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Problem with 555 LED flasher (not flashing)

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lefam

New Member
Hi guys!

I am having problems with my first 555 LED flasher. The LED is not blinking. It just stays ON when I feed power to the circuit.

Details of the circuit:

PIN8 connected to +9V
PIN1 connected to ground
PIN2 connected by a wire to PIN 6.
Resistor R1 connected between PIN8 and PIN7
Resistor R2 connected between PIN7 and PIN6
Capacitor C connected between PIN2 and (ground)

LED protected by a 330 Ohm resistor connected between PIN3 and ground.

I tried the circuit with these configurations:
C=1000uF, R1 = 1k and R2 = 5k
C=1000uF, R1 = 5k and R2 = 1k
C=1000uF, R1 = 100Ohm and R2 = 1k

C=10uF, R1 = 1k and R2 = 5k
C=10uF, R1 = 5k and R2 = 1k

I don't think I am not wiring well the circuit. If the configuration of C,R1 and R2 should lead to a flashing LED maybe my 555 chip is dead.

Please help me to understand why my LED is not flashing. What should be the values for R1 and R2 for a 1Hz flashing LED? Does it matter for R1>R2 and vice-versa?

Thanks in advance
 

audioguru

Well-Known Member
Most Helpful Member
Pin 4 should also be connected to pin 8 which is +9V.
The LED will stay turned on if the capacitor polarity is connected backwards or if the capacitor is shorted. The (-) wire of the capacitor must be connected to ground (pin 1 of the 555).

With 1k, 5k and 1000uF then the frequency will be low so that the LED lights every few seconds.

Edited a typo error.
 
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lefam

New Member
audioguru said:
The LED will stay turned on if the capacitor polarity is connected backwards or if the capacitor is shorted. The (-) wire of the capacitor must be connected to ground (pin 2 of the 555).

Sorry, I did not understand. The (-) wire of the capacitor (the shorter leg with a silver strip) must go to PIN2 or ground? Or are you saying that PIN2=GROUND?

I understand that the (-) wire of the capacitor should be in PIN2 and the (+) in ground. Is it?
 
A 555 timer works in a really simple way, when the capacitor charges to 2/3 of the voltage supply it is sensed by pin 6 and switches its output to low and discharges the capacitor through pin 7 via an internal PNP transistor. Once the capacitor is discharged to 1/3 of the voltage supply, it is sensed by pin 2 of the chip and switches the output to its high state and the capacitor begins charging again. Pin 4 is the RESET pin, and if it is held low, the output of the 555 will also be low, so pin 4 should be connected to pin 8. You should also check pin 5 with a volt meter, if it is close to 9V then your duty cycle is 100% and the output will always stay high. You can try adding a capacitor of any value to pin 5 to stabilize it, or try holding it in a voltage divider (between 2 series 10k resistors).

What AG is saying is that if you're using a polarized capacitor then you may have installed it backwards (in which case the capacitor will act more like a diode) since the capacitor is not charging, the output stays high and this can also happen if the capacitor is shorted. Check your connections one more time and see if any of this helps. (by the way, a 1000uF capacitor is very large and will take some time to charge)

If everything checks out once you have confirmed your connections, and the 555 still does not oscillate, then your chip may be bad and you should try replacing it.

Hope this helps; Vince
 

audioguru

Well-Known Member
Most Helpful Member
Sorry, I did not understand. The (-) wire of the capacitor (the shorter leg with a silver strip) must go to PIN2 or ground? Or are you saying that PIN2=GROUND?

I understand that the (-) wire of the capacitor should be in PIN2 and the (+) in ground. Is it?
Sorry, I made a typo error. The (-) wire of the capacitor connects to ground (pin 1 of the 555). The (-) wire has the nearest side of the capacitor marked with a bunch of --- lines.
 

MikeMl

Well-Known Member
Most Helpful Member
Since you are a newbie, the most valuable piece of information I will pass on to you is to READ THE DATA SHEET (**broken link removed**).

