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555 flasher isnt flashing

messy

Jan 12, 2017
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I made a post a long time ago about 555 timers and I considered resurrecting that post as this question is basically the next step along but as the questions are different i thought it'd be tidyer to make a new post.

I'm trying to teach myself about 555 timers and have read through all the basic tutorials I can found. Having done that I created the attached circuit on a breadboard to check I understood what I had read (apologies for the mess, I couldn't rearrange the chip pins on the drawing tool).


I was expecting the LED to flash at a frequency of about 7Hz but what I actually get is a steady on LED. I thought maybe it was just blinking too fast too see so I swapped R2 from a 10k to 100k hoping this would reduce the freq to a little under 1Hz but still a steady on LED.

How I thought this would word - when the output is high C1 would charge and once it hit its threshold voltage the output would switch to low and connect the discharge pin to ground, discharging C1 until it hits the trigger voltage - setting output high, disconnecting discharge pin and charging C1 again.

Since the LED is contestant on that must mean that for some reason either C1 never reaches its threshold or for some reason the chip doesn't realize its reached it?.....right?


Have I done something wrong or missed something?
 

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messy

Jan 12, 2017
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also just noticed the info by the led - mine isnt a LTL-307ee, its just the only LED icon it seemed to have
 

WHONOES

May 20, 2017
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If the diagram is straight from your simulator, then the negative end of V1, the 9V supply, needs to be connected to ground as is the cathode of D1 and the end of C1, otherwise it simply won't work.
 

messy

Jan 12, 2017
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aah, apologies for my crappy drawing - thats just meant to be the +V, the ground line is at the bottom (Ill just draw the diagram in paint next time).

Grasping at straws here but could it be an issue with the polarity of the LED? (I'm at work and cant test till I get home).
 

messy

Jan 12, 2017
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I turned the LED around and it didn't light at all (as id expect tbh)

I wont lie, I pulled it apart and re-did it a bunch of times when making it, particularly around R2 (100k).
 

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kellys_eye

Jun 25, 2010
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The 555 output is pin 3 - your images show pin 3 connected to +Vcc and pin 4 as the output.......
 

messy

Jan 12, 2017
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The 555 output is pin 3 - your images show pin 3 connected to +Vcc and pin 4 as the output.......

I think that must just be the picture angle, pin 4 is a short black wire and is connecting to +V
Pin 3 is the very long green wire connecting to the beige resister which in turn connects to led - ground
 

WHONOES

May 20, 2017
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Are the outer power line strips continuous? The breaks in the red and blue line seems to suggest they are not. If not, that could be you problem.
 

messy

Jan 12, 2017
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the power lines are indeed continuous, the led is getting power and is steady on (about 2.5 volts i think? will check again when i get home).

The power lines are separate from the central component section and run the length of the board.
The central tracks run in lines across the width of the board with a break down the very center. - You can see the chip straddles the break.
 

WHONOES

May 20, 2017
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Tried you circuit in my simulator and it works fine at a shade under 7Hz.
What voltage level is pin 3 on the 555 stuck at? Also check that you have the correct voltages on the supply, ground and reset pins, particularly the reset as if it is floating or connected to ground, it will not work.
 

messy

Jan 12, 2017
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when i measure across the led legs i get 2.5

2.55 on pin 3 output
4.29 on pin 4 reset
2.57 on pin 1 ground
2.55 on pin 8 +V
4.29 on pin 2 trigger
4.29 on pin 6 thresh
9 on pin 7 dischrge

0.11 across the led resitor
0.11 across R1
4.68 across r2
4.3 across the cap

all values are steady
 
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kellys_eye

Jun 25, 2010
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Check your timing component values. You are using a 100k pot where your schematic shows 10k and the 1k resistor value may be suspect (I can't see the colours properly). What about the timing capacitor? Correct value? Correct polarity?
 

Alec_t

Jul 7, 2015
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Pin 1should be 0V and pins 4 and 8 should be 9V (assuming you are using a 9V supply).
 

messy

Jan 12, 2017
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the 10k was switched to 100k to lower the frequency. I believe with a 100k it should now be about 1Hz?
I am using a 9v battery
The capacitor is def 10uF and polarity is correct

I re-checked the values and they are the same however I may be using the multimeter wrong.
When reading the value of the pins im holding the red (V) probe to the chip pin and the black gnd probe to the negative terminal of the battery - is that correct?

The multimeter dial is set to DCV 20
 

messy

Jan 12, 2017
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i removed the beige led resistor since it didn't seem to be doing much, v over the led went up .03 as you'd expect,

i read the resistance of the 100k at max resistance and with the MM set to 200k i was getting a reading of 44 for some reason.

swapped it back for the original 10k which is reading 10 on 20k setting which seems better. Hasnt changed anything tho, output is the same.





it must be something im doing (or not doing) as I have another circuit on breadboard for a basic atari punk console (synth generator) and thats doing exactly the same - its giving a continual tone rather than an on/off square wave.

I actually did the led one in an attempt to figure out what the problem was :p
 
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Alec_t

Jul 7, 2015
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When reading the value of the pins im holding the red (V) probe to the chip pin and the black gnd probe to the negative terminal of the battery - is that correct?
Yes, providing battery negative is actually connected to your circuit correctly.
 
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