LED reverse polarity

LesJones

Joined Jan 8, 2017
4,174
I can see a few problems with the schematic in post #7
1 - There is nothing to limit the current through the LEDs so you are likely to exceed the current rating of the LEDs or the 200 mA maximum current rating of the 555s.
2 - The two red LEDs in series will tend to clamp the voltage to about 3.6 volts ( 2 x 1.8 volts.) As these are connected in parallel with two blue LEDs in series the blue LEDs will not light as they will need about about 6 volts (2 x 3 volts)
You need two current limiting resistors. One for the red LEDs and one for the blue LEDs. The values need to be to be calculated to give the required currents through the LEDs.
I am not convinced that the light output from the LEDs will be enough for use in daylight. (Assuming that you are running the 555s at their maximum output current rating of 200 mA and you have 100 mA through the red LEDs and 100 mA through the blue LEDs the power input to the red LEDs will be 0.36 watts and the power input to the blue LEDs will be 0.6 watts)

3 - U1 will not oscillate as there is nothing to discharge the timing capacitor. You need to connect pin 7 to the top end of C2
Edit. I am was wrong about point 3. I now see that the top end of R3 is connected to U1 output. (I was assuming it was connected to + 12 volts.)
Les.
 
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eetech00

Joined Jun 8, 2013
3,858
Thanks a lot for those who help. I was wondering if it can make the LedView attachment 241167 stays lit up. This circuit makes Led flashing mode. Im planning to install it with our local rescue vehicle for more visibility. The problem is that the leds cathode and anode are connected. If I put 12v dc directly on the Led it will fry the circuit and the Led. Is there a way to make the Led stays lit up and not flashing? Like a flashing mode and steady mode? I want to DIY it since our budget is tight
Thanks
Which LEDs do you want to light steady? Both RED and BLUE? or?
 
The circuit has so many problems. Parallel LEDS are generally a no no.

In all likeliness, the LEDs used would be designed to operate at 12 volts.
I think the 555 is good for 200 mA, but that's a limit, so less than 2.4 Watts per color. As @crutschow pointed out ist has aysmetrical output source and sink currents.
 

eetech00

Joined Jun 8, 2013
3,858
Which LEDs do you want to light steady? Both RED and BLUE? or?
Here is an alternate circuit. The attached circuit lights both sets of LEDs steady on. This can be changed if needed.
I've used one CD4093B chip with a schmitt oscillator for a flasher.
The "Flash/Steady" toggle switch selects "Flash" when closed. The RED and BLUE leds will flash alternately.
The "Flash/Steady" toggle switch selects "Steady" when open. The RED and BLUE leds will light steady on.
I've shown 350mA LEDs but a higher/lower power LED can be used with suitable current limit resistors. While the LED's could have been wired in series, I chose two parallel strings to provide redundancy. The mosfets will operate slightly warm to the touch without a heat sink, but I would use heat sinks anyway, in case a higher wattage LED is used.

1623778891798.png
 
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LesJones

Joined Jan 8, 2017
4,174
Here is a suggestion to force the original circuit (In post #7) to have the LEDs on.
150621.jpg
When the switch is closed it forces U2 pins 2 and 6 high overriding the timing resistor. This causes it's output to be low. It also causes the transistor to conduct forcing U1 pins 2 and 6 low. This causes it's output to be high. So now the LEDs should be on.
I think eetech00's circuit is MUCH better than the the circuit in post #7

Les.
 

BobaMosfet

Joined Jul 1, 2009
2,110
Hi, is it possible to light a LED even in reverse polarity? Like passing through a circuit. Im applying on automotive. Thanks
For a correct answer, we would need to know much more about the LEDs in your specific question- without their parameters nothing said here is applicable to your case.
 

crutschow

Joined Mar 14, 2008
34,281
For a correct answer, we would need to know much more about the LEDs in your specific question - without their parameters nothing said here is applicable to your case.
That's a very broad statement.
What type of LED would significantly change the answers given to the question?
 

MisterBill2

Joined Jan 23, 2018
18,167
I like that circuit in post #30, which will work with any kind of LEDs just by changing the current limit resistor. It has a very definite advantage of allowing one side to be commoned to the 12 volts return. (ground on most vehicles.) It has the additional benefit of having one side of the mode switch be common. That makes it much simpler to install.
 

BobaMosfet

Joined Jul 1, 2009
2,110
That's a very broad statement.
What type of LED would significantly change the answers given to the question?
When I posted it, the specific LED the TP referred to was unknown- I haven't had time to review the thread to see if was answered. TPs need to post specific questions otherwise it's a little like asking ' will this bumper fit my car if I turn it over?' - without telling us the car make/model. Maybe it will, maybe it won't. Why throw out a lot of answers until you have enough facts to give one specific answer? IMHO
 

MisterBill2

Joined Jan 23, 2018
18,167
When I posted it, the specific LED the TP referred to was unknown- I haven't had time to review the thread to see if was answered. TPs need to post specific questions otherwise it's a little like asking ' will this bumper fit my car if I turn it over?' - without telling us the car make/model. Maybe it will, maybe it won't. Why throw out a lot of answers until you have enough facts to give one specific answer? IMHO
My response that the circuit in post #30 will work with any type of LED still holds. With very high powered LED arrays it may need a change in the power MOSFET types, but that is a component change, not a circuit change. The reason this is true is because the LEDs are the load of the circuit , not an active part of it.
 
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