If you had read the data sheet, Fig 4 would have showed you that an unused pin 4 should be tied to Vcc (+9V). (your likely biggest error)
If you had read the data sheet, Fig 4 would have showed you that an unused pin 5 should be bypassed to GND with a 10nF capacitor.
If you had read the data sheet, Fig5&6 would have showed you that your values for R1, R2 and the Ct are ok.
If you had read the data sheet, Page 10, ADDITIONAL INFORMATION, first paragraph is HUGELY IMPORTANT. (Your second biggest error)

My suggestions in order of importance:
1. Add the missing pin 4 wire.
2. Check polarity of timing capacitor; Plus should go to pins 2,6. Minus to GND
3. Add a power supply bypass capacitor, 1 to 10uF, Plus to pin 8, Minus to pin 1, short leads.
4. Add the 10nF bypass capacitor between pin 5 and GND.
5. Try another chip

Check to see if the astable oscillates after each step above...

When copying any circuit posted on the Web, be very suspicious. Most have serious errors. IMHO, they are created by computer nerds who are really good at creating fancy web pages, but don't know which end of a soldering iron to pick up.

Test to see if it oscillates after each step above...
 
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lefam

New Member
Yes, I realized that I was not connecting correctly the timing capacitor.
So I bought a new chip and tried again attaching correctly the timing capacitor and adding the PIN4 wire. But it is still not flashing.

I used C=220uF, R1=2k, R2=10k

I am pretty sure all the PIN connections are ok.
I suspect the capacitor is dead. When I measure the capacitance using my multimeter it gives a wrong value. Does it mean the capacitor is dead?

When I use the same capacitor to make a LED fade out circuit it works. The interesting thing is that I used to make the circuit with the capacitor attached backwards and it worked but took a very long time (>1min) to completely turn off the LED.
When I attach the capacitor correctly, the LED fades out and turns off in 3 seconds.

Today I will buy new capacitors (I suspect I killed all of them).

The lack of the 10nF bypass capacitor may be the problem? Or just the timing capacitor?
 

ericgibbs

Well-Known Member
Most Helpful Member
Yes, I realized that I was not connecting correctly the timing capacitor.
So I bought a new chip and tried again attaching correctly the timing capacitor and adding the PIN4 wire. But it is still not flashing.

I used C=220uF, R1=2k, R2=10k

I am pretty sure all the PIN connections are ok.
I suspect the capacitor is dead. When I measure the capacitance using my multimeter it gives a wrong value. Does it mean the capacitor is dead?

When I use the same capacitor to make a LED fade out circuit it works. The interesting thing is that I used to make the circuit with the capacitor attached backwards and it worked but took a very long time (>1min) to completely turn off the LED.
When I attach the capacitor correctly, the LED fades out and turns off in 3 seconds.

Today I will buy new capacitors (I suspect I killed all of them).

The lack of the 10nF bypass capacitor may be the problem? Or just the timing capacitor?

hi lefam,
Do you have a circuit diagram to post, lets see how you have it connected..??
If yes, mark the polarity connections for the 220uF on the drawing, just in case you have it reversed.
 

lefam

New Member
It is not reversed. (+) ON PIN2 and (-) on ground.
(-) is the shorter leg with a (grey/silver) strip on its side.

Can you recommend me nice programs to draw schematics?
 

ericgibbs

Well-Known Member
Most Helpful Member
It is not reversed. (+) ON PIN2 and (-) on ground.
(-) is the shorter leg with a (grey/silver) strip on its side.

Can you recommend me nice programs to draw schematics?

hi,
A free download is expresspcb,,, its got a schematic and pcb layout option.

https://www.expresspcb.com/
 
Last edited:

kp2010

New Member
post your schematic here and if u want i have flashing leds project with me its fun....i can share here
 

audioguru

Well-Known Member
Most Helpful Member
I copy and paste parts of datasheets and other schematics into Microsoft Paint program. Straight lines are drawn with the SHIFT key held down.

With a 220uF timing capacitor and 1k for R1 and 10k for R2 the LED will blink about every 3 seconds using the formula on the datasheet.
If the polarity of the timing capacitor is connected backwards then the LED will probably be turned on continuously.
 
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