Flashing LED Police Lights - noise?

[RandomGlitch]

Hi there,

I'm building a "police" themed fuzz (pun intended!) for a friend. In addition to police-themed livery I'm planning on having red and blue alternating flashing LEDs as status indicators.

Now I've built a little circuit based on a 555 timer.  It's a very simple circuit, 101 electronics stuff really.  Having read threads on here I have a couple of concerns about putting it in a fuzz pedal, that I hope people could advise me on.

(1) Will the flasher circuit generate clicking noises in my fuzz? If so, is there any way I can prevent it?

(2) What is the best way to switch the circuit? Cut the power to it or just switch the LEDs and leave the timer circuit powered up? I'm not concerned about power use as it will be DC power adapter run, no battery.

Many thanks!
Random

[armdnrdy]

If your going to use a 555 use a CMOS version. LMC555, TS555, IMC7555, etc.

The regular 555 takes more power and will most likely cause the noise that you are inquiring about. I'd use a CMOS version and throw a decoupling cap (.01uf, .1uf) as close to the 555s power supply as possible for good measure.

Check out the data sheets for the above 555s.

[armdnrdy]

The answer to your other question is: switch the ground to the LEDs.

[Jazznoise]

Try paralleling the led with a cap in series with a resistor, this should soften up the spikes on the signal rail. With all these things, try and keep the runs short. The caps should be sitting next to the legs to stop any runs radiating HF nonsense onto other rails.

I'm curious to know what sort of Fuzz you'd associate with the police. They were always more of a raggae thing to me! 

[RandomGlitch]

Thanks armdnrdy, Jazznoise. No idea if my 555's are CMOS, but I will check it out. 

I already have a 10nF to pin 5 for decoupling do you mean a resistor in series with that?.

I've seen other 555 circuits with a 10-100uF cap from 9v to ground (I do this on my pedal builds anyway) - do you mean do this as well?

I guess baddies are more likely to be associated with fuzz! Cops are more wah-wah maybe... if you live in the 1970's anyway!

Thanks.

[armdnrdy]

Thanks armdnrdy, Jazznoise. No idea if my 555's are CMOS, but I will check it out. 

Check the model number on your 555 and then look up the data sheet.

Pin 5 is the control voltage pin. You need to decouple the power pin. ( pin 8 )

[Jazznoise]

This is a rough idea of what I'm saying. Obviously depending on the speed of the LED's the filter value and the LED series resistor (which I in my sleep deprived wisdom omitted) would need to change.

[armdnrdy]

It is well known that a standard 555 creates a major current spike when the output changes state. The current spike of the Cmos 555 is very miniscule in comparison. I would not use a standard 555 in an audio application. The Cmos version was created because of this limitation. The standard 555 can drive a larger load compared to the Cmos but, that's the trade off in this case for a circuit friendlier devise.

[RandomGlitch]

Thanks guys - I just checked - my 555's are cmos.  I'll play around with those other ideas too.
Will post pics when I'm done!

[Toney]

 Have you looked at an astable multivibrator?
They often two BJTs and two LEDs. The switching is perhaps a softer "cascade" between the two states and may be a bit friendlier re switching noise, assuming you keep the current fairly low.

[Mustachio]

I built a mxr blue box and used one of those RGB led's from radio shed. Its a 3 lead LED and you can wire it up for different cycles. But if you just clip the lead on the right I believe, and wire it up normal it just flashes red and blue (and a light blue but you can barely tell). 

That's a cheap easy way to do it . Just a suggestion you may have other ideas on how  you want it to look like having 2 LED's, one on each side.

[RandomGlitch]

That's a cool idea! I was thinking I want the effect of movement side-to-side like a police car, perhaps your idea coupled with a CD4017 chaser circuit I've seen would be neat.

Potential for some brain-frying led displays right there, haha!

I feel a few Nightrider/Cylon themed pedals coming on! 

[Jdansti]

Here ya' go:

http://www.555-timer-circuits.com/knight-rider.html

[Mustachio]

hehe I've seen a few of those knight rider circuits their cool, that link says not found though.

I built this vu meter not long ago that does sort of a kit(knight rider inside the car) when hes talking style  <<<<<>>>>>  it was fun gonna put it on a few newer builds soon too !

http://www.diystompboxes.com/smfforum/index.php?topic=98118.60

[Jdansti]

Thanks. The last "L" got deleted.

I'd like to try one too. I remember as a kid, thinking it would be cool to have a car with those lights on the front!  At my age now, maybe just as a third tail light.