Model lighthouses hold a wide fascination and many owners must think how nice it would be if, instead of just sitting there, the model actually flashed. The problem is that lighthouse models are likely to be small with little room for batteries and circuitry and the tea-light shown in the picture above is a good example where there is just room to squeeze in a PP3 battery or a small stack of lithium button cells along with a very small circuit board.
The internet abounds with LED flashers. Many are based on the 555 chip and hence can be expected to consume around 10 mA of current which would flatten a small battery within days. After some desultory playing with components on a breadboard I stumbled upon the CMOS circuit that is the basis for this article. This circuit is 5000 times better than a 555 and consumes 2 microAmps which means that an alkaline 9 Volt PP3 battery should last 31 years although this is academic as it is well beyond the shelf life of the battery. A stack of 3 X 2032 lithium cells also giving 9 Volts will last a mere 12 years.
To achieve this performance some rules are broken and electronics professionals will raise an eyebrow if not two.
Step 1: The Basic Circuit 1
It may be helpful to get the circuit going initially on a solderless breadboard and besides the breadboard you will need:
1 X CMOS CD4011 quad NOR gate. (We are using the IC as a quad inverter so a CD 4001 will also work.)
1 X 4.7 Meg Ohm resistor. (Up to 10 megOhm can be used for longer cycle times.)
1 X 10 Ohm resistor.
1 X 1000 microFarad electrolytic capacitor.
1 X 1 microFarad non polar electrolytic capacitor. (1 microFarad ceramic capacitors may be used but they are a little harder to source.)
2 X high efficiency white LED's.
2 X 2N7000 N channel FET.
1 X 4.7 microFarad electrolytic capacitor (tantalum would be best.)
1 X 9 Volt battery such as a PP3.
The schematic above shows the basic circuit. A CMOS CD 4011 has all pairs of gate inputs tied together making it a quad inverter. Two of the gates are wired as an astable with the timing defined by the 4.7 megOhm resistor and the 1 microFarad non-polar electrolytic capacitor resulting in a cycle time of three to four seconds. The time can be easily doubled by the addition of another 1 microFarad capacitor or more in parallel and the 4.7 megOhm resistor can be increased to 10 megOhm so long cycle times are feasible. The remaining two gates are wired as inverters fed from the astable section and their antiphase outputs feed the respective gates of the 2N7000 FET's which are wired in series across the supply line. When the last inverter in the chain output goes high the one before will be low and the top 2N7000 conducts charging up the 4.7 microFarad capacitor via one LED giving a flash. When the last inverter in the chain goes low then the bottom 2N7000 conducts allowing the 4.7 microFarad to discharge through the other LED giving another flash. The output stage consumes zero current outside of the transition times.
The 10 Ohm resistor and the 1000 microfarad capacitor in the power supply line are just for decoupling and are not vital but are very useful in the testing stage.
Electronic purists will point out the the output stage is not good design because any dithering or uncertainty at the point where the circuit switches could result in both 2N7000's being switched on briefly at the same time resulting in a short across the power supply. In practice I find that this is not happening and would show up in the current consumption, see later.
The circuit as shown was found to consume an average of 270 microAmps which is creditable but far too high for our purpose.
Step 2: The Basic Circuit 2
The picture above shows the circuit assembled on a solderless breadboard.
Step 3: The Enhanced Circuit 1
The circuit shown in the schematic above looks to be almost identical to the previous one. Here the addition of just one component effects a transformation in performance that is as drastic as you are ever likely to see in simple electronic circuitry.
A 1 MegOhm resistor has been placed in series with the supply to the CD4011 IC. (Electronics professionals will say that this is something that should never ever be done.) The circuit continues to operate BUT the average consumption drops to some 2 microAmps which equates to a life of 31 years for an alkaline PP3 cell of 550 mA hours capacity. Incredibly, the output voltage is still high enough to reliably switch the 2N7000 FET's.
The picture above shows the added resistor ringed in red.
Measuring the average current drawn by this circuit is a daunting task but a quick test is to remove the battery and allow the circuit to run down on the charge in the 1000 microFarad decoupling capacitor if you have fitted it--the circuit should run for five or six minutes before one of the flashes gives up.
