LM3909 – An IC Just to Flash an LED

So during my placement year I was getting really into old electronics, and old IC’s, especially those no longer in production. We were also on a project where we were trying to design a circuit that would flash an LED for a short period of time from the charge on a small super capacitor. The big issue we had was how to minimise current flow, and power an LED at really low voltages, less than 2V. This on the face of it seems like a simple problem, until you start to think about it.

Million Mile Light
Products like the Million Mile Light, a flashing low powered, high brightness LED indicator need to flash for long periods of time on very little charge, much like the problem I faced. Credit: Million Mile Light

There are two go to ways that most engineers would go with to make a flashing LED with a constant flash rate. First is to use a small microcontroller, such as an ATTiny, or a Pic12F series, and use software to flash the LED. This seems good on the surface (and it is what we used in the end product) but it has a big drawback, it can only output a voltage less than the power rail. some versions of the PIC12LF’s can function down to 1.8V, perfect for our power supply needs, but LED’s need upwards of 2.7V (usually) before they start to light, so although our micro will work the LED wont. The second go to way to make an LED flash would be to use the classic 555 timer, one of the most manufactured chips of all time. There is a good reason it is famous, it is extremely versatile. You can decide the frequency based purely on the capacitor and resistor choices. We still have a similar drawback though, a 555 timer needs at least 4.5v as a power supply. So with our potential sub 2V power supply, neither the IC or the LED will turn on. That is one way to conserve energy!

schematic
A cut out from the datasheet, with a basic view of the circuit inside the IC, which I will go through in another post, and a pinout diagram, very useful for prototyping. Credit: National Semiconductor.

This is where the LM3909 came in to play. You have to remember that this chip was developed prior to 1995 (so it is older than me) when the electronics market was very different. Battery technology was not the same, and nowhere near as cheap. It was much more common for people to want to use off the shelf single use batteries such as AA, C and D batteries, or even coin cells in most projects. If you wanted something with a little flashing light on it there were plenty of applications for it. There are buoys in the ocean, store signs and displays, and Christmas lights, all of which would benefit from minimising weight of batteries, but lasting for serious amounts of time. Just as a reference, you could get to 4.5V (to power a 555) by using 3 AA batteries, but the voltage across them would soon dip below this, so you would need at least 4 in most applications. 4 AA batteries take up a lot of space, and weight, not great for many of these applications. Plus most of the chips we have discussed use a fair amount of power, the 555 uses at least 3mA while running, not including the dissipation in the resistors, and all of the power charging the capacitor wasted.

1.5v schematic
A snippet of the datasheet, showing the simplest connection diagram, and a graph of typical current consumption with relation to the battery voltage. It also has a great table describing how long standard batteries tend to last in this configuration, up to 2 years! Credit: National Semiconductor.

So how does the LM3909 solve these issues? well it makes use of a clever concept similar to the 555 of charging up a capacitor. The difference is that the 3909 uses that charge in the capacitor to flash the LED. Although it is slightly more complex than the below schematic, you can think of it as there being a switch inside that oscillates between two states. We will go through how it actually works in a future post. To start with the capacitor is in series with the battery, and in parallel with the LED. The LED wont light, but the capacitor charges up to near the power supply voltage. Once charged, the switch inside flips, and now the power supply, charged capacitor, and LED are in series with each other. To the LED it now sees the capacitor (charged to 1.5V) plus the 1.5V power supply, equivilent to 3V, more than the forward voltage it needs to turn on. As there is a very small resistance, the LED will be on as long as the capacitor has some charge, which isn’t very long as it will discharge fairly quickly. This is the “flash”, as once the cap is discharged the LED will turn off, and the switch will flip back. The capacitor starts charging again, and the whole process restarts. This goes on for as long as the battery has power to give.

Rob Paisley
A great description of the basic principle of how the LM3909 works, charging the capacitor up, and then releasing all that energy through the LED, with increased voltage. Credit: Rob Paisley.

A couple of points to note, the timing and the brightness of the flashing is based upon the capacitor you use, which is quite clever. There are two settings, depending in the pin you put the capacitor in will also double (or halve) the time the cap will take to charge. This means slower flashing, but longer lifetime. Having a smaller capacitor will mean faster, but less bright flashing, and a bigger cap will therefore be slower and much brighter flashing. The design of the chip also means that only two external components are needed for it to work, a capacitor and the LED, compared to the many resistors and extra cap needed on things like a 555 timer. The fact it can use less than 1.5V power source means we can use a single AA battery to power this device, and according to the data sheet it can last up to 6 months on one battery! I have one on my desk that has lasted longer than this.

my LM3909 circuit
My version of this circuit fit into a AA battery box, with it being powered by a single AA battery. It has a switch meaning I can turn it on and off. Poundland LED lights are a good source of these!

