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schematic

simulate this circuit – Schematic created using CircuitLab

I need an LED flasher circuit to operate inside small model lighthouses. Ideally the flash needs to be white and the battery needs to be physically small but last for years.

Conventional wisdom (and holy writ,) is that you should not put a resistor in series with the Vdd pin of a CMOS logic circuit however I have working before me a highly efficient LED flasher circuit that does just this.

The circuit (that I rather stumbled upon,) uses one 4001 IC. The IC has two gates configured as a conventional astable and uses the remaining two gates as inverters in series which provide anti-phase outputs. With no resistor in the supply line and a supply voltage of 9 Volts this part of the circuit consumes 170 micro Amps which is not surprising for a CMOS oscillator at this supply voltage. This is unsustainable for a small battery to supply for any length of time. The oscillator cycle period is 12 seconds.

With a massive 1 Meg Ohm resistor between the supply line and the Vdd pin the circuit still runs but now with a cycle time of 5 seconds. The circuit now consumes around a micro Amp and, incredibly, the 'On' voltage available at the output of the two inverters is above 6 Volts and is available to feed two 2N7000 FET's with no need for level shifting. The two FET's are wired in series and fed in anti-phase so that when one is switched 'on' the other is 'off'. At the source/drain junction between them the output voltage cycles between 0 and 7.1 Volt.

The output of the two FET's is connected to two white LED's connected back to back in parallel and these in turn to an electrolytic 'bucket' capacitor which is connected to ground. When the output goes high it charges the capacitor via one LED giving a flash and when the output goes low then the capacitor discharges via the second LED giving another flash. The two LED's can be physically mounted close together so as to appear to be a single light source and to fulfil the requirements of a model lighthouse.

The intensity of the flashes can be varied by selecting different values of the bucket capacitor and I have tried values from 4.7nF which gives a just discernable flash and up to 47 microfarad at which value the flashes are very bright. At 4.7nF the current consumption is 1 micro Amp and at 47 microfarad the current consumption is 21 micro Amp. These values are roughly one thousandth what would be normal for a 555 circuit and would reduce further with longer cycle times.

The knotty problem of measuring such low currents is solved here by having the 100 Ohm sense resistor with a parallel 3 Farad super capacitor in the supply line. (This capacitor is one that I have to hand--a smaller super capacitor would probably do.) After a lengthy period, (say half an hour for very low current values,) the voltage across the resistor can be measured with a suitable milliVolt meter and the current calculated. These two components can be removed when not required.

My question is "I would like to think that this circuit could have widespread application but is it just too outrageous to recommend?" (I should add that my qualifications lay outside electronics but having had a ham radio callsign for nearly sixty years I have a long history of cruelty towards and misuse of innocent components.)

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    \$\begingroup\$ Have you accounted for the leakage current through your capacitors, particularly the 3F capacitor? This may add significant error to your current estimates. \$\endgroup\$ – Elliot Alderson Jan 15 at 18:31
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    \$\begingroup\$ A one megohm resistor there is certainly quite unconventional. If it works, and this is a one-off, I imagine you're fine, but if it's a production device I'd say don't do it; try to find another way to get such low power. \$\endgroup\$ – Hearth Jan 15 at 18:33
  • \$\begingroup\$ There are much simpler and more efficient ways to do this. Can you define the Candella intensity , minimum number of LEDs and expected duration of the pulse needed , or like a Xenon flash? then one can determine the optimal battery power to sustain years of usage. with wireless rechargeable abilities even. \$\endgroup\$ – Sunnyskyguy EE75 Jan 15 at 18:35
  • \$\begingroup\$ That is a real simple circuit, try bread-boarding it. In my experience, spice simulators don't simulate correctly when you do such things to the power pins of logic chips. Also, they don't simulate capacitor leakage and absorption well. Experiment! PS. since you don't have current limiting on the LEDs, you'll need to make sure that the energy pulse is small enough to prevent over heating the die. \$\endgroup\$ – Aaron Jan 15 at 18:39
  • \$\begingroup\$ This circuit is very unconventional and also relies on components that are there but not in the schematic! For example: how can C1 charge/discharge? If you look at the schematic one side of C1 is connected to an input so no current can flow. So why can it still work? Because C1 charges/discharges through the ESD diodes that are between all pins of the CD4001 and its supply pins. That probably also explains why despite the 1M resistor in the supply this thing still "works". I think it is designed in a poor way. There is no reason why it would perform better than a more conventional circuit. \$\endgroup\$ – Bimpelrekkie Jan 15 at 18:45
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(Because this is on the unanswered list...)

This is just too outrageous to recommend. There are better ways of making efficient flashers, and with the resistor in the power lead, this one is going to be problematical.

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They don't make the LM3909 anymore. Or they do, sorta... it's available as the NTE876. That old part used a ‘flying capacitor’ approach to boost up the LED voltage. It was popular in traffic barrier flashers some 40 years ago. Even if you don’t use this part it gives an idea on how to do what you’re trying to do so it’s worth studying.

At any rate this page shows an interesting and efficient ‘joule thief’ flyback approach to blinking an LED. This is another popular project. Link: https://www.electroschematics.com/10896/nostalgic-led-driver-upshot/

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