555 timer flashing LED circuit

[matt_hau]

Can someone please help me out here? I am trying to build a flashing LED using an ultra bright LED. I am getting conflicting results from various online calculators.

I am using an NE555P and trying to drive an ultra bright 3.4V 20ma led from a Vcc of 12vdc. I want the led to flash at a rate of approx 600 times a minute to emulate machine gun fire. Will the below circuit with the values of RA 56K, RB 56K, C 0.001uF and LED resistor 470R work ?

I would also like to be able to drive 12v solenoid from the timer with the option to activate the solenoid at the same time as the led and also opposite timing to the led.

 

[mneary]

What are your conflicting results?

 

[matt_hau]

555 timer

What are your conflicting results?

I am just not sure if I am reading the values correctly on the calculators, some are in farads or micro farads and i have done the conversion on a spreadsheet to double check,

When i built the circuit initially with RA and RB using 500ohm and 100uF capacitor the led came on and stayed on. I recheck it on another calculator and I get 50K and .001 uF.

Confusion is setting in.

 

[mneary]

500R and 100uF gives a an output of maybe 20 Hz, which should flicker or maybe even appear to stay on at 1/2 brightness. Try 1.5k ohm.

If you want more sensible results try starting the calculators with 0.1 uF.

Most solenoids will have a hard time reacting to 10 pulses per second.

 

[matt_hau]

I want to achieve this.

**broken link removed**

 

[mneary]

It may appear that the steam is being interrupted at the same rate as the LED, but I think it only looks that way because of the strobe effect.

Any clarity on the calculations?

 

[matt_hau]

It may appear that the steam is being interrupted at the same rate as the LED, but I think it only looks that way because of the strobe effect.

Any clarity on the calculations?

The gas is being interrupted giving the cracking sound,
I have tried to slow down the frequency of the flash but the led still stays on constantly. I did notice that when i touched the underside of the 555 the light went out and the when i touch again it comes back on. There is no bad solders on it so I am thinking my finger is causing a discharge of the cap causing the volts to go low therefore turning of the LED??????

Some other diagrams I have seen show a cap from pin 2 to earth as well as jumped across to pin 6 or a 0.01uF cap from pin 5 to earth. Should I be doing this?

 

[ecerfoglio]

.....................

Some other diagrams I have seen show a cap from pin 2 to earth as well as jumped across to pin 6 or a 0.01uF cap from pin 5 to earth. Should I be doing this?

In your original post, pin 2 has a label "To pin 6". That indicates that you should connect those pins together.

In the "clasical" astable (oscilator) 555 configuration, pin 2 and 6 should be connected together - The timing capacitor goes from the junction of pins 2 and 6 to ground, there is one timing resistor between pin 7 and the junction of pins 2 and 6 and another timing resistor from pin 7 to +V.

The circuit will work without the 0.01uF capacitor between pin 5 and ground, but it is good practice to use it, as well as a 0.1uF power supply decoupling capacitor between pin 8 and ground.

 

[fingaz]

Hi,

If the led's flashing at 600 times per minute (10 times per second) I wouldn't have thought you'd notice, won't it just look like it's flickering slightly?

 

[matt_hau]

Hi,

If the led's flashing at 600 times per minute (10 times per second) I wouldn't have thought you'd notice, won't it just look like it's flickering slightly?

Agreed, I tried to slow down to 4 times per second but had the same result. New set up to try out will be Ra 1K Rb 91K and C1 1uF, That should give me a frequency of 7.88Hz with a 50% duty cycle.

 

[AllVol]

Have you connected pin 2 to pin 6 as directed?

 

[matt_hau]

Have you connected pin 2 to pin 6 as directed?

Yes, had done form the start. I have not done the caps on pin 5 and 8 yet though.

 

[Krumlink]

Just in case if your not absolutely stumped yet:

http://www.kpsec.freeuk.com/projects/flashl.htm
It is strip board so it is connecting horizontally.

 

[matt_hau]

Just in case if your not absolutely stumped yet:

http://www.kpsec.freeuk.com/projects/flashl.htm
It is strip board so it is connecting horizontally.

Thanks for the link it looks good and is pretty much what I have already got. I have just read though that with DC voltage I should run a decoupling capacitor. I will give the decoupling a go tomorrow and try the bigger RB resistor to slow down the frequency as per the link.

very frustrating but I am learning lots.

 

[matt_hau]

I finally got it!! RA 1,000K RB 91,000k C 1nF. It flashes perfectly at about 7 cycles per second and I have even hooked it to a sugarcube relay (any risks in that?)

I found one site that suggested the minimum RA should be 3K and worked from there to achieve the desired flash rate.

 

[audioguru]

A relay gives a very high voltage spike whenb it turnes off. A reverse-connected diode across the relay coil arrests the voltage spike so it doesn't destroy the 555.