All in all I can see why National Semiconductor decided to make this chip, it filled a gap, and was used widely for a long time. Developments in battery technology, and more complex designs needed for the applications this was for has meant that they no longer make the LM3909, but they are still available on Ebay and some Chinese manufactures make them. There is also a design out there to make a discrete version of the LM3909, and I may try that for a future post, as it looks interesting.

Thank you for reading, take a look at my other posts if you are interested in space, electronics, or military history. If you are interested, follow me on Twitter to get updates on projects I am currently working on.

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The Genius of Bob Widlar

National Semiconductor Ad
A famous National Semiconductor ad based on the Widlar Salute and methedology

If you are at all interested in early IC design, especially that in the start of silicon valley, it’s likely that you will have come across the name Bob Widlar. If you have not heard of him then this post may shed some light on an early pioneer of the semiconductor industry. Not just a great hardware engineer, arguably a legendary one. Shaping integrated circuit designs for over a decade he created circuits still in use today, and some of the most famous chips ever. Including the uA702, the first linear IC operational amplifier and the LM109, the first high power voltage regulator. Although a great engineer he was famous for his pranks, and odd office habits. He definitely would not like the current state of corporations, with a bohemian look on life Bob Widlar can definitely be described as eccentric.

Bob widlar salute
Bob Widlar showing the official Widlar salute.

In the late 1960’s and 70’s the semiconductor industry was like something out of a scene in a wild west film. The bars around Silicon Valley were packed day and night with engineers creating innovative circuits and designs left and right, and Bob was right there in the middle of it all. I think a key point to note is that he was partial to his alcohol, for better or worse there are accounts that he wouldn’t make a speech until the had his allotment of scotch or wine. This wasn’t uncommon for the time though, everyone around him was likely the same. The History of Semiconductor Engineering (a very expensive book) describes, “Bob was a fiercely independent individual, very happy to be by himself, and he did everything in a stunning way, which was absolutely natural to him, but completely weird to so-called ‘normal people’.” Basically he didn’t care what other people thought about him. If you want to change an entire industry you have to upset a few people on your way, so this mindset might be best.

Bob Widlar disliked digital circuitry
It could be said that Bob widlar was not a fan of digital circuitry.

There isn’t much known about his early life, and reportedly rarely spoke about it. We do know thought that electronics played a huge role in his early life as his dad was a self taught radio engineer. His father worked at a local radio station so Bob had access to ultra-high frequency transmitters. At 15 he was featured in his local newspaper as an electronics designer who could fix radio and TV sets. Allegedly he also played pranks on the local police using radios, but there is no known details. The passion for electronics continued on when he joined the United States Air Force in 1958. He was responsible for teaching fellow recruits in the use of electronic equipment such as radios. During this time he actually wrote a book, his first, Introduction to Semiconductor Devices. This seems to be a slightly different person to the famous side of Bob Widlar. Some say that his “liberal mind” wasn’t a good fit for the military environment, but his early performance reviews suggest otherwise. His superiors noted his superior electronics and communications skills, they also noted that he had an above average ability to use clear concise words to express himself, and always strived for perfection. In areas of improvement he was recommended to stop dramatising his frustrations at inefficiencies that exist”. This might be closer to the famous widlar. He then left the service in 1961 for unknown reasons, and joined the Ball Brother Research Corporation in Boulder, Colorado. There he helped develop analog and digital equipment for NASA. He was simultaneously studying for a degree with the University of Colorado and graduated in the summer of 1963.

