# Bicolor leds

#### ek4313

Joined Dec 20, 2014
21
I recently purchased some 2 lead bicolored red and green blinking leds #LTL-2935SJW. When I tested them using a 3v coin cell they will only display one color. What I mean by that is when I place the leads on the battery, one + and 1 - they will display constant red no blinking. When I reverse the leads they will only display constant green no blinking. What do I need to do to make them to blink between red and green? Thanks

#### WBahn

Joined Mar 31, 2012
26,398
I don't find any LEDs with that part number.

I did find a bicolor LED with Part ##LTL-293SJW

http://www.digikey.com/product-detail/en/LTL-293SJW/160-1038-ND/121782

As expected, it's just two different colored LEDs back-to-back in the same package. If this is what you have, then to make them blink you will need to apply a suitably modulated waveform to them.

Also, keep in mind that putting an LED directly across a battery is risky. You are relying on the internal impedance of the battery being sufficiently high to limit the current to something that the LED can handle. Sooner or later you will get a nasty surprise.

#### ek4313

Joined Dec 20, 2014
21
I don't find any LEDs with that part number.

I did find a bicolor LED with Part ##LTL-293SJW

http://www.digikey.com/product-detail/en/LTL-293SJW/160-1038-ND/121782

As expected, it's just two different colored LEDs back-to-back in the same package. If this is what you have, then to make them blink you will need to apply a suitably modulated waveform to them.

Also, keep in mind that putting an LED directly across a battery is risky. You are relying on the internal impedance of the battery being sufficiently high to limit the current to something that the LED can handle. Sooner or later you will get a nasty surprise.
Yes that is what I have and thanks for the info. This is my first attempt at electronics and I was naive enough to think a two lead bicolor LED circuit was simple as configuring + to + and - to - and I would have a blinking LED. I do know it is best to have an appropriate resistor to protect the LED but that's about it. Do you know where I can find a schematic or can you tell me how to configure this circuit ? Thanks

#### WBahn

Joined Mar 31, 2012
26,398
What, exactly, do you want your circuit to do? What do you want to see (or at least perceive) and what kind of control do you want over the behavior. If you just want it to blink red at a fixed rate that is slow enough to easily see the flashes (say 0.1Hz to 10Hz or so) as soon as you hook up the battery and for that to change to green when you flip a switch, then that is pretty easy to do with a 555 timer and a small DPDT switch (and there are other ways to do the switching using transistors as well). If you want something more elaborate then the circuit will be more, well, elaborate.

#### ek4313

Joined Dec 20, 2014
21
What, exactly, do you want your circuit to do? What do you want to see (or at least perceive) and what kind of control do you want over the behavior. If you just want it to blink red at a fixed rate that is slow enough to easily see the flashes (say 0.1Hz to 10Hz or so) as soon as you hook up the battery and for that to change to green when you flip a switch, then that is pretty easy to do with a 555 timer and a small DPDT switch (and there are other ways to do the switching using transistors as well). If you want something more elaborate then the circuit will be more, well, elaborate.
Yes, sorry that I wasn't more specific. It just reveals my lack of knowledge. This is going to be used as part of an outdoor Christmas display and what I would like to end up with is this http://makezine.com/projects/dark-detecting-led/ except using a bicolored LED that changes from red to green etc. at the rate of what I read described as a "breathing" color change. It can power up as soon as the 3v coin cell is placed into it's holder and I will depend on the phototransistor to aid in the the switching aspect. Any help will be appreciated but it would need to be written in a "circuits for dummies format". Thanks for your interest.

#### WBahn

Joined Mar 31, 2012
26,398
I almost gave up on the site you linked because it wanted to install over a dozen cookies on my machine. Normally after blocking six cookies I give up and go elsewhere.

Have you built the circuit in the diagram yet? That will be your first step. It will only work for one color at a time, but it's a first step. The circuit needed to change colors will be more complicated. By a "breathing" color change I'm assuming you mean something that fades in and out.