I have had some success by inserting a 100 Ohm resistor plus 3 Farad super capacitor, (observe polarity,) in parallel into the supply line and allowing several hours for equilibrium to be reached. Using a milli-Voltmeter the voltage across the resistor can be measured and the average current calculated using Ohm's Law.
Step 5: Some Thoughts at This Stage.
I have committed the cardinal sin of placing a resistor in the supply line of a CMOS IC. However the IC is standing alone and not part of a logic chain and I would suggest that we are using this single IC simply as a collection of complemetary CMOS transistors. It may be that we have here a poor man's ultra low power relaxation oscillator.
The 'bucket' capacitor that charges and discharges through the two LED's can be increased to provide a brighter flash but with values in the hundreds of microFarads it may be a wise precaution to add a small resistor in series with the LED's to limit peak current and 47 or 100 Ohms is suggested. With larger capacitor values the flash may get a little 'lazy' as the last part of the capacitor charge dissipates through the bottom LED although you may consider that it provides a more realistic lighthouse experience. The current consumption will rise of course maybe even to twenty or thirty microAmps.
Step 6: Making a Permanent Version of Your Circuit 1.
We have done the easy part but should have proved that the circuit works and can now be committed to a permanent form to go into our lighthouse.
This will require elementary electronic tools and assembly skills. The components needed will depend on how you choose to do this part and the skills that you have. I will show a couple of examples and give further suggestions.
The picture above shows a small double-sided Prototype PCB stripboard point to point circuit board. These are available on EBay in a number of sizes and this one is one of the smallest. Also shown is a square of plain printed circuit board with a wire attached and this will form one connection for our battery which is to be a stack of three lithium button cells. With this type of board I find that it is not possible to bridge adjacent pads with solder as the solder runs down through the holes--you must bridge with wire.
Step 7: Making a Permanent Version of Your Circuit 2.
In the picture above we see that construction is well under way. Note that two 1 microFarad capacitors were used for timing and three 2025 lithium button cells are ready to be sandwiched between the battery end connectors.
Step 8: Making a Permanent Version of Your Circuit 3.
In the picture above we see the finished article ready to be installed in a lighthouse. Note that the three lithium cells have been connected in series positive to negative up to the top positive which is connected to the square of plain PC board soldered to the red lead. The stack of cells has then been bound together tightly with self-amalgamating tape. You will find examples of this method of making batteries from multiple button cells elsewhere on the Instructables site.
Step 9: Making a Permanent Version of Your Circuit 4.
In the picture above we see another version assembled on stripboard which is the modern version of Veroboard. This is fine but modern board is unforgiving of mistakes and will not stand much soldering and desoldering before the copper strips lift, so do get it right the first time! The battery is an alkaline PP3 which at 450 mA hours capacity calculates to a rather academic 31 years life.
Step 10: Making a Permanent Version of Your Circuit 5.
Here the stripboard circuit plus PP3 battery have been cocooned in plastic packing material and wedged into the tealight holder which allows our assembly to be inserted up into the lighthouse.
For a simple circuit like this you can also make your own printed circuit board with a printed circuit pen but you have to be able to etch it, preferably not in the kitchen! Lastly a small sheet of plain printed circuit board can be the subject of 'dead bug' construction which can give the smallest and most robust construction of all of the examples.
Step 11: Last Thoughts.
This circuit is so cheap to make as to be disposable . It can be made so small as to go into a small glass jar and then even potted in resin or wax if the LED's are left in the clear. In such a robust form there may be a multitude of potential uses. I would suggest that it could be a valuable safety item in caving and especially cave diving where a number of these could illuminate a way out of a cave or from the inside of a tortuous wreck. They could be left in place for years.
The bucket capacitor may be made smaller lowering the power consumption to a level where the circuit could be driven by a 'pile' battery of dissimilar metal plates interleaved with electrolyte pads. This might even result in an assembly that could be placed in a 'time capsule' to be dug up some fifty years later!