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Problem with 555 LED flasher

lefam

Nov 18, 2010
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Nov 18, 2010
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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
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
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25,505
Do you mean like this?

555ast.gif


Did you miss the connection to pin 4?

With the 10uF capacitor, and the resistors you have selected, the frequency may be high enough that the LED appears to be on all the time.

With the 1000uF capacitor, the time period is probably in the range of 5 to 10 seconds, except in the case when you use a 100R resistor.
 

lefam

Nov 18, 2010
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Is there any difference making R1=5k, R2=100R vs R1=100R, R2=5k?

Is it compulsory to connect the PIN4 to +Vs?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Is there any difference making R1=5k, R2=100R vs R1=100R, R2=5k?

The charge path is through both resistors, the discharge path through only one of them.

So the difference is in the discharge time as you are always charging via both the 5k and the 100R resistor, but discharging through 100R in the first case, and 5k in the other.

Is it compulsory to connect the PIN4 to +Vs?

The datasheet says that you should connect it to V+ for astable operation. That's good enough for me.
 

lefam

Nov 18, 2010
43
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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?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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The lack of a capacitor on the control pin is probably of no consequence (it can often be omitted). And that pin can be left disconnected.

I can't tell what a wrong reading is that your meter might give. What reading does it give, on what range?

Connecting an electrolytic the capacitor around the wrong way will almost certainly make it exhibit excessive leakage. That would explain both the long time to turn off a LED in the "dimmer" circuit, and the lack of oscillation of your 555.

Your long period of oscillation (in this case 3 seconds) demands that the leakage of the capacitor be quite low. Connected backwards, the leakage could quite easily have prevented it charging to 2/3 V+.

And what is your supply voltage, by the way?

Connect the capacitors to your power supply (the right way around) via a 1k resistor and measure the voltage across the 1K resistor. This voltage should fall to zero, or very close to it, after about 5 to 10 rc periods (r in ohms * c in F, gives rc period in seconds). The voltage across the resistor falls by 67% each rc period. If it does not fall to a very low value, the capacitor may be permanently leaky.
 

lefam

Nov 18, 2010
43
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*steve* said:
And what is your supply voltage, by the way?
9V

*steve* said:
Connect the capacitors to your power supply (the right way around) via a 1k resistor and measure the voltage across the 1K resistor. This voltage should fall to zero, or very close to it, after about 5 to 10 rc periods (r in ohms * c in F, gives rc period in seconds). The voltage across the resistor falls by 67% each rc period. If it does not fall to a very low value, the capacitor may be permanently leaky.
I will try it. Nevertheless I will buy new capacitors.
 

lefam

Nov 18, 2010
43
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Nov 18, 2010
Messages
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Hi guys. The LED is flashing now.
I realized a few days ago that I was not connecting correctly the battery. That fact killed my first 555 ICs. I bought new ones and my LED is flashing now.

Thank you. I learned a lot with you guys.
 
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