555 Timer Frustrating Me

Twinfun2

Hey Everybody!

I have been using this page as a guideline to learning how to use a 555 timer:

http://www.kpsec.freeuk.com/555timer.htm

I have been trying to make an LED flasher by using a 555 Timer for the first time, but after 2 days, I have been unable to see my problem. I am using the astable mode schematic seen in the astable mode section of the page I have been using. I am using C = 10uF Capacitor,
R1 = 10k, and R2 = 100k. I understand that this should yield about 40 blinks by the light according to the chart I am using.

My problem is that after I build the circuit onto my breadboard, the light constantly stays on, and does not blink. I have also noticed that the 555 gets terribly hot, but I have no idea why. Also, I saw posts with people who were able to get voltage ratings on each pin. How would I do that, and is an analog multimeter accurate enough?

Thanks in advance! Don't hesitate to ask me any necessary questions.
Brandon

negitron

The 555 should not get hot at all and the circuit diagram you linked to should work as drawn. Re-check each and every connection (assume nothing and double-check the pinout on the data sheet for your specific device); if you're absolutely certain they're correct then it's possible the 555 is bad. If you still can't get it working, a good-quality photo of your breadboard setup might be helpful to us.

Twinfun2

I have no access to a digital camera at this time, but when I do, I'll most definitely post those images. I used the timer in another circuit layout that drives a PNP transistor to make a speaker buzz like a siren, and that worked without any issues. Also, in my last post i meant "about 40 per minute", which I'm sure was pretty obvious anyway.

Any recommendations will be most appreciated, as I cant be 100% sure if anything is correct if it isn't working as planned, or even if its wired correctly.

Again, I will try and post images as soon as possible.

Thanks

negitron

Silly question: Are you wiring the LED between pin #3 (Output) and ground (0V) or between pin #3 and +Vs, and are you including an appropriately-sized current-limiting resistor?

Twinfun2

Between Pin #3 and +Vs, with a resistor just before the LED.

(*Awaits emotional slap in the face *)

negitron

There's your problem, I suspect. Connect the LED instead between pin #3 and ground with the cathode to ground. Assuming a +5 V output level and a 2.0 V drop across the LED (typical for standard red and green LEDs,) use a (5-2) / .010 = 300-ohm resistor (standard value 270-ohms is close enough) in series on either side.

Twinfun2

LED still doesn't blink sadly; solid red light.

I am using a 6v (4aa) power supply, if it helps or explains anything.

Redbelly98

Does it matter? The duty cycle is nearly 50%, so either way should work.

A few more questions to help with the troubleshooting:

• What is the value of the current-limiting resistor (the one in series with the LED)?
• What is the value of the source voltage V_s?
• Is there a 0.01 uF cap between pin 5 and ground, as shown in the circuit here:
  http://www.kpsec.freeuk.com/555timer.htm#astable

negitron

Depends on what +Vs is and what the logic level output of pin #3 is. If +Vs - Vout is more than the Vf of the LED, it will always be biased on. Best practice is to connect through to ground.

An additional question: which flavor of 555 are you using? There are, for example, CMOS versions and TTL versions and they are not necessarily directly compatible.

Redbelly98

Okay, I was assuming V_o=V_s when the output is high -- that has been my (limited) experience. I was unaware that this is not in general true.

Or better yet, provide the manufacturer and exact part number.

negitron

Looks like for most flavors of 555, Vout(high) = Vs - ~1.7 V.

chroot

I don't have much time at the moment to thoroughly consider the deductive paths the others here are taking you down, but I'm only concerned right now about your 555 timer getting very hot. I won't go into details, but often the only way for this to be possible is to run the part way above its rated power supply voltage, activating its ESD structures and drawing as much current as your (presumably) small wall-wart transformer can provide.

Look up the datasheet for your exact 555 part on the manufacturer's website, and find its intended power supply range. Also, please explain to us how you're powering the part.

(It's also worth mentioning that activating the ESD structures on a chip generally derates it -- damages it measurably, making it less reliable -- and continuously powering it above its intended supply range could destroy it pretty quickly.)

- Warren

negitron

He did say 4 AA cells for a total of 6 V. This seems to be well within the tolerances of all the common types of 555.

Twinfun2

1) 330 Ohm
2) 6v
3) Yes

EDIT: I am using an NE555N; also tried NE555P, but yields no difference. The data sheets for each model seem to allow 6v, where max volt is around 15v-18v for each. I am unsure of finding the exact manufacturers of each timer except by searching for the entire model number shown on the timer itself. The NE555P has nothing but NE555P on it, while the NE555N has "H2L937", which yields no results on Google.

chroot

Okay, from the look of it, the Fairchild NE555N does not have a current-limited output driver, and does not provide a plot of output voltage versus current (shame on them). I can only assume that if you short the output to ground or +Vs, you will draw enough current to damage the device. Check for this.

- Warren

Twinfun2

I bring up again, how would I check for loose connections on my breadboard, and how to check ratings on each pin out properly with my multimeter.

In other topics on this forum, many people have posted that information.

Do I simply need to identify the voltage on each pin?

negitron

Well, you can easily check the voltages on each pin, but you can't so easily check the currents. Best thing to do is to go to each wire and gently pull on it; if it comes out of the breadboard very easily, it's probably too loose and you should use a different connection point. You can also use your multimeter on ohms (or better still in audible continuity modem, if you have it) and ohm out between circuit terminals which should be connected with the circuit unpowered.

However, since you have an overheating problem, it's probably not an open connection which is the issue, but a short. Check for continuity between points which should NOT be connected and correct as necessary.