Collection Schematic

This circuit activates a relay upon detecting the absence of light on an LDR (light dependent 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 multivibrator, the Type 555 IC in this circuit functions as a comparator. To explain this rather I unusual application, it is necessarily to note that the operation of a 555 is normally as ( follows: the output goes high 1 upon receipt of a trigger (start) pulse on input pin 2. This pulse is a voltage whose level is lower than of the supply voltage. The output goes low again when the voltage at the second input, pin 6, has briefly exceeded of the supply level.

In the present design, the second 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 setup is accounted for by the accompanying Table, taken from the 555’s data sheets. In principle, the supply voltage for the circuit must equal the coil voltage of the relay. Do not / t apply more than 16 M however as this may damage the 555. The current consumption of the circuit is 4mA, exclusive of the relay at a supply level of 12 V Components R2 and C1 ensure a delay of about l0s before the relay is energized, so that the circuit is rendered insensitive to rapid changes in the light intensity Basically the circuit has no hysteresis effect. However, when the supply is not regulated, the actuation of the relay y will lower the supply level somewhat. This lowers the internal threshold of the IC, since the trigger point is defined as of the supply level (pin 2).

Therefore, the hysteresis of the circuit can be dimensioned as required by fitting a resistor in series with the supply. It is also possible to tit a resistor between pins 5 and 7 of { the 555, as shown in the circuit diagram. The amount of hysteresis is inversely proportional to l the value of the resistor, and 100K is a reasonable starting point for experiments. The sensitivity of the trigger circuit can be controlled if R1 is replaced with a 1M potentiometer or preset.