Discrete Low Voltage Flashing LED Circuits - Page 1
Forum section: electronics / projects

author: Dabbot
author_rank: Regular Contributor
timestamp: December 04, 2021, 01:59:54 am
content: Consider this a place to share and discuss methods of flashing an LED from a <= 1.5V power source (single alkaline / NiMH cells, etc).Discrete components only and preferably tested. Feel free to include discrete versions of the LM3909 and similar.Here is my own. This circuit uses a PUT connected transistor pair, which operates a third transistor to discharge a storage capacitor in series with the LED and the power source.When C2 charges high enough to trigger the PUT, R6 diverts the majority of current from C2 to switch on Q1 and flash the LED. It works down to 0.8v. Tested with 548/558 and 2222/2907.The user can choose C2 up to 10uF and R4 as low as 47K when setting up the timing. R2 and R3 can also be modified to accommodate different choices of C1 and charge rate, as long as R2/R3 is roughly maintained.Having R4 connected to Q1's collector helps reliably reset the PUT, but if a sufficiently large value of R4 is chosen, it can be connected directly to the positive supply, allowing R2 and R3 to be the same value, e.g. 4K7.Plenty of testing went into selecting R6, R7 and R8, with one of my goals being the ability to drop in different jellybean transistors and maintain roughly the same performance, while sticking to E12 resistance values.(Edit1: Insert image)(Edit2: Updated schematic)

author: magic
author_rank: Super Contributor
timestamp: December 04, 2021, 04:00:57 am
content: Blinking aside, are you sure that there are LEDs that will work from 1.2V supply?Alright, I see the trick

author: Dabbot
author_rank: Regular Contributor
timestamp: December 04, 2021, 04:08:41 am
content: Yes. This is sitting on a breadboard nearby and can comfortably run most LEDs.

author: ledtester
author_rank: Super Contributor
timestamp: December 04, 2021, 04:33:34 am
content: The first one I remember seeing years ago was in Bill Bowden's collection of circuits:His collection used to be hosted on Compuserve user pages. It has since moved to:http://www.bowdenshobbycircuits.info/index.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 04, 2021, 04:40:04 am
content: Submitting for Ye Olde blocking oscillator ("joule thief").It works down to whatever Vbe is (so, ~0.7V for Si, less if you're willing to allow vintage Ge), and startup can be had from a JFET or depletion MOS oscillator which can start up as low as the 10s mV.(There are SiGe RF transistors, but the Ge is employed in the B-C junction IIRC, greatly enhancing performance, but not affecting Vbe much compared to Si.)A relevant property of the blocking oscillator is, when gain is adequately high, it can kick on with very little bias indeed, and then do a single cycle, or burst of cycles, depending on the values of base resistance and capacitance. When the time between bursts is quite long, well, it visibly flashes.I once made a reasonably-scaled joule thief, that is, capable of about a watt into an illumination grade LED:Of course you'll notice that's a 1.5V cell (or 1.2V, I most often have a NiMH in it), so it's doing the business. It's more of a flashlight than a flasher, but I'll get to that in a moment.That's a 3A low-Vce(sat) transistor (such as PBSS303NX), which boasts quite excellent hFE and Vce(sat), even up to quite high currents. And, so it seems, down to low currents as well. The switch selects between open, 1k and 100R for the base bias, giving "off", low and high power settings. If I touch my finger across the switch terminals while in the "off" position, I can make it blink at some fractional Hz as my skin leakage (~Mohm) charges the base bypass cap, which eventually kicks over and fires off one pulse. And being the pulse is a few watts, it's quite brightly visible despite being only some microseconds in duration.I think you'll have a hard time going lower (in voltage) with a charge pump architecture, but inductors are fine down to whatever. Eventually the problem is getting enough power at all; you need very thick semiconductors indeed to obtain any kind of useful power from a handful of thermocouples, say (~10s mV, so even 100mW needs some ~A!). The advantage of a charge pump is, it doesn't need to switch fast, just solidly enough to not lose a bunch of charge in the process.Another difficulty is the "valves" in the "pump" -- doing it the easy way, with diodes, loses you a ton of Vf. (Consider the losses in a voltage multiplier or Cockroft-Walton stack.) This is another case where integrated circuits shine; it's no problem to load up a couple dozen CMOS switches, complete with level shifting driver circuitry. Have fun doing all that on the breadboard (where you can't even get 4-terminal MOSFETs, except for a very few outliers).Tim

