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.