555 timer problem. HELP!!!?

Question

trying to build a strobe light using a 555 according to this schematic: http://i.imgur.com/xAVqb.png in case you can't read the writing on the pic, R1=680 ohms, R2=3.5K and C=22uF according to a 555 simulator I found online, this should produce a square wave with a duty cycle about 50% and a frequency of about 8.5 Hz.
The output of the 555 goes to a 2N3904 that controls power to an LED array. The power to the chip is an 18V wall wart, that feeds to the chip through (initially 68 ohms, that burned so now only 22 ohms, the only higher wattage resistor i had around) Because of the resistor burning (oh, and the lights didn't flash, just went on constant) I figured something was wrong- I took some measurements, the whole circuit (timer and LEDs) is pulling almost an amp. The LEDs alone, connected directly to power normally only pull a few hundred milliamps and I know a 555 should only pull a couple mA, so what's wrong? Did I fry my 555? I haven't spotted one yet, but if there were a solder bridge somewhere, where might one be to cause this?
And please no "OMG you're a f*cking idiot, you did this this and this wrong" answers, I'm new to building electronics, this is me trying to learn.

Answer 1

Do the math. Do the math. Do the math! A 555 should draw about 10mA at 15VDC so to drop even 3 Volts you need a 300 Ohm resistor. But then the voltage will go up and down as it switches so a better idea is say a 7812 regulator. In both cases, the reduced voltage needs a decoupling capacitor. I also hope you put a series resistor on the 2n3904 base and another in series with the LEDs. Other wise the resulting currents are undefined ie stuff will burn out in an instant.

Answer 2

It is most likely that ou "killed" the 555. Next time, feed the chip with a regulated supply (7805 or 7812) and use the raw "18 volts" for the LED/transistor circuit. A resistor on the base of a generic NPN in the range of 1k to 10k is always a good thing. Also, build the circuit on a solderless breadboard while designing/experimenting...saves time and frustration! Build the circuit in two parts. Test the 555 half seperate from the LED driver half. Make sure they work independantly, the put them together. The 22uF electrolytic should be fine as long as the negative lead is connected to the ground buss.



BTW.. you can make a non-polarized cap from two back-to-back electrolytics....just double the values so they will be correct when combned in series.

Answer 3

Bit bothered about your LEDs pulling a "few hundred milliamps". You may be asking too much of the 2N3904 as well which is only rated at Ic=200mA continuous so a medium sized power transistor such as a TIP31 (Ic=3A) would be more robust. A current limiting resistor of 1kΩ between pin 3 of the 555 and the base of the transistor definitely won't hurt and is seriously recommended.The 22μF electrolytic is fine with the negative terminal to ground, the circuit doesn't require a non-polarised capacitor in that or any other position.



You can check the frequency using the calculator at http://www.csgnetwork.com/ne555timer2calc.html which for the 2KΩ pot at minimum is 17.82Hz at 59% duty cycle and 8.54Hz at 54.4% duty cycle at its maximum position. Above 8.54Hz may be difficult to see so maybe a 5KΩ might be better to slow things down. Using the larger pot will give you down to 4.79Hz with a duty cycle of 52%.



So your shopping list is :



555 (get two, save a trip if things go wrong again, cheap enough)

7812 12V regulator, the 1A TO220 cased version and not the 78L12 low power version

1μF electrolytic capacitor for smoothing the +12V output from the 7812

TIP31 NPN medium power transistor

(optional) 5kΩ pot



Connect all parts of the rebuilt LED flasher circuit to the new +12V rail and not to the unregulated input from the wall wart. http://pdf1.alldatasheet.com/datasheet-pdf/view/4475/MOTOROLA/7812.html

Answer 4

The maximum supply voltage is given as 15 volts. A wall wart rated at 18 volts would give a higher voltage at less than it's maximum current. A single series resistor of 68 ohms would not be sufficient to reduce the voltage to a safe level, so you have probably fried the 555.



http://en.wikipedia.org/wiki/555_timer_IC#Specifications