author: Dabbot
author_rank: Regular Contributor
timestamp: December 04, 2021, 05:14:10 am
content: Quote from: ledtester on December 04, 2021, 04:33:34 amJust built the bottom right one. Wasn't able to get it to work, but I'll keep investigating.Edit: Italmostworks. When I switch it on I get a single pulse, but that's it.- Tried adjusting the 1.5M resistor up and down- Tried different transistors (2222/2907)- Using a non-polarized cap for the 1uF just to be sure- Tried changing the supply voltage- Introduced some series resistance into the supply

author: Dabbot
author_rank: Regular Contributor
timestamp: December 04, 2021, 05:26:01 am
content: Quote from: T3sl4co1l on December 04, 2021, 04:40:04 amExtra points for this versatility.

author: Dabbot
author_rank: Regular Contributor
timestamp: December 09, 2021, 03:22:28 am
content: Quote from: ledtester on December 04, 2021, 04:33:34 amI got this one to work with small modifications. See attached.Q3 could also be driven from the other side of C1.There are other variants of this circuit which use Q2 to perform the charge pump switching, instead of a dedicated transistor.(Edit: Correction to schematic)

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2021, 09:36:24 am
content: 1m is too lower resistance. Don't you mean 1M?Note that LTSpice is case insensitive, so 1Meg should be written for 1M. I know you're just using it for schematic capture, rather than simulation, so it's not important here, but it's important to know for future reference.How about using the classic two NPN astable circuit?

author: Dabbot
author_rank: Regular Contributor
timestamp: December 09, 2021, 09:52:55 am
content: Quote from: Zero999 on December 09, 2021, 09:36:24 amGood catch. Corrected.

author: Dabbot
author_rank: Regular Contributor
timestamp: December 09, 2021, 11:09:11 am
content: Quote from: Zero999 on December 09, 2021, 09:36:24 amI built this up on the breadboard. Worked straight away for a nice bright blinky.It does pull a fair bit of quiescent current, but that could be mitigated by lowering C1 & C2, and raising R1, R3 and R4.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 09, 2021, 06:12:25 pm
content: Related, but definitely not single cell:https://www.seventransistorlabs.com/Images/Tube_Shift_Register.pdfhttps://imgur.com/gallery/OF4jAxhRelevant, the ratio of resistor and capacitor values get kind of weird, for large ratios (and thus extreme duty cycles). At some point, increasing the ratio fails to increase duty, but increases frequency instead -- because the discharge cycle of the larger one is cut short by the short charge cycle of the other, something like that. With the values shown, I got a... what was it, something like 10us pulse every some Hz? Dang, I forget exactly what the timing was anymore...And mind, that's for tubes, so, a FET-like multivibrator. This should be harder with BJTs, as you're limited on base bias resistor value as well. Maybe doesn't matter since the architecture above (BJTs) is ultimately limited by C3 into D1, but interesting anyway.Heh, or a more relevant but even more esoteric method, instead of neons, I suppose you could make a relaxation/multivibrator that drives LEDs directly, using tunnel diodes. That's probably the least-component solution. Good luck with finding them though.Edit:...Can you make a low voltage lambda diode with logic-level N/P MOS? Hmm... that might be worth a try. Don't think I have any low enough in voltage to try it at 1.5V though.Tim

author: Zero999
author_rank: Super Contributor
timestamp: December 13, 2021, 10:12:42 pm
content: Quote from: Dabbot on December 09, 2021, 11:09:11 amThis one should use less quiescent current.

author: StillTrying
author_rank: Super Contributor
timestamp: December 13, 2021, 11:44:41 pm
content: Quote from: Zero999 on December 13, 2021, 10:12:42 pmIt doesn't work for me, both transistors stay turned on.

author: Dabbot
author_rank: Regular Contributor
timestamp: December 14, 2021, 12:59:13 am
content: Quote from: Zero999 on December 13, 2021, 10:12:42 pmThis one works for me. Draws around 20 microamps between flashes and operates down to just over a volt.

author: magic
author_rank: Super Contributor
timestamp: December 14, 2021, 06:39:27 am
content: Quote from: StillTrying on December 13, 2021, 11:44:41 pmIncrease R3 perhaps?And I think it could simply go to ground as well.

author: Dabbot
author_rank: Regular Contributor
timestamp: December 14, 2021, 08:05:46 am
content: Quote from: magic on December 14, 2021, 06:39:27 amYou can take R3 to ground (and thus get rid of R1), but make sure you increase it a lot. At least 470K. The reason it's there with the 22K is to reliably reset the circuit.I've also noticed this circuit stops functioning at higher voltages, so verify your supply voltage.

