Light-Sensitive Trigger

From an idea by R de Haan This circuit activates a relay upon detecting the absence of light on an LDR (light depen- dent resistor). It is particularly well suited to control outside lighting as used for driveways and garage entrances. Contrary to its normal use as an astable or monostable multivi- brator, the Type 555 IC in this circuit functions as a compara- tor. To explain this rather unusual application, it is nec- cessary to note that the oper- ation of a 555 is normally as follows: the output goes high upon receipt of a trigger (start) pulse on input pin 2. This pulse is a voltage whose level is lower than "/3 of the supply voltage. The output goes low again when the voltage at the second input, pin 6, has briefly ex- ceeded 2/3 of the supply level. In the present design, the sec- ond input is not used, but the output of the chip can none the less revert to the low state, since pin 6 is connected direct to the positive supply rail. This set-up is accounted for by the ac- co...