Three State LED Driver Circuit - Page 1
Forum section: electronics / projects

author: AJ3G
author_rank: Regular Contributor
timestamp: April 18, 2022, 12:22:13 pm
content: All:I am wondering if someone can help me with terminology. I believe there is likely an IC that will do what I want, but when I search the term in the subject line, I keep seeing LED Drivers which are designed to run strings of LED’s. I am not interested in driving many LED’s all the time, as I am using it as an indication of DC Voltage.Listed Below is a General Description of the circuit I wish to design:1. Turn Three Separate LEDS on Depending on the Voltage input to a circuit.2. The LED in Question is three state LED, with Red, Green, and Yellow (Both Red and Green Illuminated).3. The LEDS are 2VDC4. The function I seek is to apply an input voltage between 0 - 10VDC. Between 2 - 4VDC Green Illuminates, Upon reaching the 4 - 6V DC RED and Green illuminates, Making Yellow. At >6V Red Illuminates.5. In short, I want a status indication of Yellow, Red, or Green dependent on input voltage detected by the circuit.After review of the online material I keep getting IC’s that are designed to be LED Drivers, which are not appropriate for the application I have specified above. I am certain there are IC’s that will do 90% of the job, but I think I am searching the wrong term.Was hoping someone here could set me down the right path so I can chose the appropriate component for this application.Rich

author: ledtester
author_rank: Super Contributor
timestamp: April 18, 2022, 12:53:13 pm
content: Have a look at the venerable LM3914.Alternatively, look for "battery level indicator" circuits. A common design uses a single LM339 like this one:https://www.electronics-circuits.com/battery-level-indicator/A video on designing with the LM3914:Make your own LED Battery Level Indicator -- GreatScott!https://youtu.be/rZq2v7two1s

author: CaptDon
author_rank: Super Contributor
timestamp: April 18, 2022, 01:00:42 pm
content: Those LED's were often driven with a tri-state buffer I.C. and 5vdc logic supply. One side of the LED went to the center tap of two resistors. Let's say 220 ohms each. (In actual practice the resistors were different values to give equal brightness of red vs. green) The top of the resistor stack went to +5vdc, the bottom of the stack to ground and the +2.5vdc 'center point' to one side of the led. Now, the tri-state buffer operation, When the output was logic low one color would light, when the output was logic high the other color would light, when the output was a square wave 100Hz to 1Khz usually it would light yellow, and when the tri-state buffer was disabled the LED was dark. So you may need to get creative with your circuit.

author: themadhippy
author_rank: Super Contributor
timestamp: April 18, 2022, 01:17:04 pm
content: 2 comparators would get you most of the way there,1 to turn the green off if its over 6 volts and the other to turn the red on if its over 4 volts, make the green a window comparator with an on range of 2-6v and its job done

author: Ian.M
author_rank: Super Contributor
timestamp: April 18, 2022, 01:42:20 pm
content: The devil is in the details!The first thing you need to know is your LED's pinout and internal connections as I can think of six different ways two LED dies can be connected inside a two, three or four lead package, not counting trivial reordering of the pinout. If your LED has a datasheet or product page, link it. If you haven't chosen a LED yet, a three lead common anode package would offer the most flexibility as it would let you easily drive it from logic, open drain comparators or, with the addition of two small signal diodes, from three outputs of a LM3914.

author: CaptDon
author_rank: Super Contributor
timestamp: April 18, 2022, 05:52:29 pm
content: Ian, you are correct in your assessment of the different varieties of bipolar LEDs. The circuit I described was for the 2 lead LEDs and not for any other configuration. We only used the 2-lead devices at the company I worked for and we did internal QC/QA so I ended up with dozens of fall-outs for my home projects also. It was nice to have worked for a company back in the 80's that would allow us engineers to sign out equipment to borrow and give us access to fall-out parts (Usually physical blemishes or falling outside our very rigid specs for 'industrial controls'. Our criteria was that the parts had to be as good as the 'typical' specs and not poorer or slower across our industrial temperatures without resorting to the 54XX TTL parts with their weird pinouts, higher prices and limited future availability).

