Instructables

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.
Ploopy4 months ago

Cool!

agis682 years ago
very cool....how did you make it the circular shape of the PCB?
TinkerJim (author)  agis682 years ago
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.
batman962 years ago
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 (author)  batman962 years ago
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.
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 (author)  jiovine2 years ago
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 !
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
yoyology2 years ago
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 (author)  yoyology2 years ago
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.
bijikenyot2 years ago
toys kids ...... i bet only $5 in shop
Well documented. Be sure to submit this in the Hurricain LASER contest if you haven't already.
Awesome idea!