Help with flash frequency

Thread Starter

burtonwisher

Joined Aug 9, 2011
4
I need some help with a small circuit that I've made (as in the attached schematic).
The flash frequency of each pair of LEDs is fine, but the rate that it alternates between each pair is too slow. Each pair currently flashes rapidly for over a second before the other pair flashes, and I'd like this to be about half a second.
Can anyone tell me what I need to change to make this happen?

Many thanks
 

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Wendy

Joined Mar 24, 2008
22,164
OK, I had a bit more time to study what you are wanting to do. Basically it will work, but the signals will not sync to each other, which I assume is something you want.

You may have other problems, a typical standard 555 only goes to around Vcc - 1.3VDC. This could cause some issues. It is because a 555 uses a transistor to ground one way, but different type of transistor configuration to the positive volts out.

Something else, you can reduce the current for the circuit simply by putting the pairs of LEDs in series and eliminating one chain each for one side.

http://forum.allaboutcircuits.com/showthread.php?t=37409&highlight=police&page=4
Same thread, but more specific including a YouTube video...
http://forum.allaboutcircuits.com/showpost.php?p=247476&postcount=38

http://forum.allaboutcircuits.com/showthread.php?t=17499
Check post #22
Note: this thread violates current Terms of Service. For an RC car it would be OK.

http://forum.allaboutcircuits.com/showthread.php?t=16484
 

SgtWookie

Joined Jul 17, 2007
22,220
Here's an easy way to make your circuit adjustable using a pot (potentiometer) wired as a rheostat:



This is almost exactly like your current circuit, but I have changed R1 from 33k Ohms to 2k Ohms, and have added VR1, a 20k potentiometer.

This gives you an output range of 3.1Hz to 0.3Hz (one pulse every ~3.4 seconds).

If you don't have or cannot find a 20k pot, you could use a 10k pot and reduce C1 from 100uF to 47uF, and get about the same range of adjustment.

If you don't want it to be adjustable or you just want to make the minimum changes possible, then either replace R1, the 33k resistor, with a 3.3k resistor, or change C1 from 100uF to a 10uF capacitor. No guarantees that it will be exactly 2 Hz due to variations in components (capacitor tolerances mainly), but it'll be close.

If you want the LEDs to flash one complete cycle in 1 second, then either change R1 to 6.8K, or C1 to 20uF-22uF.
 

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Last edited:

SgtWookie

Joined Jul 17, 2007
22,220
Oops - Bill, I simulated it, and it does work. The timer on the left alternates between the pairs of LEDs, and the timer on the right flashes them quickly (~13Hz).

With their existing R1/C1 values, they should have been getting around 5 seconds (0.2mS) per cycle.
 

Wendy

Joined Mar 24, 2008
22,164
I think I've already expressed my reservations about the OPs design. Tain't mine, which is why I posted all the links. To do what I think he wants to do will take 3 555s, more or less.

What I think I am seeing is two separate free wheeling oscillators, on selects which LEDs are lite, the other determines the flash rate, with no sync between them.
 

SgtWookie

Joined Jul 17, 2007
22,220
Yep, that's basically it. Two free-running bistable multivibrators.

The flash cutoff/start might be ragged when the left 555 toggles, but it works nonetheless.
 

Thread Starter

burtonwisher

Joined Aug 9, 2011
4
Many thanks to all for the valuable input. I'm clear on what I need to do to speed up the rate the pairs alternate, and I've also got some interesting thoughts on the design of the circuit.

My need is for a circuit that flashes a pair of LED's quickly (~10 Hz) for about half a second, then immediately switches to flash the other pair of LED's for half a second, and so on. The schematic I've used is one I found online, and it works on a breadboard but does have a couple of issues that I have noticed, namely that the right hand pair of LED's seem to be slightly dimmer and flash slightly slower than those on the left. It's only a slight difference but noticable to the eye, and ideally I'd like both pairs to flash with exactly the same frequency and with the same brightness. Could be an error iIve made building it though?

I'll have a look at the links provided and look at the option they contain, and any thoughts in addition to these would be welcome.

Thanks again
 

Wendy

Joined Mar 24, 2008
22,164
It can be done simply with precision. The most popular method is with a 4017 and one 555, it duplicates the classic double sided flash of a police car very precisely, and is extremely simple to boot.

The only reason that design works even halfway is they are depending on coupling through the power supply, which is a very iffy way to do it.

The difference in LED intensity is inherent in how 555s work, but can easily be compensated with by changing the resistors limiting the LEDs current.
 
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