# 555 Timer Circuit LED Flasher - Wont Flash

#### Shamrock330

Joined Dec 9, 2018
3
Hi all,

I am currently working on a project in which I am required to construct a 555 timer circuit that makes an LED flash on/off in 1-2 second periods. This circuit is powered by taking a 240AC with 2 12v secondary windings and converting it to 15-16V DC by using a full wave rectifier.

So far, I have managed to construct both of these circuits. The result I am being given is an LED that stays on when it is supposed to flash. The breakdown of the components used in this project is:

555 Timer Circuit:
R1 (value of 1K)
R2 (value of 330K)
R3 (value of 1K)
Capacitor C1 (value of 4.7 microfarads)
555 Timer
LED

Apologies in advance for the rather poor diagram/plan:

Following calculations made using CircuitDigests astable 555 timer LED flasher calculator, the R1 and R2 values of 1 and 330K respectively and the C1 value of 4.7uF were shown to give an on/off period of 1.04 seconds (within required range). However I am getting a constant ON from the LED. I have so far replaced the 555 pin 3 times and replaced the capacitor twice and ensured the polarity was correct. The DC rectifier circuit has also been run through an oscilloscope (as seen below).

(testing the 555 circuit using an independent power supply)

If my rectifier circuit is also giving the same result as the power supply, would I be right in saying that it is functioning correctly? The rectifier circuit does not include smoothing but from research appears to work just fine for this project.

#### dl324

Joined Mar 30, 2015
15,423
Welcome to AAC!

This is what I transcribed from your stripboard wiring diagram:

We don't like to see CV (pin 5 floating), but that shouldn't be causing the problem you're seeing.

Put your scope probe on pin 2 to see if the capacitor is charging/discharging.

You should get a solderless breadboard to test circuits before soldering things.

#### absf

Joined Dec 29, 2010
1,968
I simulated your circuit on proteus and I think it was working OK.

The ON time is about 1.1 seconds but the OFF time is roughly about 3 mS. That's why it appears as if it were ON all the time. Our eyes would not be able to catch the blink of only 3mS.

Make the OFF time slightly longer (>100mS) and you'll be able to see it.

I just make R1 22K then was able to see the blink.

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#### AlbertHall

Joined Jun 4, 2014
12,186
The problem is with the mark/space ratio of the oscillator. The LED will be on for about one second and then it will be off for about 3ms (1/330 seconds). The capacitor charges through R1+R2 (1k+330k) and then discharges through R1 (1k). To make the on and off times roughly equal exchange R1 and R2.

#### Shamrock330

Joined Dec 9, 2018
3
Welcome to AAC!

This is what I transcribed from your stripboard wiring diagram:
View attachment 165465
We don't like to see CV (pin 5 floating), but that shouldn't be causing the problem you're seeing.

Put your scope probe on pin 2 to see if the capacitor is charging/discharging.

You should get a solderless breadboard to test circuits before soldering things.
I simulated your circuit on proteus and I think it was working OK.

The ON time is about 1.1 seconds but the OFF time is roughly about 3 mS. That's why it appears as if it were ON all the time. Our eyes would not be able to catch the blink of only 3mS.

Make the OFF time slightly longer (>100mS) and you'll be able to see it.

View attachment 165466

I just make R1 22K then was able to see the blink.
The problem is with the mark/space ratio of the oscillator. The LED will be on for about one second and then it will be off for about 3ms (1/330 seconds). The capacitor charges through R1+R2 (1k+330k) and then discharges through R1 (1k). To make the on and off times roughly equal exchange R1 and R2.

Thank you! I changed the R1 value to 22K and as expected this made the ON/OFF sequence visible. This worked when testing the circuit on an independent power supply however the light continued to stay ON when the circuit was being powered via the rectifier circuit so the problem seems to be with that now.

#### AlbertHall

Joined Jun 4, 2014
12,186
In the picture you give in post #1 there is no reservoir capacitor after your rectifier. You need to have 200uF to 1000uF there to make a nice smooth DC voltage.

#### AnalogKid

Joined Aug 1, 2013
10,170
If you reverse R1 and R2, you should have near 50/50 on and off times.

ak

#### Dodgydave

Joined Jun 22, 2012
10,583
Looking at your transformer , it's wired for 12V-0-12Voutput without any smoothing capacitors, put a 1000uF electrolytic capacitor across the DC .

#### jbeng

Joined Sep 10, 2006
84
The one thing I would do to see the LED flash is to connect the circuit like my image shows. You'll probably be able to see the flashes if you do that. Otherwise, it's "ON" most of the time and "OFF" only briefly, as AlbertHall stated. By reversing the polarity, so to speak, it will now be "OFF" most of the time and briefly flash "ON".

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#### Shamrock330

Joined Dec 9, 2018
3
In the picture you give in post #1 there is no reservoir capacitor after your rectifier. You need to have 200uF to 1000uF there to make a nice smooth DC voltage.
If you reverse R1 and R2, you should have near 50/50 on and off times.

ak
Looking at your transformer , it's wired for 12V-0-12Voutput without any smoothing capacitors, put a 1000uF electrolytic capacitor across the DC .

The one thing I would do to see the LED flash is to connect the circuit like my image shows. You'll probably be able to see the flashes if you do that. Otherwise, it's "ON" most of the time and "OFF" only briefly, as AlbertHall stated. By reversing the polarity, so to speak, it will now be "OFF" most of the time and briefly flash "ON" .

Thanks! I added a 1000UF capacitor as advised along with a zener diode (with a 1K resistor) and managed to get it working with an output voltage of 12.9V.

#### ScottWang

Joined Aug 23, 2012
7,305
The label of resistors (R1, R2 should be exchange) was wrong when it is comparing with the normal label, a wrong label looks quite strange to me, specially when I used it over 35 years had a fixed image.

R1 = 22K and R2 = 1K, the ON/OFF duty cycle will be more equal, because R2 for charge current only, but the charge current and discharge current both will flow through R1. (see the circuit in #2)