author: magic
author_rank: Super Contributor
timestamp: December 14, 2021, 08:33:50 am
content: This solution is quite sensitive to R3 and β of the transistors.Dependence on PNP beta could be eliminated with addition of a diode (better: a diode-connected PNP), like in this circuit that Zero999 "borrowed".https://www.eevblog.com/forum/projects/a-simple-traffic-warning-light-drives-me-nuts!/

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2021, 08:45:38 am
content: Quote from: magic on December 14, 2021, 08:33:50 amHere's the correct link.https://www.eevblog.com/forum/projects/a-simple-traffic-warning-light-drives-me-nuts!/Yes, I did take it from that thread. It's an old circuit. I remember it from the Forrest Mimms books I bought from Tandy years ago and that thread reminded me of it.Unfortunately the circuit in his books often omitted a series resistor with the frequency adjustment potentiometer, which would smoke the transistor if it was set too low and adding base resistor to Q2 is also a good idea.https://en.wikipedia.org/wiki/Forrest_Mims#Author

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 14, 2021, 09:18:58 am
content: Here are a couple of circuits I tested years ago... I didn't saved any note on the performance but I found something about in old messages I posted on local (Italian) newsgroup.This should work with LEDs of any colour. The light pulse lasts 100-150ms and the frequency is about 1Hz with 500µA of battery current. Low fwdV LEDs should work with 1.2V or less.I could not find any note about this single capacitor flasher. LTspice simulation did not work at 1.5V but I think it depends on the LED model.

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2021, 09:54:58 am
content: Quote from: not1xor1 on December 14, 2021, 09:18:58 amIt looks interesting. What LED model did you use?I just use many standard silicon diodes in series, to get the appropriate forward voltage to model an LED. Insert the standard LED into the schematic. Right click on the symbol. Where it says value2, enter n=3, meaning three in series, for a standard red LED. The good thing about this feature is decimals can be used, so 4.5 can be used for a blue LED. Using m instead of n, connects them in parallel.This works for other components.https://www.analog.com/en/technical-articles/ltspice-combining-multiple-model-instances-into-one-symbol.html

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 14, 2021, 11:52:07 am
content: Quote from: Zero999 on December 14, 2021, 09:54:58 amQTLP690CQuoteI know... but had not tried... it looks like it works better... I changed a few values inthis:

author: Dabbot
author_rank: Regular Contributor
timestamp: December 14, 2021, 12:04:53 pm
content: Quote from: not1xor1 on December 14, 2021, 09:18:58 amI gave this one a try. It barely works at 1.5V. I did get improved performance changing the 6K8 to 10K.Quote from: not1xor1 on December 14, 2021, 11:52:07 amThis one works much better.

author: StillTrying
author_rank: Super Contributor
timestamp: December 14, 2021, 07:52:08 pm
content: For some reason I have to keep lowering the 1.5V to <1.43V for it to oscillate.

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2021, 09:12:44 pm
content: I changed the resistor values and moved R1, as suggested by the responses to my post. The simulator shows it will work from 1V to 1.6V, which should be good for an AA cell. I'll have to breadboard.

author: StillTrying
author_rank: Super Contributor
timestamp: December 14, 2021, 09:58:06 pm
content: Quote from: Zero999 on December 14, 2021, 09:12:44 pmIt worked that one when I made the .asc match the schem. it seemed to work at a bit slower pulse rate without the 47k.

author: Dabbot
author_rank: Regular Contributor
timestamp: December 15, 2021, 01:08:30 am
content: Quote from: Zero999 on December 14, 2021, 09:12:44 pmOperates down to just under a volt from my testing.For other transistors I tested, namely 2222/2907, 3904/3906 and 548/558, it stops working around 1.1V.Also, the .asc you uploaded is for the original version.

