LM3914 circuit for prop help needed

Thread Starter

Nick321

Joined Jul 30, 2017
9
Hello, first post, new member and very new to circuit design and creation, so please be gentle :)

I design and build sci-fi inspired ray gun props and would like to incorporate LED details into my designs.

In researching what I would like to incorporate, I have found that a LM3914 driven circuit would achieve the desired effect, but I have no idea how to design the circuit. I have looked at the data sheets, and scoured the forums already, but I don't know enough to figure out how to modify them to fit what I am looking for.

Essentially, I would like there to be a tri-color, 10 LED bar on the side of the ray gun, that would light 0-10 of the LEDS as a potentiometer knob or slider is turned (in bar mode, not dot mode). I would also like to incorporate an on/off switch into the circuit (to help with battery life), and power the whole thing with a 9v battery (fits best into housing/design). In my head it seem like this is possible and probably pretty easy to achieve, but I can't get my mind around the design of it.

Any help or advice would be greatly appreciated.

Thanks in advance. -N.
 

bertus

Joined Apr 5, 2008
22,120
Hello,

Keep in mind that using the current taken from the battery will be higher when more leds are on using the bar graph function.
The 9 Volts battery will have a shorter life when more leds are on.
You could consider to use high efficiency leds and drive them with very little current to save battery life.

Bertus
 

Reloadron

Joined Jan 15, 2015
7,079
Keeping in mind what Bertus has mentioned a 9 volt battery while compact in size is going to be limited in the current it can supply. That said the LM3914 responds to the amplitude on an incoming wave form. You will need a signal to drive it. You may want to consider a sawtooth wave form generated using a simple little circuit. Here are several examples to look over. The rate of change on the sawtooth will determine the speed at which the LEDs change. Another possible is maybe a 4017 Decade Counter Circuit. Again the LEDs will progress or "chase" along, frequently called LED Chaser circuits. Either chip will need a driver chip to drive it, the 3914 or the 4017. The speed the LEDs change will be a function of the applied driving signal.

Ron
 

JohnInTX

Joined Jun 26, 2012
4,709
A 555 astable with a constant current source on the timing cap will make a sawtooth wave. Feed that into the 3914. Zit Zit Zit. Trigger switch is the power switch. You could probably dispencs with the constant current and just do a simple astable as well.

Feed the sawtooth into the control voltage of another 555 astable for a zoop zoop sound.

Or you could use a PIC...

;)
 
Last edited:

Thread Starter

Nick321

Joined Jul 30, 2017
9
What is the effect that you wish to achieve with the 3 colors ?
Hi Bernard, what I would like is pretty simple. I would like the leds to light in sequence (bar mode) as the potentiometer is turned or slid. I think of it in terms of a movie scene where someone needs a more powerful blast from the ray gun, so they dial up more power, which is indicated by the led graph. Like they want max power so they turn it all the way up until the red LEDs are lit. Really doesn't need to function any more than that. I'd like to put the switch in to be able to turn it off when not in use. Plus I think switches look cool . Hope this makes sense.
 

Thread Starter

Nick321

Joined Jul 30, 2017
9
Hello,

Keep in mind that using the current taken from the battery will be higher when more leds are on using the bar graph function.
The 9 Volts battery will have a shorter life when more leds are on.
You could consider to use high efficiency leds and drive them with very little current to save battery life.

Bertus
Thanks for the reply bertus. This would not be something that would be switched on frequently or in use that often. More just a cool display piece that I can say "oh hey, check this out! It can also do this!"
 

Thread Starter

Nick321

Joined Jul 30, 2017
9
Keeping in mind what Bertus has mentioned a 9 volt battery while compact in size is going to be limited in the current it can supply. That said the LM3914 responds to the amplitude on an incoming wave form. You will need a signal to drive it. You may want to consider a sawtooth wave form generated using a simple little circuit. Here are several examples to look over. The rate of change on the sawtooth will determine the speed at which the LEDs change. Another possible is maybe a 4017 Decade Counter Circuit. Again the LEDs will progress or "chase" along, frequently called LED Chaser circuits. Either chip will need a driver chip to drive it, the 3914 or the 4017. The speed the LEDs change will be a function of the applied driving signal.

Ron
Thanks for the suggestions Ron and John. Both are excellent ideas, but more than I think I need. Basically what is happening in the first 15 seconds of this video
is all I'm looking to achieve with this project. Driven by a 9v with a toggle switch in the mix.
 

Thread Starter

Nick321

Joined Jul 30, 2017
9
Our pleasure & have a good one. :)

Ron
Well, turns out I don't have it figured out. I was reading another breakdown of the chip, and I think the resistors I was using were not of high enough value. After doing the math, I came up with using resistors of 3k and 12k ohms (as R1 and R2, respectively). Does that sound correct? Thanks again for the continued support.
 

Bernard

Joined Aug 7, 2008
5,784
R1 = 1.2k & R2 = 3.6k seem about right for a 5V ref. An 8k R in series with 10k Pot. from +9V to ground
should give an input from 0 to +5 V. If last LED does not light, drop 8k to 7.5k ?
 
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