#### ek4313

Joined Dec 20, 2014
21
I almost gave up on the site you linked because it wanted to install over a dozen cookies on my machine. Normally after blocking six cookies I give up and go elsewhere.

Have you built the circuit in the diagram yet? That will be your first step. It will only work for one color at a time, but it's a first step. The circuit needed to change colors will be more complicated. By a "breathing" color change I'm assuming you mean something that fades in and out.
Sorry about the link. No I haven't built the circuit yet but your right about the fade. If fade is too complicated, blink will be ok as long as it's not a rapid flashing frenzy. Are fade, blink and rapid flash achieved using the same procedure just a different timer or are all three accomplished differently ?

#### WBahn

Joined Mar 31, 2012
26,398
Sorry about the link. No I haven't built the circuit yet but your right about the fade. If fade is too complicated, blink will be ok as long as it's not a rapid flashing frenzy. Are fade, blink and rapid flash achieved using the same procedure just a different timer or are all three accomplished differently ?
Blink and rapid flash are the same thing only at different rates -- in both cases the LED goes from 'on' to 'off' very quickly and when 'on' stays at the same brightness. The differences are how long the 'on' time is and how long the 'off' time is.

There are a number of way to accomplish fading. Perhaps the simples (at least conceptually) is to have a triangle wave voltage across the LED/resistor combination. As the voltage increases the LED gets brighter and as it decreases it gets dimmer. Things are trickier, but not much, with your bicolor LED. If you want to go with a digital approach, then you can achieve brightness control using what's called PWM - pulse width modulation - which is actually very simple and straightforward. The easiest way to do that is probably with a microcontroller, but it can be done in pure hardware, too.

What kind of components do you have handy? Any small-signal transistors? Assorted resistors? 555 Timer ICs? Opamps of any flavor? Do you have anything like a solderless breadboard?

Once we have a feel for what you have (or can get), we can give you some small circuits that will walk you up to something that does what you want (or something close).

#### ek4313

Joined Dec 20, 2014
21
Blink and rapid flash are the same thing only at different rates -- in both cases the LED goes from 'on' to 'off' very quickly and when 'on' stays at the same brightness. The differences are how long the 'on' time is and how long the 'off' time is.

There are a number of way to accomplish fading. Perhaps the simples (at least conceptually) is to have a triangle wave voltage across the LED/resistor combination. As the voltage increases the LED gets brighter and as it decreases it gets dimmer. Things are trickier, but not much, with your bicolor LED. If you want to go with a digital approach, then you can achieve brightness control using what's called PWM - pulse width modulation - which is actually very simple and straightforward. The easiest way to do that is probably with a microcontroller, but it can be done in pure hardware, too.

What kind of components do you have handy? Any small-signal transistors? Assorted resistors? 555 Timer ICs? Opamps of any flavor? Do you have anything like a solderless breadboard?

Once we have a feel for what you have (or can get), we can give you some small circuits that will walk you up to something that does what you want (or something close).
I bought some components to experiment with not knowing that the bicolored LEDs would not change color by just connecting + and -. I have some LTR-4206E phototransistors, 2n3904 transistors, 12 ohm 1/8 watt resistors and the LTL-293SJW bicolored LEDs. I have no 555's , breadboard or opamps. . (not sure what opams are). All of my components came online from Mouser. The only outlet that I have locally is Radio Shack.

#### WBahn

Joined Mar 31, 2012
26,398
I'd invest in a handful of 555's and some opamps (the LM384 is a decent choice for playing around with stuff like this, but most cheap opamps will do fine; the ubiquitous one that everyone always talks about is the 741, but I'd stay away from that one because it is so seriously outperformed by all the modern baseline stuff it isn't funny -- it's just that lots of textbooks used it in their examples back when it was all-but the only practical choice for students and the like). I'd also get some 2N3906 transistors (the PNP counterpart to the 2N3904). One thing you can get from Radio Shack (or used to be able to, anyway) is an assortment pack of resistors; they used to sell 500 resistors that ran from 10Ω (maybe even 1Ω) up to 10MΩ and the price was quite reasonable. They also used to carry assortment packs of capacitors, which is a handy thing to have. DigiKey and Mouser also sell assortment packs/kits, so check their prices, too.

