I want to build safety vest for my dog. I have planned to use Arduino Pro Micro 3.3V and 18650 battery to power it. There would be 1 unit for each side of the dog. 1 unit consist of 3 POWER LEDs, microcontroller, battery, power switch, FETs and charging pcb. See schematich attached. N-channel fet would be driving LED. Power of each led is 3W and threshold voltage is something between 2.5V to 3.2V depending on LEDs color. This is something i have planned. Would it better to use 2 batteries in series to rise voltage to 7.4V or is 3.7V enough?
LEDs are not lighted all the time. They are blinking continuously each led separately BLUE-RED-WHITE over and over. So they are not warming too much and current stays low enough.
You must have current limiting resistors in series with the LEDs and ideally a stable supply voltage for them so the current does not change with change of battery voltage. I think I would use a boost converter to 5V for the LEDs and then the correct resistors for each colour.
There would be 1 unit for each side of the dog.
Is that necessary? Can you build one with LEDs for both sides of the dog all in one unit?
So they are not warming too much and current stays low enough.
Right idea, wrong method. You limit the current with resistors, then they can stay on all the time without drawing too much current.
Yes, there has to be those limiting resistors. It doesn't matter if brightness of LEDs varies depending of supply voltage, i can live with that. But if it's easy to use that converter it might be useful.
It might be possible to use just one unit, this is something what i have to think more.
Sorry, I meant that when led is blinking like i want it draws current only something like 10% vs if it would stay on all the time. This will help to handle the heat what led is producing. I want that unit to be as small and light as possible.
Is there some fet circuits where is 3 or more n-channel fet in one SMD component?
Sorry, I meant that when led is blinking like i want it draws current only something like 10% vs if it would stay on all the time. This will help to handle the heat what led is producing.
I think you missed the point; LEDs produce very little heat if you power them correctly, probably so little you won't notice. If you supply them with too much current they produce a lot of heat - for a second until they go POP!
Idea is to get very bright illumination , that is why i want to use 3W leds. Power is voltage x current so it will take ~1A to get 3W with 3V voltage. I already test it and leds went hot in seconds if they are on all the time. That is why i want to blink them like police car lights etc.
Idea is to get very bright illumination , that is why i want to use 3W leds. Power is voltage x current so it will take ~1A to get 3W with 3V voltage. I already test it and leds went hot in seconds if they are on all the time. That is why i want to blink them like police car lights etc.
Did you have resistors in series with them? Were they drawing 1A as you say?
It was just a quick test with laboratory power supply without resistors. And yes, i know, it's very easy to brake them That 1A is just theoretical value. I think maximum current should be less than 800mA with these CREE XP-E leds. If i drive those with 100mA or so they are so dim. With 800mA they are very bright and that is what i am looking for.
I have dozens of these leds and its possible to test these without resistors but its risky.
At first i have to decide which fets do i use, maybe IRL540? I am not very familiar with fets. What i have understood is they should be logic level n-fets.
Im not sure how it affects if i dont use that 3.7->5V convert ? Without converter supply voltage varies something between 3.5 to 4.2V. Leds will go dimmer when voltage decreases but i'm ok with that. I believe that 3500mAh battery will last at least 24 hours.
At first i have to decide which fets do i use, maybe IRL540?
Max drain current for those is 28A, which is a bit overkill/ IRL530 max drain is 17A, still overkill, just not so much. Other than that I don't know without checking various specifications. You need to make sure they are logic level, that is critical (as I know to my cost!)
I'm not sure how it affects if i dont use that 3.7->5V convert ? Without converter supply voltage varies something between 3.5 to 4.2V.
The problem you have is that the current through the LEDs varies according to the difference in voltage between the LED Vf and the supply, which is a lot more in percentage terms than the change in supply voltage alone. For example:
Supply voltage 3.5V, LED Vf 3.2V, desired current 1A
Resistor needed (3.5 - 3.2) / 1 = 0.3 / 1 = 0.3 Ohms
Supply voltage 4.2V, LED Vf 3.2V resistor value (from above) 0.3 Ohms
LED current (4.2 - 3.2) / 0.3 = 1 / 0.3 = 3.33A
This is why you need to keep the supply voltage reasonably constant or use a proper constant current source. The closer the supply voltage gets to the LED Vf the worse the problem gets. Obviously you can work the maths the opposite way to my example and select an appropriate resistor for the maximum voltage, which will give dim LEDs when the battery is flat, but not excessive current when it is charged. Try it and see. Do consider also that at night the LEDs don't need to be very bright to be visible.
