LEDs & Hall Effect Switch Circuit Advice Please!

[ElFern]

Hi there - I’m new to the forum and after some help. I’ve popped the same thread in the Sensors etc forum but thought here could be useful too.

I’m a prop maker and don’t usually work with electrics but I’ve got a job that requires me to build a few circuits that can each power 2-3 diffused white 5mm LEDs using button batteries (as the circuit needs to be hidden in a small area) with a gradual Hall Effect switch and accompanying magnet.

Looking for info on:

• How many batteries, and of what voltage, do I need? I understand most standard LEDs have a forward voltage of 3.0 - 3.4 so I think thats what I’d most likely be using.
• How many resistors do I need, of what type, and where do they go in the circuit?
• Where does the gradual Hall Effect switch need to go in the circuit?
• How do I know what to put in series or parallel? I want it to have as long a battery life as possible while keeping the lights bright.
• How do I know how strong the magnet needs to be?
• Also any links to Amazon or elsewhere online where I can buy a specific:
  • Gradual Hall Effect switch
  • Magnet that would work
  • Small battery pack/case that fits the right type and amount of batteries

This is really outside my normal work so any advice would be greatly appreciated!

Thanks in advance - ElFern

 

[bertus]

Hello,

The forward voltage of a led depends on the color.
Have a look at this chart:

Also the forward voltage is dependend on the current:

Bertus

 

[ElFern]

Hi Bertus,

Thanks for this, I will be using white LEDs so probably the Pale White on the above chart.

Also would 20 gauge wire be affective for this?

ElFern

 

[Bluejets]

Button cells next to useless ,not enough capacity.
Workout resistor value from ohms law.
Hall effect I think you intend to use may not work as you think.
Size of magnet would depend on distance from the hall effect, specs will show it.
Wire size depends on the length of cable run and the current draw. If only a couple of feet, the probably 26g is plenty.

 

[Harald Kapp]

What do you mean by "gradual Hall effect switch"?
A switch is on or off, there's no gradual in the operation. Do you by chance mean to dim the LEDs depending on the distance of the magnet from the sensor? Then a switch won't do. You will need a sensor for the magnetic field and a controller to dim the LEDs depending on the strength of the sensed magnetic field.
The strength of the magnet depends on the distance to the sensor and the sensitivity of the sensor. A strong magnet will allow for a greater distance, a weak magnet will require to be near to the sensor for activation.
Assuming your complete circuit (sensor, controller, LEDs) works from 5 V (e.g. an off the shelf power bank), you can drive one LED (@3.6 V) from 5 V. With 20 mA current the resistor required would be (5 V - 3.6 V) / 20 mA = 70 Ω. A 75 Ω resistor is a good match with a standard value.
Dimming would be achieved by puls width modulation (PWM).
The controller could be e.g. an Arduino.
You will need a driver transistor for each LED, see here or more simplified here. Typically the base resistor would be chosen for a base current + 1/10 of the collector current. In the above example that would be 2 mA and the resistor required will be (5 V -0.7 V) / 2 mA = 2150 Ω. A standard 2 kΩ or 2.2 kΩ resistor would be well suited.

I may interpret your request completely wrong. If you only want to light up an LED when the magnet comes near, you could use simple reed switches. You may also forget about resistors etc. when you use e.g. a CR2032 button cell. While this is not good practice and I wouldn't recommend it for long term use, it works well for a prop that is used probably only for a short time. The construction would follow that of a LED throwie with the addition of the reed switch in one of the connections between button cell and LED.

May I suggest that you describe in detail what you want to achieve (not how) so we can propose a solution?
Your Idea of solution to your requirement may not be the best way to tackle that task.