Electronic Paperweight

Intro: Electronic Paperweight

Imagine a paperweight that not only holds piles of paper in place, but perpetually blinks ! 

STEP 1: Basic Components

A solar cell, supercapacitor, and a new ultra low power 555 chip make it possible !

STEP 2: The CSS555 Micropower Timer IC

The heart of this novelty is the CSS555 timer integrated circuit. It is a micropower version of the venerable 555 and 7555 chips that the electronics world knows and loves.  It has the exact same pin configuration and functions the same way.  It actually can be user-programmed to work in various modes, but for the project described here, it is used in the standard 555 mode as it comes from the factory.   Its key feature for the paperweight is the tiny amount of power that it needs to function. In this application, the circuit takes only about 6uA to carry out its timing tasks.  The biggest current draw occurs when it blinks the LED which is only about 1/2% of the time.  Thus on average, the circuit uses a tiny amount of power.  This enables the paperweight to blink all night long on the energy it stores in its supercapacitor via a solar cell during the day or under your desk lamp.

STEP 3: Astable Operation

The basic circuit used here is the standard 555 astable setup.  At the start of an ongoing cycle, the timing capacitor CT starts charging up through resistors RA and RB; the discharge pin is open and the output pin is high (i.e. at the supply voltage VDD).  When the rising capacitor voltage reaches the upper  trip point, which is 2/3 of the supply voltage VDD , the output and discharge pins go low, i.e. are connected to ground or 0V.  The capacitor voltage then drops via resistor RB until the lower trip voltage, 1/3VDD, is reached which brings the trigger pin to the lower trip voltage and that causes the output pin to go high again and the discharge pin to disconnect.  The cycle then repeats itself as before.  Note that the reset pin must be connected high for operation; setting this pin low stops the timer and the output pin goes low.

STEP 4: Voltage Waveforms

The diagram shows how the voltage cycles at the output pin and the timing capacitor.  After the initial energizing of the circuit, steady state operation is established  and the capacitor voltage cycles between  1/3 and 2/3 of the supply voltage.  On its upward rise, the voltage at the output pin is high, VDD; on its downward discharge, the output voltage is low, 0V.

The lengths of the on and off periods are determined by the values of RA, RB, and CT.  The on time is inherently longer than off because the resistance for charging the capacitor is RA+RB, whereas the discharge resistance is just RB.  In order to achieve an on time less than the off time, we connect the LED with its cathode to the output pin and its anode - through a suitable resistor - to the supply voltage.  In this way, the LED goes on when the output is low, and is off when the output pin is high. This gives a duty cycle of RB/(RA+2RB), which is the ratio of time the output pin is low to the total time of a cycle.  The timing capacitor contributes to the complete cycle time, or period, according to the following expression:  0.695(RA+2RB)CT.

These illustrations are taken (with permission) from the data sheet that Custom Silicon Solutions has prepared for these chips.  They have also prepared an Applications sheet showing how these chips can be used for a variety of purposes.  These information sheets are extremely well done - very clear and well illustrated.

STEP 5: The Paperweight Circuit

The values of the components used in the original paperweight circuit are:

RA = 10M
RB = 47K
CT = 0.22uF
RL = 47
CS = 1.0F

These values give a period of about 1.5 seconds and a duty cycle of just under 1/2%. Between flashes, the circuit draws under 6uA.  So the average power consumed is very low and the blinker will go for around 40 hours on the energy stored in the supercapacitor CS.  This carries it through the night and even through a whole day when you are not at your desk. It won't quite make it through a weekend though, unless you are important enough to have an office with a window !

Bypass capacitor CB is not needed in this application because the large storage capacitor CS more than takes care of squelching any voltage glitches that might occur.  The diode D blocks CS from draining through the solar cell when it is dark, and can be any small diode such as 1N914, 1N4148, BAT41, etc.  The resistor RL limits the current draw through the LED, which should be a high or super bright type for the best flash.  Note that in the circuit diagram the Reset pin is connected directly to the supply voltage.