author: AJ3G
author_rank: Regular Contributor
timestamp: April 18, 2022, 11:54:19 pm
content: Hi Ian:While there is no data sheet, I was able to characterize the part using a DC Supply. This part is a two Anode Lead with a Common Cathode Lead. The marking on it suggest that there are two color (RED/GREEN) inside a semi translucent lens. When both LED’s are turned on you get a yellow color.I left the current measurements at the office, but it’s clear from the testing I conducted this is a 3V0 Part. I wrote down the current values at work, and walked out without taking them with me. I need to double check, as I think it may draw more than the 30mA limit on the LM3914, but this could be corrected after I review what it looks like with a current limiting resistor.This LM3914 amongst the recommendation of themadhippy are definitely much better than the returns I was getting after consulting google before.Rich

author: Ian.M
author_rank: Super Contributor
timestamp: April 20, 2022, 09:34:27 am
content: The next question is does the circuit have to be powered from the input voltage its testing? If so, its going to be very difficult as the 2V minimum at which Green should illuminate is not only below the LED's Vf, but there is also no headroom for a current limiting circuit so there will be a massive brightness change between whatever minimum it actually turns on at, and 6V where it must turn off again.If an auxiliary supply is available, what voltage is it and is is stable enough to use as a reference?Also what's the tolerance on your 2V, 4V and 6V thresholds?N.B. the LM3914 doesn't 'play well' with common cathode LEDs as its designed for individual or common anode ones.

author: AJ3G
author_rank: Regular Contributor
timestamp: April 20, 2022, 10:31:52 am
content: Hi Ian:I was just looking over this information a few moments ago. Each LED is consuming >70mA, so there is no chance for the LM3914 to work as is for this application. The input voltage would be varying, and as such, there is also no stable supply. I had not planned on another stable supply for this project to use as a reference, which I also noted as problematic for the LM3914At the moment, I am thinking it may be a suitable solution to utilize relays with varying coil voltages. I would be happy with a tolerance of 5% but could stand to have more slop on the change over of the LED’sThe basic circuit description is:1. An auto transformer is used to drive the primary side of a HV transformer.2. The transformer has two secondary windings, One is 480VAC and the other 6.3VAC4. I want to use the rectified secondary 6.3V winding to indicate status of the 480VAC Winding. So the 6.3VDC will be wired in a manner to provide the LED Feedback of what is going on on the HV Secondary winding.5. HV Secondary side will vary but has the potential of providing >600VDC after rectification and filtering. I am trying to have the LED let me know when I have reached a threshold where it could be lethal to touch a DUT connected to the 600VDC Adjustable supply.In other words, you can think of the different colors as a warning as follows (generally)1. Safe to touch (GRN)2. Might want to think twice about touching anything (YLW)3. Do Not Touch DUT - (RED)Rich

author: Ian.M
author_rank: Super Contributor
timestamp: April 20, 2022, 11:20:15 am
content: You've got problems. I believe UL/ANSI/IEC 61010 sets 30V RMS as the limit for SELV/PELV AC, which is only 6.25% of your nom. 480V secondary voltage, so the 6.3V winding will only be delivering a fraction of a volt, and probably nothing after rectification. Lighting the green LED without independent power to the voltage sensing and thresholding circuit is therefore extremely problematic.Meanwhile, just for LOLsI've put together a LTspice sim of a discrete transistor circuit that comes fairly close to meeting your original requirements before you gave tolerances, if you don't care at all about temperature stability. It hasn't got a hope of turning on the green LED properly at 2V though. IMHO it isn't fit for purpose for a safety critical application.

author: AJ3G
author_rank: Regular Contributor
timestamp: April 20, 2022, 01:18:14 pm
content: Hi Ian:I am beginning to think I may have to add a small supply from the primary side of the transformer. Not the direction I wanted to go but I do think having an annunciator to give feedback about the DC Voltage output is worth the effort. Unfortunately, I have been bit with Voltages in the 600VDC range when I was in my teenage years, and it's something I will avoid at all cost. A little extra effort to not get touchy with the DUT's while lethal voltage is present is probably time and money well spent.Rich

author: Ian.M
author_rank: Super Contributor
timestamp: April 20, 2022, 01:33:08 pm
content: Yes, an auxiliary permanent regulated supply for the voltage monitor is definitely the way to go. Add a DPST-NO lockout relay in the feed to the transformer, with its coil powered by the auxiliary supply, so it cant produce output voltage if the auxiliary supply has failed.Then its just a matter of some comparators driving the gates of two P-MOSFETs to switch power to the LEDs at your required thresholds, using the auxiliary supply to power the threshold reference divider, e.g. as per attached sim.I'd also recommend fitting a large neon indicator (+ appropriate dropper resistor) direct to the HV secondary as a backup to the fancy electronic indicator.