Beware a 3W led is very bright and may dazzle a motorist and not have the desired effect. - he might run you over .
Have a look at how pushbike rear lights work , they are very bright with clusters of small leds , taking little power .
Agree!. Some of the safety LEDs I see bikers using are painfully you can't look at them bright, and they are nowhere near 3 watts.
I am reminded of a letter in New scientist a few weeks ago from a lawyer about a court case where a driver went straight into a cyclist waiting in a queue of traffic. The gist of it was that the driver could not see the cyclist because of bright rear lights on cars, including fog lights (it wasn't foggy) making the cyclist's lights blend in and become invisible. The driver was acquitted of dangerous driving.
Thanks for good comments! Like i wrote earlier this is going to be a vest for hunting dog. Dog is used to chase game in forest and vest is used to prevent crash with cars when crossing a road in dark. Many hounds are died by traffic here in Finland. That's why i want to build new kind of vest for huntin dogs.
PerryBebbington:
Max drain current for those is 28A, which is a bit overkill/ IRL530 max drain is 17A, still overkill, just not so much. Other than that I don't know without checking various specifications. You need to make sure they are logic level, that is critical (as I know to my cost!)
The problem you have is that the current through the LEDs varies according to the difference in voltage between the LED Vf and the supply, which is a lot more in percentage terms than the change in supply voltage alone. For example:
Supply voltage 3.5V, LED Vf 3.2V, desired current 1A
Resistor needed (3.5 - 3.2) / 1 = 0.3 / 1 = 0.3 Ohms
Supply voltage 4.2V, LED Vf 3.2V resistor value (from above) 0.3 Ohms
LED current (4.2 - 3.2) / 0.3 = 1 / 0.3 = 3.33A
This is why you need to keep the supply voltage reasonably constant or use a proper constant current source. The closer the supply voltage gets to the LED Vf the worse the problem gets. Obviously you can work the maths the opposite way to my example and select an appropriate resistor for the maximum voltage, which will give dim LEDs when the battery is flat, but not excessive current when it is charged. Try it and see. Do consider also that at night the LEDs don't need to be very bright to be visible.
PerryBebbington:
Max drain current for those is 28A, which is a bit overkill/ IRL530 max drain is 17A, still overkill, just not so much. Other than that I don't know without checking various specifications. You need to make sure they are logic level, that is critical (as I know to my cost!)
The problem you have is that the current through the LEDs varies according to the difference in voltage between the LED Vf and the supply, which is a lot more in percentage terms than the change in supply voltage alone. For example:
Supply voltage 3.5V, LED Vf 3.2V, desired current 1A
Resistor needed (3.5 - 3.2) / 1 = 0.3 / 1 = 0.3 Ohms
Supply voltage 4.2V, LED Vf 3.2V resistor value (from above) 0.3 Ohms
LED current (4.2 - 3.2) / 0.3 = 1 / 0.3 = 3.33A
This is why you need to keep the supply voltage reasonably constant or use a proper constant current source. The closer the supply voltage gets to the LED Vf the worse the problem gets. Obviously you can work the maths the opposite way to my example and select an appropriate resistor for the maximum voltage, which will give dim LEDs when the battery is flat, but not excessive current when it is charged. Try it and see. Do consider also that at night the LEDs don't need to be very bright to be visible.
Thanks! Maybe IRLZ44 will do the job? I think i will make some test with that fet and try few different resistor values. I already have arduino, battery and charging pcb.
Kikemppa:
Thanks for good comments! Like I wrote earlier this is going to be a vest for hunting dog. Dog is used to chase game in forest and vest is used to prevent crash with cars when crossing a road in dark. Many hounds are died by traffic here in Finland. That's why I want to build new kind of vest for hunting dogs.
At the risk of spoiling you fun building this, do you know that such things already exist? Light Hound
Yes, i have seen those but those are not even close that bright and robust what i want them to be. Correct vest already exist and i want to make my own robust light unit for that. It should handle water, cold weather, snow, dust etc. That is my vision. Let's see what happens
I got some IRLI530N fets and made a test setup. Everything seems to work and LEDs are bright. Next i have to make sure my charging circuit works and battery gets fully charged. Then i have to test how long it will work with 3500mAh battery and what happens when battery voltage gets low.
If everything is working like they should be i have to ask my buddy to design a pcb for this. I am going to use arduino pro micro 3.3v and Cree LEDs with aluminium star board. Enclosure is going to be machined PMMA.
That 1A is just theoretical value. I think maximum current should be less than 800mA with these CREE XP-E leds. If i drive those with 100mA or so they are so dim. With 800mA they are very bright and that is what i am looking for.