You will want a breadboard or two (or twelve) and some suitable connect wire -- #22 or #24 solid works well with most boards (I prefer #24 as it doesn't stress the board contacts as much).

Do you have any kind of a bench power supply? You can get by for sometime with batteries, but it sure is handy to be able to flip a switch and get power. There are lots of cheap used stuff out there to be had, but even new adjustable supplies can be had from DigiKey and Mouser for under $40. BTW, a bench power supply is a great beginner project, so you might stick with batteries until you get comfortable with the basics and then build yourself a really simple supply (and those can be made dirt cheap in a few hours) and then, over time, build better supplies with more bells and whistles. Thread Starter #### ek4313 Joined Dec 20, 2014 21 I'd invest in a handful of 555's and some opamps (the LM384 is a decent choice for playing around with stuff like this, but most cheap opamps will do fine; the ubiquitous one that everyone always talks about is the 741, but I'd stay away from that one because it is so seriously outperformed by all the modern baseline stuff it isn't funny -- it's just that lots of textbooks used it in their examples back when it was all-but the only practical choice for students and the like). I'd also get some 2N3906 transistors (the PNP counterpart to the 2N3904). One thing you can get from Radio Shack (or used to be able to, anyway) is an assortment pack of resistors; they used to sell 500 resistors that ran from 10Ω (maybe even 1Ω) up to 10MΩ and the price was quite reasonable. They also used to carry assortment packs of capacitors, which is a handy thing to have. DigiKey and Mouser also sell assortment packs/kits, so check their prices, too. You will want a breadboard or two (or twelve) and some suitable connect wire -- #22 or #24 solid works well with most boards (I prefer #24 as it doesn't stress the board contacts as much). Do you have any kind of a bench power supply? You can get by for sometime with batteries, but it sure is handy to be able to flip a switch and get power. There are lots of cheap used stuff out there to be had, but even new adjustable supplies can be had from DigiKey and Mouser for under$40. BTW, a bench power supply is a great beginner project, so you might stick with batteries until you get comfortable with the basics and then build yourself a really simple supply (and those can be made dirt cheap in a few hours) and then, over time, build better supplies with more bells and whistles.
Ok. I will gather the needed items and let you know when I have ever thing that you suggested. Mouser is about 5-7 business days out via regular ground delivery. Radio Shack is only a mile away. I just hate paying 2 or 3 times the price. But sometimes shipping cancels any savings. I'll check. As far as a power supply.... 2,3 or 4 bank of AA's, single 9v or will a coin cell work since that is what I would like to power up with in the end? Once I get the components and configuration down pat I will need to build about 20/25 individual units. Thanks

#### WBahn

Joined Mar 31, 2012
26,398
Ok. I will gather the needed items and let you know when I have ever thing that you suggested. Mouser is about 5-7 business days out via regular ground delivery. Radio Shack is only a mile away. I just hate paying 2 or 3 times the price.
When I was starting out Mouser and DigiKey were outrageously priced, frequently higher than Radio Shack, for onesies and twosies. As a student and hobbyist I hated them -- because I was broke and it was coming out of my pocket. But my time was basically worthless, so I learned about all the surplus stores in town and kept piles of scrap that I would liberate components from. As a practicing engineer the reverse became true and I came to love them because they almost always had what you wanted and they would sell things in onesies and twosies, which has considerable value when taking into account the value of time both on the project and of the engineer. I really came to appreciate the cost and value of the service they provided. None-the-less, it has been a real joy to watch the market forces drive Mouser and DigiKey both down to reasonable prices for low quantities over the last decade or so.