author: Zero999
author_rank: Super Contributor
timestamp: December 15, 2021, 09:15:00 am
content: Quote from: Dabbot on December 15, 2021, 01:08:30 amI've updated the .asc file in my post.I know this thread is about discrete circuits, but have you tried a low voltage CMOS logic gate oscillator?The 74AUP2G14 will work down to 0.8V, which is below half the forward voltage of a red LED.https://www.farnell.com/datasheets/1994607.pdf

author: cliffyk
author_rank: Frequent Contributor
timestamp: December 15, 2021, 01:49:43 pm
content: Quote from: Dabbot on December 04, 2021, 01:59:54 amFlashing at what frequency, duration, periodic or random?This is just a plain ol'flickering LEDrunning at 3.0 to 3.3 V directly wired (no resistor) to 2 AA cells:(click image for video)

author: Dabbot
author_rank: Regular Contributor
timestamp: December 17, 2021, 06:46:48 am
content: Quote from: cliffyk on December 15, 2021, 01:49:43 pmYes.Quote from: cliffyk on December 15, 2021, 01:49:43 pmHere we're focusing on simple circuits which can blink an LED from 1.5V or lower, which requires a charge pump or boost converter.

author: Zero999
author_rank: Super Contributor
timestamp: December 17, 2021, 05:32:03 pm
content: A Joule Thief circuit can be used to drive a self-flashing, or flickering LED.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 18, 2021, 10:50:15 am
content: Just for reference:https://www.eevblog.com/forum/projects/led-blinker-that-operates-below-1mv-hold-my-beer/new/#new

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 19, 2021, 03:42:35 am
content: Quote from: T3sl4co1l on December 04, 2021, 04:40:04 amI managed to build one with just a chinesium 2N3904, values were all guessed or found experimentally, and the core was just the first one I found in the junk box, so I'm sure it can be improved on. I'm not sure what the 10 ohms on the emitter is doing (discovered accidentally trying to scope the current waveform), but it makes it brighter. Flashes much faster and dimmer without the 1nF and 10R, pretty bright with the 1n (flattens the pulse?), almost makes me see spots with the 10R added. Not sure why it likes such a big base capacitor, but it does, I think it's just firing a single pulse (I need to get a better oscilloscope), so the big cap puts more energy through the base?Next I think I'll try to make it blink a neon just to up the difficulty.

author: Zero999
author_rank: Super Contributor
timestamp: December 19, 2021, 12:35:29 pm
content: Yes, it's possible to combine both the low and high frequency oscillator into one circuit, so it produces bursts of higher voltage pulses to flash an LED. This is known as a squegging oscillator.https://en.wikipedia.org/wiki/Squegginghttps://www.eevblog.com/forum/beginners/weird-one-transistor-led-blinker-circuit/msg1981934/#msg1981934https://www.circuitlab.com/circuit/zjy5g2/simple-jt-flasher/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2021, 02:15:31 pm
content: The large capacitor tends to prefer squegging over single pulses; it has to do with the relative time constants of the inductor, drive current and hFE, and bias resistor and bypass capacitor. When the bypass cap is adequately discharged during a cycle, it will tend to do single pulses. A smaller value then gives shorter pulses, and larger values give burst operation.I find base resistance tends to impair function; it seems excessively strong base drive isn't a problem after all, meanwhile maintaining drive strength until end of pulse seems more beneficial. This may have a different priority for squegging (burst mode) operation, not sure.Squegging can be somewhat discouraged by damping the output network. Consider: if the base capacitor doesn't discharge much during a cycle, then when the flyback pulse finishes, the voltage rings down, overshooting somewhat. And if this overshoot turns on the transistor, it latches on and goes for another cycle, and so on until the capacitor is too discharged to start up after that last cycle. Hmm, ah yes, the series limiting resistor should encourage that then -- by limiting base current, at the expense of weaker drive (but if drive strength is adequate, given desired load and pulse width, it should still be fine).I forget what the transformer ratio on mine was, might've been a bit shy of 1:1. It's a powder core, something like 1uH total, maybe even less, I'm not sure. You certainly need more than a few hundred mV at the base to get solid switching action, and a few volts seems optimal. Over 7Vpp will cause avalanche and runaway operation (avalanche provides bias current independent of the pull-up -- something like a diesel engine ingesting its own oil!).Looks like you got a powder core as well (yellow-white?) so that will tend to be well damped, and should be free from saturation. (A common mistake is using something like a ferrite bead; the saturation current even for just a few turns, may be too low to be practical even for a 20mA LED.) Downside is the poor efficiency. Ideal is higher Q material, or gapped ferrite. Rewinding a ferrite spool type inductor is a reasonable  option.My flashlight BTW does something like 60% efficiency, between 1.5V input, and like 3.6V output -- as measured with a schottky diode and filter cap instead of the LED. Which has more capacitance, so there will be some difference in switching behavior, but likely still representative.Tim