There are a wide range of applications for light sensitive switches: lighting, entrance door, automatic staircase, automatic opening of doors to the action of a ray of light, alarm systems, etc… Many of us are familiar with photosensitive switches made with a single transistor that is controlled by a photoresistor placed between the base and the recharger, depending on what is desired: normal functioning “closed” or normal “open” the switch.
This configuration allows easy installation more complex, involving the use of operational amplifiers, especially those of type 741, which are very cheap. Another way to optodetect, less known, uses a bridge assembly, which operates on the principle that when the current is zero diagonal axle, means that the bridge is balanced.
This last principle is used in the sensitive switch installation you will be presented next. Photocell is placed in a bridge circuit and a comparator is used as detector bridge balanced. The comparator’s output control thyristor through a transistor. In this circuit, protective measures must be taken because it is isolated from the network.
Power supply circuit is taken from the bridge rectifier D1…D4 is filtered and stabilized by R1, C1, and D5. The bridge circuit is difficult to identify in the given diagram, but it is made of R2 ….. R4, P1 and photoresist (LDR). IC1 is used in the comparator configuration and power level will be about 1.8 V when potential entry vice versa (negative) exceed that of the entry non vice versa. Resistance R5 conducts a hysteresis of about 1V to prevent thyristor to oscillate at T1 and threshold lighting.
Switching point of the light sensitive switch may be adjusted from P1. With the potentiometer set the minimum (minimal resistance) lamp will light a crepuscular light. If you want more flexibility, replace P1 with one that has a value of 1 MΩ. If desired operation reverse position in the scheme of the photoresistor can be changed with the group P1/R4. LA1 lamp will be extinguished, in this case, after dark.
Some practical advice: if you want to use higher power lamps then diodes D1 …. D4 need to be replaced with other type 1N5404 and thyristor TH1 will be equipped with a heat sink. With these changes, the circuit can control currents up to 3A.
Maximum gate for TH1 is 250 µA, which means that you must use a very sensitive thyristor. It can use any type of photoresistor. Remember precautions to be taken due to lack of isolation mounting over the network. It is important that the entire circuit to be protected in a plastic box, of any shape, provided with a hole in the upper side, in order to allow to the photoresistance “to see”. Assure that the entry cable and the exit cable are fixed. This precautions will assure you from accidents.