# Problem with 555 LED flasher (not flashing)

#### lefam

Joined Nov 30, 2010
33
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?

#### retched

Joined Dec 5, 2009
5,207

#### macgyver

Joined Dec 7, 2010
1
what's the voltage reading in PIN3 of the 555?

#### bertus

Joined Apr 5, 2008
22,290
Last edited:

#### retched

Joined Dec 5, 2009
5,207
good call mac and bertus.

This is a good reason why a drawn schematic is better than a spoken (typed) one.

Also lefam, are you building this on a solderless breadboard?

#### Wendy

Joined Mar 24, 2008
23,464

#### bertus

Joined Apr 5, 2008
22,290
Hello Bill,

In his opening post he has a list with the values of R1,R2 and C.

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
Bertus

#### Wendy

Joined Mar 24, 2008
23,464
My bad, so he did. Tired computing will do that to you.

Assuming he is using the same schematic (a large assumption), the first set of values will be around 0.13Hz, or every 7.6 seconds. 555 circuits also take a lot longer on the first flash, so it will be over 10 seconds.

I'm betting on a simple wiring error, though there is another possibility.

Really large caps, when they are old, can be finicky. The resistors values are low enough that it shouldn't matter, but there is always the possibility of a bad cap.

#### lefam

Joined Nov 30, 2010
33
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 connection to +V. 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?

#### Wendy

Joined Mar 24, 2008
23,464
The chips are probably fine. We must have the schematic you are using before we can help, it is an absolute. After that I can tell you the voltages you should see at the various pins.

I write a lot about this little chippie, it seems complicated at first, but it really isn't.

Are you using a protoboard? This (as defined by me) looks a lot like this...