Nuts and Volts - February 2016

Standard 555 Micropower Operation

The CSS555 is pin-for-pin compatible with the customary 555 series timers as Figure 1 indicates. Therefore, application circuits have the familiar 555 topology as, for example, in the astable circuit illustrated in Figure 2.

However, the micropower nature of this IC allows using external components with values that demand much less power. For example, the LED blinker in the electronic paperweight in Figure 3 uses the above astable circuit with timing resistors RA = 10M, RB = 47K, and a 0.22 µF timing capacitor. It blinks perpetually on the energy stored in a 1F capacitor charged from a small photovoltaic cell taking in typical desktop lighting.

Another application of the CSS555 in the standard (unprogrammed) mode is the solar engine shown in Figure 4. Here, the circuit is a sort of monostable arrangement with a more or less constant voltage (about 1.4V usually) on the trigger and threshold pins, which is supplied by a photodiode (a suitable LED, actually). The supply voltage to the circuit varies as the storage capacitor Cs charge from a solar cell, or discharge through the load when the output pin goes high.

It goes high when the supply reaches three times the trigger voltage, and goes low when the supply drops to 1.5 times the photodiode voltage.

The original concept for this solar engine circuit was devised by Manfred Schaffran and Wilf Rigter in 2003 when only the 7555 timer was available. The CSS555 greatly improves the efficiency since the circuit now takes under 5 µA during periods when the storage capacitor is charging up. The various available 7555s required anywhere from 50 µA to 180 µA. A CSS555 equipped solar engine has operated perfectly from a tiny solar cell supplying a mere 15 µA.

The little robot “Walker” in Figure 5 is equipped with this circuit, a good size solar cell atop his head, and, presently, a 0.22F capacitor. He periodically waddles around in a little circle (one of his arms/legs is longer than the other) whenever he has gleaned enough energy from

The CSS555 is a micropower programmable version of the 555 family of timer ICs. It operates at a current under 5 μA and a supply voltage from 5.5V down to 1.2V.