If you wish to have your paperweight (or other flasher application) only work when it is dark, then include the phototransistor TP as shown in the circuit diagram.  The type is not critical ;  Vishay TEPT5600 or Ledtech LT9593 work great.   Alternately, a photodiode will work as well, and even some LEDs have sufficient output in bright light to effectively short the capacitor and halt operation; test some LEDs with your digital voltmeter to find one that puts out 1.4V or so and it should work in this circuit.

STEP 6: Construction Details

A Google search will reveal a host of suppliers of phenolic stripboard.  The CSS555 Timer IC is available from Jameco .  Be sure to find a supercapacitor of 1 Farad or better, with a voltage rating of at least 5V.

STEP 7: Breadboard First

Once the electronic components have been gathered, breadboard them together to make sure everything will work to your satisfaction.  It is not a pleasant task to fix a circuit once it has been soldered together.  Let the breadboard setup run for a week or two in the location and under the lighting conditions in which it will eventually reside. 

STEP 8: Solar Cells

The solar cell to use should be the thin film on glass type because these have good voltage under indirect or ordinary room lighting - even fluorescent.  They are not as easy to find as the silicon crystal type. Calculator batteries are the right kind, but their output is too low in voltage and current - calculators these days do not need much power !  Imagesco.com have some very nice indoor type cells, but they are larger than would fit inside a paperweight of the size used here.  Well...a larger  paperweight with several blinking lights would be neat...or...a set of blinking book ends !  Smaller indoor type cells have been available at Deal Extreme - one of these is shown in the photo.  A cell with a nice strong output for this application can be taken out of a solar keychain flashlight like the one shown in the last two photos.  It is a bit larger than the one used in the prototype, but could maybe be squeezed into a small paperweight.  You could even use the rechargeable battery from the flashlight instead of the supercapacitor, but the battery will only stand up to a thousand or so recharge cycles, whereas the capacitor has virtually unlimited life.  The output voltage of the solar cell for the lighting conditions to which it will be exposed should not exceed the 6V absolute max. rating of the  CSS555 chip or the max. voltage rating of the storage capacitor which is typically 5.5V; the blocking diode will reduce the peak voltage of the cell somewhat.  You could insert anther diode or two in series to drop the voltage of the cell if necessary.  All this should be sorted out during the breadboard trials.

Comments

[Dashing Rainbow Dash]

I did something similar, but instead of using the mentioned 555, I used a modified joule thief circuit. It flashes twice a second, and runs on 3 farads. the solar cell comes from a cheap garden light, and the blocking diode is a geranium. It was basically made from scrap electronic parts

[tcm84]

Nice instructable!

Some 5V memory backup supercapacitors have a large internal resistance, so CB capacitor might increase LED brightness. I think 47uF would be good.

That supercapacitor might not
have an infinite life. I have seen some Elna and other brands of
supercapacitors leaking. One reliable brand is Tokin/ Nec. I have
collected about 40 of them from different equipment. I have never seen
one going bad, the oldest ones i have are two 5V and 0.47F from 1980s. They are pretty big, 15mm x 35mm.

[geocacher2]

I made one cast in epoxy resin. It's been working for a month now. (yes, that's a big bubble on the solar cell.) I think I'll cast one in layers to eliminate bubbling.

[geocacher2]

Any idea how to prevent those tiny solar panel pads from popping off. They are quite easily pulled loose. I was thinking of adding a large blob of E6000 glue over my wiring connection to add some physical strength.

[TinkerJim]

Are you referring to the "pads" on which the wire leads are soldered? If so, then glue on the wires is a prudent idea. I usually place a dollop of glue (epoxy or sometimes even Tacky Glue) on each wire coming off the cell as a "strain relief" so the pads themselves do not experience stress. And when a pad does come loose before being treated as above, I have found that a product called "Wire Glue" can be used to attach wire to the cell and restore the electrical connection, and when dry (and checked), then the strain relief glue can be added.

[geocacher2]

Fixed it. My breadboard wiring was messed up. With just a quick charge from a couple AA batteries, it is now blinking all night long.

