Background: I'm a software engineer who has done a bit with Arduino and simple circuits but have not worked much with low-level circuits with transistors etc. I'm trying to make a necklace for one of my daughters, where an LED glows when she touches a part of it. Ideally, the harder she presses it, the more it glows. It is powered by a small 12V battery, and the bright green LED I got came pre-wired with a 470Ω resistor. I am willing to buy different parts if necessary, but it would be difficult to replace the LED (because it is embedded with epoxy into a crystal)

When the LED+470Ω resistor is hooked up to the 12V battery, it is very bright, and measures 15mA.

I learned that this 2N2222 transistor can be used to act as a touch sensor, and built a little prototype to test it:

- The two bent headers are where i touch to test it.
- the LED's positive side is connected to battery +, the negative side is connected to the transistor's collector only
- transistor base is only connected to the left header
- transistor emitter is connected to battery -

This works - it glows brighter the more you squeeze the headers but I do not understand it well and the LED glows far dimmer than when directly connected to the battery:

Current when LED+resistor is directly connected to battery: 15mA
Current in above circuit when the two headers are touched together: about 1mA

My Questions:
- Is there an easy way to make this function better so that the LED gets brighter? (The LED has a forward current rating of 20mA)
- Does this circuit slowly drain power over time when the transistor is 'off'?
- If the transistor's base and collector were accidentally connected directly with metal, would the transistor break?
- Can you think of a better way to do this?

Because this is going into a necklace, the fewer components, and the smaller/easier to work with, the better.

I appreciate anyone taking the time to even read this; if anyone has any answers or suggestions, it would be greatly appreciated.

 

Welcome to AAC!

You're never going to get the results you want from such a simple circuit.

I don't see any current limiting resistors. Is the 470 ohm resistor you mentioned in the LED encapsulation?

Post a schematic for the circuit. Most of us prefer that to tracing breadboarded circuits because we can get essentials from a schematic with a glance or two.

 

In your breadboard photo, I do not see the 470 ohm resistor in series with the LED.
I also do not see where the right header connects, to the transistor's collector?

If you use a piece of metal to connect the base to the collector then the transistor will be connected as a series diode so the current will be almost maximum (you said 15mA).
If you connect the base to the collector with the high resistance of your dry skin then the transistor turns on only a little with about 1mA.

The A23 battery is very small so it will not light the LED for much time before it is dead. You might see the LED becoming dimmer and dimmer as the battery dies.

 

Thank you both!

I don't see any current limiting resistors. Is the 470 ohm resistor you mentioned in the LED encapsulation?

There are none. There was a 470 ohm resistor, but when i found that this circuit only lit the LED dimly, I switched it for a 220, then 100... until I just removed it, and it seemed fine.

The A23 battery is very small so it will not light the LED for much time before it is dead. You might see the LED becoming dimmer and dimmer as the battery dies.

I was concerned about that but they have about 60 mAh, so I think that means at 15mA power draw it would last 4 hours with the LED on full all that time. That's pretty good for this because this is intended for a necklace that only lights up when it is touched, which will be a few seconds here and there i imagine. So my concern is now more about how much power it consumes when in the 'off' state, since it will be off for hours-weeks between uses. my multimeter isn't sensitive enough to measure it apparently.

I had someone suggest using a 'darlington pair' for this, so i gave it a shot. here's the circuit (the prototype board is now even messier and harder to see what's going on, so actually tried making a diagram this time... )


It works pretty well... it is too sensitive but i am going to try playing with resistor values in the red areas to try and fix that.

the next problem will be trying to figure out how to put so many components together on a necklace without it looking janky What would solve that is if i could find some really fine but strong-enough wire, ideally one with two conductors through it thin enough to string through beads... i don't think that exists however. I did find some very thin stainless-steel wire wrapped in nylon that seemed great until i attempted to solder it and decided against ever attempting to solder stainless steel again.

 

You did not read the datasheet of an Energizer A23 miniature battery.
Its capacity is 60mAh only when its load draws only 2mA. It is 26mAh at 15mA.
The voltage continues to drop as it is used. Its mAh ratings are when the 12V has dropped to 6V (fairly dimmed).

Why not use one small darlington transistor instead of two normal size transistors??

 

Why not use one small darlington transistor instead of two normal size transistors??

Because I didn't know that that was a thing I am very new at this. Do you know of a type that would work? It would be great if i could cut down on components

It is 26mAh at 15mA.

Ah, well that's too bad. it should still suffice for this though I think. Next time I'll know to try and look for a datasheet when looking up parts.

 

Here's my option. The sensitivity can be adjusted by the size of the resistor in the transistor gate.

 

I'm just curious but, wouldn't using a MOSFET in the configuration in post #7 cause the LED to go to full brightness based on the time you touch the contacts, instead of determining the brightness by pressure?

 

Junk the 12V battery, you are wasting most if it’s energy. A 3V button cell, like CR2032 would be smaller and last longer.

Bob

 

Here's my option. The sensitivity can be adjusted by the size of the resistor in the transistor gate.View attachment 238520

I've never used a MOSFET before, so any pointers about them are welcome. I've ordered some 2N7000s to give this a try, thank you.
What does the downwards-facing triangle with the word Body over it mean?

 

Junk the 12V battery, you are wasting most if it’s energy. A 3V button cell, like CR2032 would be smaller and last longer.

Bob

This has piqued my interest... i spent an embarrassingly long time making custom battery holders for the A23 12V battery out of an acrylic tube, springs and epoxy... but is this true - would a smaller, lower voltage battery actually last longer? I was thinking that generally a larger battery would last longer, and it being higher voltage wouldn't matter because i think of the resistor as slowing down power usage... but from your comment... is the resistor just converting it to heat?