadboard with the LED and limiting resistor. The voltage regulator is for the 22V10 that will take the 555 timer as an input. IC's usually require a 5V input for high.

    The circuit wasnt working before I attached the regulator and the regulator is working as it should, it has an output voltage a few volts lower than the input voltage. Something is wrong with the connections and components around the 555timer, it is outputting a static voltage. The LED is working fine, its just lit up all the time instead of lighting up and blinking
     
  20. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
    7
    the 555 timer has an input from the 9V battery, its middle pin is ground and the last pin is the output which leads into the 555 timer circuit as the 9V rail
     
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