# 555 timer strobe light not working

1. Apr 14, 2010

### xxslackerxx

Hi, I have a circuit that I am working on that I adapted from the 6v version here http://electronician.blogspot.com/2009/09/555-timer-project-15v-led-flasher.html I have a 12v supply with a total of 48 leds 24 on each bank. I have gotten previous version to work with single leds but now that I have added more only one bank lights and it stays lit with out ever flashing. I have found on info on other sites that prompted me to to put relays in where the output goes to led. Any help would be greatly appreciated.

2. Apr 14, 2010

### vk6kro

When you put LEDs in series, the voltage needed to light the combination increases.

For example, if each LED needs 3.5 volts to operate, (if it was a white LED), then 10 of them would need 35 volts before you would see anything.

24 would need 84 volts.

With a 12 volt supply, you could light 3 LEDs with a series resistor of 68 ohms.
You could put more of these in parallel, though, up to the limit of what the 555 could drive.
Each time, you would need a resistor and 3 LEDs.

Before trying this, though, you might like to just make up one string of 3 LEDs and a resistor to see if it will be bright enough. You may even need to reduce it to TWO leds if yours need close to 4 volts. If you did this, the resistor should be about 180 ohms.

3. Apr 14, 2010

### xxslackerxx

I do have the leds in parallel and I have gotten this to work on the breadboard fairly nicely, but when I soldered it all onto a chip it stopped. Ive checked and rechecked solder joints but I cannot figure out what changed.

4. Apr 14, 2010

### xxslackerxx

Btw they are 20 mA leds that use 3.3v forward. used a resistor calculator online to decide on the resistor values leds are getting enough power but seems to me that no pulse is coming from the 555 but thats just a student's impression

5. Apr 14, 2010

### vk6kro

Maybe you could draw a diagram of what you are using? Just use MS Paint if you have Windows XP or draw it on paper and take a digital picture of it. Reduce it to under 100 K first, though.

See the little paper clip thing at the top of the edit screen? You can use that to attach a picture or a drawing.

Try to simplify your circuit and see if the 555 can switch ONE LED, then two, then three, etc.

6. Apr 14, 2010

### xxslackerxx

Hello,
Although my paint skills are not the best I hope this helps.

Where:
C1 = 10uF
C2 = 0.1uF
R1 = 82 ohms
R2 = ! MegaOhm Variable Resistor
R3 = 150 Ohm
R4 = 150 Ohm

Six LED's in each bank,
Each LED specs are 20 mA and 3.3V

#### Attached Files:

• ###### Circuit.jpg
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7. Apr 15, 2010

### vk6kro

Thanks for trying with Paint.

Here is how you could use relays to switch the LEds:

[PLAIN]http://dl.dropbox.com/u/4222062/led%20switching%20with%20relay.PNG [Broken]

Your diagram does not show you doing it like this.

Note the diode across the relay coil. This is to protect the driving device from spikes in voltage due to the coil.

The diagram on the right shows a transistor driving the relay if the 555 could not manage it.

The 555 output, or the transistor, only drives the coil of the relay. The LEDs are switched by the relay contacts.

Last edited by a moderator: May 4, 2017
8. Apr 15, 2010

### xxslackerxx

The original circuit used a single 555 to switch both banks without relays, I added them after I boosted the voltage and I did have diodes on the relays but when they were in nothing light up. at least without them one bank lights

9. Apr 15, 2010

### vk6kro

You have the relay in now, so can you hear it clicking?

Relays normally make a noise when they switch on or off. If yours isn't clicking, your 555 may have blown up or the diode might be in the wrong way around. The diode should not affect the normal operation of the circuit.

Did you understand this:
The 555 output, or the transistor, only drives the coil of the relay. The LEDs are switched by the relay contacts.

I can only repeat this:
Try to simplify your circuit and see if the 555 can switch ONE LED, then two, then three, etc.

You might have LEDs in the wrong way around or lots of other things could be wrong, but you need to find them.

10. Apr 15, 2010

### xxslackerxx

It clicks when power is switched on but not again. leds light when battery applied. What am I looking for if the ic is bad, or how can I test it.

11. Apr 15, 2010

### xxslackerxx

I cut leds to just one per relay still just one led lit steady. I measured output pin with voltmeter reading close to -12v steady. I don't know if that wil help you or not.

12. Apr 15, 2010

### vk6kro

The relay is not being switched.

With one end of the relay coil connected to 12 volts, disconnect the other end of the relay coil from the 555 output and connect this end to ground briefly. Does this cause the relay to make a clicking noise?

Do the LEDs turn on when you do this?