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The 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 unusual application. It is necessary to note that the operation of a 555 is normally as follows: the output goes high upon receipt of a trigger ( start ) pulse on input pin2. This pulse is a voltage whose level is lower than 1/3 of the supply voltage. The output goes low again when the voltage at the second input, pin 6, has briefly exceeded 2/3 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 pin6 is connected directly to the positive supply rail. This set up is accounted for by the accompanying Table taken from the 555's datasheets.
In principle, the supply voltage for the circuit must equal the coil voltage of the relay. Do not apply more than 16V, however, as this may damage the IC 555. The current consumption of the circuit is 4mA, exclusive of the relay, at a supply level of 12V. Components R2 and C1 ensure a delay of about 10 seconds 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 has not regulated the actuation of the relay will lower the supply level somewhat. This lowers the internal threshold of the IC since the trigger point is defined as 2/3 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 fit a resistor between pins 5 & 7 of the 555, as shown in the circuit diagram. The amount of hysteresis is inversely proportional to 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 1MΩ potentiometer or preset. Actually, within the fritzing image, I added a Red LED & 220Ω Resistor to test quickly without giving the AC supply to Load. Once you test the project successfully replace the LED + Resistor with a relay. You can use a mini breadboard or your own designed PCB to fulfill your needs,
| Resistors | Capacitors | Semiconductors & Other |
|---|---|---|
| R1 - 100 kΩ [1 Unit] | C1 - 10 pF [1 Unit] | D1 - 1N 4148 [1 Unit] |
| R2 - 1 kΩ [1 Unit] | IC1 - 555 [1 Unit] | |
| R3 - LDR [1 Unit] | Relay - 12V / 1Co [1 Unit] | |
| R4 - 10 kΩ [1 Unit] | IC Socket (8 pin) [1 Unit] |
| Reset (4) | Trigger Voltage (2) | Threshold Voltage (6) | Output (3) | Discharge Switch |
|---|---|---|---|---|
| Low | Irrelevent | Irrelevent | Low | On |
| High | <1/3 VDD | Irrelevent | High | Off |
| High | >1/3 VDD | >2/3 VDD | Low | On |
