need help making circuit!!!

Thread Starter

vane

Joined Feb 28, 2007
185
i need help making a circuit that lights up two red LEDs when the voltage supplied goes below 2 volts for brake lights on my rc car thanks
if you know anywhere where i can find an y circuit diagrams like this please help me!!!
 

Papabravo

Joined Feb 24, 2006
12,954
i need help making a circuit that lights up two red LEDs when the voltage supplied goes below 2 volts for brake lights on my rc car thanks
if you know anywhere where i can find an y circuit diagrams like this please help me!!!
A simple comparator circuit should do the trick using and LM393 dual or LM339 quad. I believe they also come in single SOT23 packages, but I don't know the part number.
 

Thread Starter

vane

Joined Feb 28, 2007
185
sorry im only good with simple led circuits what does that mean? im good at soldering but thats about it :) as it was my first post :)

it musn't come on if the voltage goes above any voltage it must switch on when its 2.5v or less maybe some sort of relay that switches when it is at 2.5v or less so that i dont need an led that runs off less than a volt thanks
 

Ron H

Joined Apr 14, 2005
7,014
Why do you want the brake lights to come on when a voltage drops below 2V? Where is this voltage coming from?
 

Thread Starter

vane

Joined Feb 28, 2007
185
well the main battery is 7.2 volts but it will be running a motor and a few more leds but as leds dont draw much capacity it wont make much difference please help me

edit: dont know if you care but they are massive 3.7mah and are nickel metal hydrides
 

Thread Starter

vane

Joined Feb 28, 2007
185
thanks id never be able to make that circuit by myself :) im sure my dad has a few of those components lying to hand, thanks

edit:could you write down all the components id need? thanks
 

Ron H

Joined Apr 14, 2005
7,014
thanks id never be able to make that circuit by myself :) im sure my dad has a few of those components lying to hand, thanks

edit:could you write down all the components id need? thanks
I'm not sure what you need that isn't on the schematic already.:confused:
 

Thread Starter

vane

Joined Feb 28, 2007
185
will that circuit work if there is like 0.01v? cause i was thinking like if you have a relay that the contacts will switch when there is the 0.01v the lights come on no matter how little power goes in cause some times when the speed controller is in brake it'll only give out 0.01v if you go into brake quickly it goes to 2v, im a bit worried about this
 

Papabravo

Joined Feb 24, 2006
12,954
incroyable! Mon Dieu!

Here's one way to do it.
Print a coppy of the schematic
Take a sheet of paper or a spreadsheet
In the leftmost column write the reference designators. e.g. R1, R2, Q1, Q2, etc.
Next to each reference designator write the value or the part type. e.g. 10K, 2N3906
Use a yellow highlighter to check off each item on the schematic

Could this process be any simpler?

When it comes to ordering the parts you have to get copies of the datasheets so you know what package the parts come in. This is actually quite a bit of work and nobody can do it for you. You absolutely must do this for yourself.
 

Ron H

Joined Apr 14, 2005
7,014
Vane, I think I got the logic backwards, i.e., the lights switch ON when the voltage is above 2V, and OFF when the voltage is below 2V. I know that's not what you want. I'm at work right now. I'll try to fix it when I get home.

EDIT: The schematic has been fixed.
 

Thread Starter

vane

Joined Feb 28, 2007
185
thanks, what voltage led do i need to use for it because as i have ranted on about in previous posts it needs to work almost like a relay as i know you probably cant get 0.01v led :)
 

Ron H

Joined Apr 14, 2005
7,014
thanks, what voltage led do i need to use for it because as i have ranted on about in previous posts it needs to work almost like a relay as i know you probably cant get 0.01v led :)
I designed it for two red LEDs, running at 10 milliamps. I have fixed the schematic (see my earlier post). The LEDs come on when the input voltage goes below 2 volts, and they go off when the voltage goes above 2.2 volts.
 

Thread Starter

vane

Joined Feb 28, 2007
185
and also what do the triangles at the bottom of the diagram? does that mean all the ones with a triangle are joined up? im not ery good at reading circuit diagrams, thanks
 

Ron H

Joined Apr 14, 2005
7,014
what is the minimum voltage that the light will light up at .eg 0.1v thanks for all you've done ill send you a vid of the lights working when i have it done just incase you want to know what is will be doing it http://www.youtube.com/watch?v=XLTmuPVp9e0&eurl=http://myspace.com/vanepico
In my last post, I wrote
The LEDs come on when the input voltage goes below 2 volts, and they go off when the voltage goes above 2.2 volts.
Doesn't that answer your question?
BTW, the input voltage comes from your speed controller.
And yes, the little triangles are called "ground" symbols, but that just means that it is the common node. As you said, they are all tied together.
 

Thread Starter

vane

Joined Feb 28, 2007
185
i dont mean to sound nagging but will the circuit work even if the speed control puts out 0.01v? its hard to explain what i mean
 
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