Electronics Q

[F-Red]

A different type of DIY. Im trying to make a modal railway crossing with flashing red Leds. Iv'e got some flashing leds and some ordinary ones, cant use two flashing leds together as the flash rate and sequence is wrong, so I had hoped to use one flashing led to drive another ordinary led. But ^&%^&% if I can make it work, if anyone can come up with a simple solution I would be most grateful. PS I know there are lots of methods of doing this from ic555 ic4011 or even two transistor oscillators, but that's not KISS enough for me.

[Frank Erskine]

There's (was?) a dedicated LED flasher chip, the LM3909. Dead simple to use!

Frank Erskine MJBC, OETKBC

[Steve P]

[Stefek Zaba]

No chance, I'm afraid: the electronics do do the flashing are all sealed up inside the body of the flashing LED; even when it's not flashing, it's wanting to draw a little bit of current for the flashing circuit, so you can't just put an ordinary LED in series with a flashing one and expect it to light up in 'sympathy' with the flasher.

You'll just have to bite the bullet and knock up the simplest 555 timing circuit you can (very few external components needed: two resistors, one capicitor, one more capacitor for glitch-free running). A super-simple circuit diagram appears at

Googling with "555 flasher" throws up hundreds of similar ones. You'd put your two LEDs, each with their own current-limiting resistor, where these circuits show a single LED, if you want them both flashing ON at the same time.

[Dave Plowman (News)]

Or a voltage regulator, like the LM78

[Andrew Gabriel]

What happens if you connect a flashing LED in series with an ordinary LED? Maybe the off-state current is still enough to light the other LED?

Also, what supply voltage to you have to hand, and do you know the part number of the flashing LED?

[Dave Stanton]

Maplins do/used to do, a 2 x transister flashing led kit. Used one for my outside alarm box.

Dave

[Ian Stirling]

Should work. You connect the flashing LED in series with the ordinary one (it'll need

2-4V (red-blue) more to work.

What happens?

[fred]

In article , Andrew Gabriel writes

I think you have the right idea andrew, a quick look at some flashing LED data shows some latitude in the supply voltage (9-12V) so should work from a 12V supply with an LED in series. A resistor in parallel with the slave LED will allow some quiescent flow without it lighting, 1k would let

1mA flow w/o any risk of lighting. 20mA seems to be a typical ON current.

A possible problem is that the flasher chip will see its voltage change from ~10 when both LEDs are on to ~12 when both are dark which may upset it, so it is one to suck and see. Other risks are intensity, spread & colour matching.

[a]

I am sure I have done this before - simply in series and the normal LED flashed at the same time as the flashing one.

[Lurch]

On Sat, 29 Jan 2005 21:44:46 +0000, Dave Stanton strung together this:

I think this is the premade unit from CPC, , not sure though as there's no picture. I might have a flick through the catalogue to confirm if I can be arsed to go and get it!

[F-Red]

Ian Stirling

Thank's for everyone replies, but the LEDs have to flash in sequence, one on other off. Just like on a real crossing ! I had hoped to do this with *ONE* transistor, ie bias for base being supplied by resistor/ Flashing led junction. But that don't work because they both flash together. I shall have to go to bed tonight and dream about this, that always provides a solution.

[Andrew Gabriel]

Ah -- I was thinking you wanted them on together. Try something like this:

+ve ---------------- | | | / --- \ \ / flasher / 680 ohms --- \ | | | ------ | NPN | | | |/ | |------| --- | |\| \ / LED / -| --- 100 \ | | ohms/ | | \ | | | | | | | | 0v --------------------

Any small low gain NPN transistor should do here which can handle, say, 20mA base current, e.g. BC635, BC639 (or BC546, BC547 but their gain is a bit high). If the separate LED does not switch right off, reduce the value of the 100 ohm resistor. Adjust the 680 ohm resistor if the two LEDs are different brightness.

[Andrew Gabriel]

^^^^^^ meant to say "increase"

Actually, try reducing 100 ohm resistor first if it needs brightening. If doing this stops the LED switching right off or you want to dim the LED, then try adjusting the 680 ohm resistor.

Without a datasheet for the flasher, I can only guess what its on and off currents are, hence the need for playing with the values.

[Andy Hall]

Do you have barriers that come down in sequence as well?

That could produce a short circuit....

[F-Red]

[e-mail] (Andrew Gabriel)

Hmm that's were I started :-? Current flows to tr/base when flashing led is ON ;-(

[Andrew Gabriel]

Indeed, which switches the other LED off.

[F-Red]

Andy Hall

Oh yes, and a little amber Led that comes on first, triggered by an approaching train.

[Bill]

In message , a writes

Yes. I have done this when I wanted to make one of the larger square LEDs flash and it worked OK, don't forget the series resistor.

[Dave Stanton]

Hi Lurch

No, mine was bought ages ago as a kit. Bit of a admission as I work 5 days a week in electronics, but it was cheaper than designing, making a pcb etc etc.

CU

Dave