His work at Ball Brothers brought him in contact with Jean Heorni and Sheldon Roberts (who invented radiation hardened transistors), some of the founders of Fairchild Semiconductor. They breached professional ethics by hiring him, a key employee of their customer. He apparently arrived at the interview intoxicated and told the R&D manager what he thought of Fairchild’s analog circuits, saying”what they are doing is bullshit”. He had a second interview and was hired even with the objections by the initial interviewers. His first task at Fairchild was to target IC reliability by improving the fabrication process. He managed to reduce the price of the planar process, and showed he could improve his own bosses designs and squeezed him out of the company. At this point Fairchild only had a lineup of three analog IC’s, all designed for the military, all amplifiers. They were all built inefficiently, like a conventional circuit with discrete devices, creating a sort of hybrid IC. The famous Gordon Moore (of Moore’s Law fame) wanted the company to favour digital IC’s as they were cheaper, easier to design and allowed high volume. Widlar opposed the strategy and held digital electronics in low esteem, famously saying “every idiot can count to one”. Along with the process engineer David Talbert, they rushed through Widlars designs for new and improved analog IC’s, changing the industry as they did so. He managed to remove the need for big resistors and capacitors in IC’s, and truly grasped the planar process. This is when he created the μA702, the first true linear integrated circuit, and the first monolithic operational amplifier.

Bob and a group of engineers at National Semiconductor.

He also created the μA709, another legendary chip. This moved Fairchild to become the leader in the field of linear IC’s. Their circuits were sold out for two years. Some say that at one point Widlar designed and Talbert made 80% of the linear circuits in the world. The problem was that Fairchild never shared the massive profits with them. So he took up a job with National Semiconductor in 1965, taking a huge amount of stock as part of the deal. He refused to do the exit interview at Fairchild and wrote one line to them “I want to be RICH!”. Oddly, Fairchild continued to pay his salary until 1966, Widlar said “Maybe they did not believe that I was actually leaving. Some people are really a little slow.” By 1966 they had set up the epitaxial process at Santa Clara, and created the industry’s first linear regulator. The LM100, a revolutionary new circuit became a flagship product, soon followed up in 1967 with the LM101, an op amp with highly improved performance due to a simple yet robust design. He followed it up with many more improvements to amplifiers, with higher bandwidth, voltage and gain. As well as the famous Widlar current source, he also managed to harness the bandgap phenomenon and built the bandgap voltage reference. This allowed the design of the LM109, a voltage regulator with a power transistor and precise voltage source on one die, something never seen before. By this time Fairchild had gone into a massive decline while National Semiconductor had rocketed up the food chain. In December 1970 he resigned from National Semiconductor and cashed in his stock for $1 million, apparently due to payment issues. He retired to Puerto Rico at the age of 33. The next four years he spent as a consultant.

Widlar current source. Original drawing from the 1967 U.S. patent.

He did come back to National Semiconductor in 1974 as a consultant. During the short stints he spent there he developed the LM12 power amplifier and and the LM10 ultra-low voltage amplifier. These have stayed in production until the 21st century, with the LM10 not even having a reasonable clone for the next decade. in 1981 he spent three years starting Linear Technologies, but this relationship eventually fell apart three years later over patent rights, and his shares were forcibly bought. For the remainder of his life he worked at National semiconductor until 1991 when he died of a heart attack at the age of 53. He had apparently taken up running late in his life and was much healthier. One of his fellow engineers Bob Pease said the damage was done in the first 20 years.

Bob Widlar standing over artwork of the LM10 power amplifier

On top of his famously eccentric nature, fighting in bars and unceremoniously leaving companies he was a well known prankster. The most famous one was the day he brought a sheep to work. The reason was to save money for the company by using it as a lawn mower. He brought it in the back of his Mercedes-Benz convertible for the day. The management not particularly pleased refused to comment. Widlar even invited some young photojournalists to document the event. After the day he left the sheep in a local bar and it was “mysteriously stolen”. On another note he apparently disliked people coming into his office and being excessively loud. He therefore built what is now known as a Hassler circuit which emits a high pitched sound whenever it hears something too loud. In the same vein he also blew up a public address speaker he found annoying with firecrackers! As an analog engineer and highly skilled with high frequency transmitters he once traced one of his problems to interference from the control tower of San Jose airport. In the Widlar way he called up the airport and demanded that they shut down the transmitter. He did have a thing about faulty components and problems, as any electronic engineer can appreciate. If he had spent a day trying to fix a fault just to find a simple component was the cause he would take it out to the workshop an pulverize it with a hammer. The practice now known as Widlarizing usually uses a sledgehammer and requires the component to be smashed so small you don’t even need to sweep it up off the floor. This was so the component couldn’t cause anyone else more problems.

Bob Widlar with the famous sheep, trying to get it to mow the lawn for him. The Mercedes is in the background, badly parked.

Thank you for reading, take a look at my other posts if you are interested in space, electronics, or general history. If you are interested, follow me on Twitter to get updates on projects I am currently working on. Most of all, thank you for taking the time to read my posts.

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