555 Circuit Problem

like

I only mention this as you say its your very first electronics project. On the breadboard, the 555 chip must be placed strategically straddling the divider section so that there is no conductivity from any pin to any other pin through the breadboard. Also, lookup the datasheet for your

555 chip to make sure its one that is OK with 9 volts. HTH Bart

I am trying out my first electronics project, a LED flasher that looks like

(The one with the 9V battery shown)

I have found numerous examples of the same circuit around the net...but this one looks the clearest.

I have C as 100uF, R1 as 1K and R2 as 22K. Using the calculator, that should have the LED flashing on and off for about 1.5 seconds each state.

The LED is set to draw 10mA on 9V

When I turn on the juice (9v --- and the reason I am writing)...the LED goes flashes as expected BUT

1. It is very dim
2. The 555 is smoking and melting the breadboard.

I have tried several combinations using various intermediate steps for wiring the same result.

Maybe someone with more experience could suggest something to look for.

like

Despite the flash, something is very wrong with your parts and / or your wiring. Please check the orientation of the 555 WRT pin 1, it is the pin to the left of the notch when viewed from above.

Did you use a fresh 555?

Did you verify the power supply is properly connected to the 555?

Is the LED shorted or connected in reverse?

Your LED needs 10ma @ 9v or 900 ohms, use 820 ohms

The resistor should be gray red brown.

--
Sounds like you might have the wiring to pins 7 and 8 reversed.

I would check your battery voltage to make sure it's not dead or below

6 volts. Also you are wasting power using a 1 K resistor for R1. When pin 7 goes low to discharge the capacitor, it supplies 9 milliamps into R1, so you have about a constant 4.5 mA loss in R1. I would use 10K for R1 and 100K for R2 and maybe 22uF for C. That will give you same times and save 5 milliamps.

There is also another problem that the 555 output will not move all the way to +9. It will only get within 2 volts, or about 7 volts on a 9 volt battery. This means when the 555 output moves high to 7 volts, you still have 2 volts across the LED and resistor connected to +9 which may turn it on. Also, you need a larger resistor for the LED connected to +9. For 15 mA current, the resistor will be 470 ohms, not 330.

There is a CMOS version of the 555 (I forget the number) that moves all the way from ground to +V on the output. The dual LED circuit may work better with that part and both resistors can be 470 ohms.

Another idea is to use 2 LEDs in series connected to +9 with a 330 ohm resistor. That way the two LEDs cannot light with only 2 volts applied.

Another idea is to use ultra bright LEDs that are bright at a few milliamps so the little 9 volt battery lasts longer. Raise the LED resistor values about 5 times and the battery will last much longer.

-Bill

As described previously, I have my first 555 project, a LED flasher that looks like

(The one with the 9V battery shown)

I have found numerous examples of the same circuit around the net...but this one looks the clearest.

I have C as 100uF, R1 as 1K and R2 as 22K. Using the calculator, that should have the LED flashing on and off for about 1.5 seconds each state.

The LED is set to draw 10mA on 9V

I got some solid core wire and was able to get things running....sort of.

Using that diagram above, I have one LED/resistor connected from pin 3 to (+) and the other from pin 3 to (-).

When I hit the juice, the two lights alternate on and off as expected at about 1.5 second intervals. However, after a few minutes the lights stop flashing and the LED connected to (+) remains solid on.

If I remove the LED to (-). The other LED blinks for a while then goes steady on. If I put the (-) one back and remove the one connected to (+) it too blinks for a while then goes to solid on.

Any ideas?

"Bill Bowden" wrote in news: snipped-for-privacy@g47g2000cwa.googlegroups.com:

I'm using an AC adaptor.

So, as a general rule is less capacitance/more resistance better here?

What might be the down side of going to

10K 1M 2.2uF

??????

I have some of those TLC555s. Might try them.

I have also been fooling around with some 556 (dual 555s). However I get some funky results.

I was using 1.2K and 12K to get a one second flash and running along side of the configuration mentioned previously. The 556 did not have the problem of the flash stopping. However, there was some "residual current". Every other flash of the 1 second LED gets followed by a very noticable dim period before final cut off. The same happens on 1.5 on/1.5 off circuit but it is less noticable.

As a general rule would one be better off with one 556 or two 555s?

I like to stay in the range of 10K to 100K. If you go higher than 100K, you have insulationn problems and the resistance drops with any moisture on the circuit board. If you go lower than 10K, you waste too much power.

I imagine a 556 would be better in most cases. But, there could be some feedback problems between the 2 circuits that might be cured by using two 555s with separate decoupling networks to the power supply. When you have several circuits on the same PC board that may talk to each other, it's sometimes helpful to run a small resistor from +V to each circuit and also a filter cap from the other side of the resistor to ground. That prevents any noise from one circuit getting into the power supply line of another circuit. Just choose the resistor so it drops a small percentage of the supply voltage. Maybe give up 1 volt on a 12 volt supply and let the circuits run on 11 volts.

-Bill

"Bill Bowden" wrote in news:1132347015.097812.48780 @g14g2000cwa.googlegroups.com:

Is that both resistors in the range 10K to 100K or just R1.

Would you see a problem with R1 == 10K, R2 == 1M, C = 2.2uf

Any idea what might cause this oddity....

I tested

R1 = 10K, R2 = 220K, C = 10uF as suggested (1.5S on, 1.5S off) and R1 = 120K, R2 = 12K, C = 10UF (1S flash)

on a 555 and all worked fine.

However, I tried it on a 556 (dual 555) and first configuration looks as expected. However, the second looks like

________^______^^_______^______^^

In other words OFF, FLASH, OFF, FLASH-FLASH, OFF, FLASH, OFF, FLASH-FLASH

I switched the pairs of resistors and got the same result on both sides.

Usually, the problems are associated with the power supply, or inputs not terminated.

Try using a 1uF cap across the power terminals of the 556 and terminate all unused inputs to ground or +v. Pins 6,8,2,12 can be grounded if not used. Pins 3 and 11 can remain open unless you want to apply an external voltage to change the trigger and threshold voltages. Pins 4 and 10 are resets and should be connected to +v if not used for the reset function.

-Bill

Yes, connect the capacitor from pin 14 to 7. Plus side goes to pin 14.

-Bill

"Bill Bowden" wrote in news:1132457511.453250.224460 @z14g2000cwz.googlegroups.com:

Do you meen connect the input (+) and input (-) with a 1uF capacitor?