# old hat needs help

Discussion in 'The Projects Forum' started by Corporeal, Jan 16, 2008.

1. ### Corporeal Thread Starter New Member

Jan 16, 2008
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man it's been years since i've messed around with electronics, and this used to be such an easy project for me to do... but I need help figuring out how to construct a 10 LED strobe circuit to be used in a model police car.

I've been told to use 558 timers in the circuit but I'm not exactly sure how to use them.

2. ### SgtWookie Expert

Jul 17, 2007
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1,728
Take a look at the attached schematic; with a minor modification or so, I think you could use it for your project just fine.

I drew this a week ago for someone who wanted an LED emergency flasher for their vehicle. No reason you couldn't use it for your model, too.

You wouldn't need the IRF510 power Mosfets, you could use 2N2222 or 2n3904's instead. You could also use the spare 4075 triple-input NOR gate to flash another set of LED's. (The symbol for the 4075 looks like a NAND gate - it's really a NOR gate. Whomever made the library didn't choose that symbol very well.)

J1, in the upper right corner, inputs power and ground, and supplies two grounds to the LEDs. The circuit would run on much lower than 12V, depending upon the particular IC's you used (check the datasheets). The IRF510s would not fire properly on less than 10V, but the abovementioned transistors will.

For the LED's, just calculate what value of resistor you'll need to limit the current over whatever voltage you'll be using. Remember to subtract the 0.6V you'll drop across the transistor.

Say you're using 6V for a supply, and one of the LEDs is rated 2.1V @15mA.
6V - 2.1V - 0.6V = 3.3V
3.3V/15mA = 220 Ohms.
You could run two such LED's in series for better efficiency:
6V - (2.1V x 2) - 0.6V = 1.4V
1.4V/15mA = 93.3 Ohms.

There's a suggested board layout, too - but of course that'll change with the transistors and addition of the gate - it's a starting place.

There's a bill of materials with Mouser part numbers and prices as of last here:

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3. ### Corporeal Thread Starter New Member

Jan 16, 2008
4
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hmm.. I'm not so sure that's going to do the job I need.

Basically it needs to work with bicolor LED's. one side is red, the other is blue.

It needs to flash one side (blue) twice, then fire red twice. both sides should continue to fire in that sequence indefinitely until I turn the circuit off.

the LED's are going to be the high output (ultrabright) type.

4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
If you were willing to compromise your light sequencing scheme so that the sequence you described happened on both sides at the same time rather than alternating between the front and rear, then you could slightly alter the circuit that sgtwookie has posted and it would work fine.

hgmjr

5. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
With an IC change, it could easily be made to flash like:
(Right) Blue Red Blue Red Pause
(Left) Blue Red Blue Red Pause
Repeat
In order to flash in the sequence you've described:
(Right) Blue Pause Blue Pause Red Pause Red Pause
(Left) Blue Pause Blue Pause Red Pause Red Pause
would require additional logic, as that is 16 discrete states and the 4017 has only 10 outputs.

Since I posted the original schematic, two errors have been discovered, but it would need to be changed anyway for your purpose.

Are your LEDs common anode or common cathode?
Or do you have a part number or datasheet for them that you could post?
Usually, blue LED's have a much higher voltage rating for a given current than red LED's do; the circuit design must take that into account.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
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You said you have 10 LEDs. Do all the reds flash twice in unison, then all the blues, or???

7. ### Corporeal Thread Starter New Member

Jan 16, 2008
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The sequence I'm trying to go after is red, red, blue, blue.

the one i'm trying for is 'wigwag pulse 2' or 'in out pulse'

two bicolours will be mounted in the headlamp area, two will be in the taillight area. four will be mounted behind the windshield, and the last two will be mounted in the grille area.

All have common cathode.

8. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Funny, but the existing circuit can reproduce InOutPulse very easily with the addition of two 3-input OR gates.

Too late for me to fiddle with it tonight though -had a long day, just got back from out of town.

9. ### Corporeal Thread Starter New Member

Jan 16, 2008
4
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hey no worries. i'm just checkin back on this thread myself.

I'll give the existing circuit a go and let you guys know how it worked out. Might be awhile because I have to wait for these LED's to come in, since they are special order.