[/QUOTE]
As far as a power supply.... 2,3 or 4 bank of AA's, single 9v or will a coin cell work since that is what I would like to power up with in the end? Once I get the components and configuration down pat I will need to build about 20/25 individual units. Thanks[/QUOTE]

Your coin cells won't last long and are usually quite a bit more expensive the AA's and the like. If you're going to use batteries, I would use AA's (or D's) for development and then test nearly polished circuits with the coin cells you intend to use -- they will have a lot higher impedance so there will probably be a difference in how the circuit behaves that needs to be observed and, if needed, compensated for.

You probably have a wall-wart supply sitting around (I have dozens that have accumulated over the years) and one thing that you could build up on your breadboard would be a real simple adjustable voltage supply. Other than the stuff you are already getting, you would just need an LM317 adjustable regulator (I'd get one in a TO-220 case, but a TO-92 will work for the short-term). You will want to be sure that your capacitor stock includes some aluminum electrolytic capacitors, say 100uF, 10uF, and 1uF.

#### ek4313

Joined Dec 20, 2014
21
When I was starting out Mouser and DigiKey were outrageously priced, frequently higher than Radio Shack, for onesies and twosies. As a student and hobbyist I hated them -- because I was broke and it was coming out of my pocket. But my time was basically worthless, so I learned about all the surplus stores in town and kept piles of scrap that I would liberate components from. As a practicing engineer the reverse became true and I came to love them because they almost always had what you wanted and they would sell things in onesies and twosies, which has considerable value when taking into account the value of time both on the project and of the engineer. I really came to appreciate the cost and value of the service they provided. None-the-less, it has been a real joy to watch the market forces drive Mouser and DigiKey both down to reasonable prices for low quantities over the last decade or so.
As far as a power supply.... 2,3 or 4 bank of AA's, single 9v or will a coin cell work since that is what I would like to power up with in the end? Once I get the components and configuration down pat I will need to build about 20/25 individual units. Thanks[/QUOTE]

Your coin cells won't last long and are usually quite a bit more expensive the AA's and the like. If you're going to use batteries, I would use AA's (or D's) for development and then test nearly polished circuits with the coin cells you intend to use -- they will have a lot higher impedance so there will probably be a difference in how the circuit behaves that needs to be observed and, if needed, compensated for.

You probably have a wall-wart supply sitting around (I have dozens that have accumulated over the years) and one thing that you could build up on your breadboard would be a real simple adjustable voltage supply. Other than the stuff you are already getting, you would just need an LM317 adjustable regulator (I'd get one in a TO-220 case, but a TO-92 will work for the short-term). You will want to be sure that your capacitor stock includes some aluminum electrolytic capacitors, say 100uF, 10uF, and 1uF.[/QUOTE]

I've never heard the term wall wart before but a quick google told what they are and your correct in fact, I have a drawer full of them. Ok, i'll get the needed items and let you know when I have everything. Thanks a lot for your interest in helping me with this because I am clueless. It's like Mark Twain said..."We're all ignorant, just about different things."

#### WBahn

Joined Mar 31, 2012
26,398
I've never heard the term wall wart before but a quick google told what they are and your correct in fact, I have a drawer full of them. Ok, i'll get the needed items and let you know when I have everything. Thanks a lot for your interest in helping me with this because I am clueless. It's like Mark Twain said..."We're all ignorant, just about different things."
My pleasure. Just keep in mind that there will be times when I won't be very responsive. The semester starts Tuesday and I wasn't expecting to be teaching at all until the Fall but got asked to teach two classes about a week ago, one of which is completely new to me. So I will be spending a lot of time trying to stay a week ahead of my students.

#### takao21203

Joined Apr 28, 2012
3,695
Radio shack charges $2 for one transistor?? #### Attachments • 206.9 KB Views: 18 #### WBahn Joined Mar 31, 2012 26,398 Radio shack charges$2 for one transistor??
Not surprising. I remember over 20 years ago that they sold 1/4 W resistors in packs of two for \$1.19.

Their assortment kits used to be pretty reasonably priced -- very few other things were.