[geocacher2]

I'm waiting on a small solar panel for my project. However, I have a battery charging the circuit, and the led is blinking ok. The capacitor I have is 1.5f at 5v. When I disconnect the battery, I don't get any further blinking of the led. I must have missed something.

I like this project !

[TinkerJim]

It is good to have the blinking part of the circuit working! Check that the capacitor is taking and holding a charge from the battery with your voltmeter. If it takes and holds the battery voltage, and is connected OK, and doesn’t have excessive internal resistance, it should blink the circuit just as the battery does, right?

[Luigi Pizzolito]

Cool!

[agis68]

very cool....how did you make it the circular shape of the PCB?

[TinkerJim]

The PCB that I used was phenolic. It can be cut with a coping saw into rough shape and then filed or sanded to a nice circle. I do not remember exactly how I did that one, but I usually cut stripboard on a bandsaw, and then finish with a disc sander, and take off the burrs with a file.

[batman96]

Couldn't you use an astable multivibrator instead of the 555 timer, I don't know how you would hook up a photodiode to it, but if you wanted it to blink all the time it would be fine, also you could use an Op Amp to give a fade in fade out effect.

[TinkerJim]

Well, the 555 circuit described here is commonly referred to as an "astable multivibrator". Maybe you are wondering if some other chip could be used to make a blinking circuit. Well, note that the requirement for a "perpetual" flasher that will fit into a small paperweight is energy economy because space for a solar cell and storage capacitor is quite limited. While there are some neat circuits for low power flashers, like for example Thomas Scarborough's CD4093BCN based flasher or Dave Johnson's two transistor circuit, the new CSS555 Timer is perfect for this application. Set up for a 1/2% duty cycle, it gives a bright flash while consuming very little power over a cycle. The average operating voltage of the paperweight circuit is around 3V and the drop across the LED is about 1.6V. So the current through the 47 Ohm resistor is about 30mA,so the blink is very bright. But the rest of the cycle time, the micropower CSS555 and timing resistors only take about 6uA. Thus the average current draw is only 156uA. This gives the long operating time (overnight, next day without light, and the next night too) on a fully charged 1F cap.

[jiovine]

I think an LM3909 LED Flasher/ Oscillator may do the trick. They're available on ebay and I know Futurlec used to have them in stock.

[TinkerJim]

The LM3909 is a wonderful chip - I used many of them way back when starting out in electronics (and still have a half dozen or so in the parts box !). Its chief feature is that it can flash an LED from a low voltage source. It uses more power than the CSS555 circuit described in this project. The spec sheet shows an average current consumption of 320 uA at 1.5V (for the minimum power circuit with lower flash brightness) which is about double the 156uA of the CSS555 circuit at 3V. So for a very rough estimate, you would get maybe about 1/2 the run time from the capacitor source. This may be enough for you - should go overnight if the cap is fully charged (of course the current draw rises with voltage). Anyway, it is very easy to set up on a breadboard to try it out !

[batman96]

I'm talking about a two transistor two capacitor astable multibibrator, it is a simple discreet component circuit, that draws very little power between flashes, I think a discrete component circuit would look cooler than a 555, they are commonly used to alternate two LED's but you can hook them up to only flash one. I just don't now how to make the photodiode trigger it. Maybe you could draw up a schematic for a discrete component circuit that does this.

like this  http://circuitalley.phpnet.us/images/astabletr.gif

[yoyology]

I'd like to see a step showing how you attach it inside the dome. Is that a glass magnifier that you've used?

[TinkerJim]

The glass dome in the photographs is a standard item available from suppliers to paperweight makers. You can glue the circuit board to the dome with tiny strands of clear epoxy at the periphery of the circuit board, or simply secure it inside with the large circular adhesive backed felt pad that comes with the paperweight. But there is plenty of scope for creativity in devising paperweights from other materials - eg a hollowed wooden or metal base with a glass or plastic cover, or a large magnifying glass fitted on top.

[bijikenyot]

toys kids ...... i bet only $5 in shop

[Jason Bedard]

Well documented. Be sure to submit this in the Hurricain LASER contest if you haven't already.