LM3914 high range flash

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Thread Starter


Joined May 23, 2008
Hi Guys, so I made a circuit a while ago, it's a 30 step (LED) analog temp sensor/display. I'm using a LM35 for the sensor, and 3x LM3914's for the display. now the standard bar mode works perfectly fine, shows the full range of 0-150 deg if I put a heat gun on the sensor so all good there. I want to add to it a bit before I put it in the car. I want it to flash when the temp is over a set range. now I've done pretty close to the circuit in the LM3914 datasheet (page 13), with the 100 ohm res on the cathode of a lower LED and the junction between R1 and C1 connected to the cathode of the same LED. now the value of the 1.2k and 470Ω resistors in the diagram are different on my breadboard mainly because it's just what I had on hand but it works so I don't think this is where any issues lie.

3 things I'm having issues with or would like clarification.

1. I've increased the size of C1 to 1000μF to decrease the speed that graph flashes. I assumed it was the cap that controls this and hey I was right there :D. problem is though that the speed varies and is quite noticeable. any way to overcome this? or should I just design a better circuit that controls the flash from the anode side of the LED's?

2. the diagram shows the 470Ω resistor going back to ref out. I assume I would need to do this for each LM3914 to get them all flashing? atm it's only the 3rd set of LED's that flash because it's only that one that's connected.

3. I will be running a PWM circuit (already designed and built) to control the brightness of the LED's, will this interfere with the flasher component? or should I just tag something on the side of the PWM that controls the flash. I'm starting to think this might be a better option.

I've probably rambled on enough, thanks in advance for any help guys.


Joined Jul 17, 2007
Hi there Bear,
It's been awhile - we started talking about that gauge back in July of 2008!

However, you've been gone for over a year, and our Terms of Service have been amended during that time; about 8 months ago; a portion of which reads:

6. Restricted topics. The following topics are regularly raised however are considered "off-topic" at all times and will result in Your thread being closed without question:
  • Any kind of over-unity devices and systems
  • Automotive modifications
  • Devices designed to electrocute or shock another person
  • LEDs to mains
  • Phone jammers
  • Rail guns and high-energy projectile devices
  • Transformer-less power supplies
And unfortunately for your project, this means that the forum will no longer support discussions on the modifications that you are making to that Charger of yours:

The change in the ToS occurred because we finally realized that we are not automotive engineers, and the people who we were trying to help were not trained technicians, and changes were being made to vehicles that were potentially very unsafe, unreliable and/or illegal. So, in keeping with the spirit of the rest of the boards' rules, where safety is the overriding concern, a decision was made on high that we can no longer support such discussions.

That may seem harsh, but we can't possibly advise when we don't have the background, test procedures, lab/test facilities, watch over the persons' shoulder to ensure acceptable quality of assembly/workmanship - and even trained, experienced engineers make mistakes that cost people's lives every day.

At any rate, if you really wish to continue making modifications, I'm afraid that you will need to seek support elsewhere.

We've compiled a short list of discussion forums where you might find some assistance; that list is in this topic:
in our Electronic Resources forum.

In the meantime, if you have other projects that are not related to automotive modifications, or other restricted items, please feel free to inquire.

A caution - wherever you may go to ask for help, make certain that you state that it is for an automotive application; as one has to be far more careful with component selection and various other things. If you try to use components that aren't rated for an automotive environment, they will likely fail much earlier than expected. If you fail to mention it's for an automotive environment, people might assume it's just for 12v, when the power can actually vary anywhere from 10v when starting, to spikes of over 60v during "load dumps".

Anyway, I wish you success with your project.

Thread Starter


Joined May 23, 2008
*sigh* yes it has been a while. had to drop all my projects for some time due to finances but finally back to being able to play a bit in my spare time... thanks for your help back then too, got me a long way and now have a much better understanding of how it all works and now I am able to just experiment with different things on the fly so to speak.

any chance that the TOS could be changed slightly to allow no risk auto mods? I don't see how a temp sensor could be potentially unsafe, considering the state of the current temp sensor anything would be better. I have to actually stick my hand up behind the dash and hold the pins on the back of the guage in a certain way to get an acurate reading. ;). that's what I get for using 40yr old parts.

I understand how mods to say, electronic ignition systems, fuel delivery systems etc can be unsafe if you don't know what your doing but there's a whole host of other things that are perfectly fine, for example would anything car audio related not be allowed?


Joined Nov 25, 2009
Unfortunately, as Wookie said, no automotive modifications are allowed to be discussed in AAC at this time. Automotive repair talk is borderline too and depends on each case.

We are facing some legal liability issues right now that have not been resolved yet. Besides, we think that cars are classified in an especially risky category, along with high voltage and directly mains-connected circuits. Since we can't ensure the careful handling of those circuits by the members, we prefer to leave them untouched.

The situation might change in the future, but not the near one, that is for sure.

Thank you for your understanding.
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