TREE

This is a very simple electronic circuit project of light sensitive and differential temperature switch circuit.   In Fig. 1 see a precision light- sensitive switch that activates when the sensed quantities go above or below pre-set values. The LDR can be any cadmium – sulfide unit that has a resistance in the range 500R to 20k at the required trip level. The RV1 adjust LDR at normal light level. In Fig. 2 see a differential temperature switch circuit using ordinary silicon diodes as temperature – sensing elements and responding to differentials of a fraction of degree. RV2 can be used to apply an effective offset of several degrees to the two diodes. To adjust the circuit, apply the required differential temperature to the diodes and then adjust RV2 so that the relay just turns on. The circuit responds to the relative temperatures, rather than the absolute temperatures, of the two diodes.

Circuit diagram of the light-sensitive fire alarm is shown in Fig. 1. It is built around L14F1 photo transistor (T1), BC547 transistor (T2) and NE555 timer (IC1). T1 can sense both infrared and visible light. NE555 based a stable multi vibrator is used for audible warning. Resistors R3 and R4 and capacitor C1 decide the frequency. Here, frequency is around 272Hz, which can be changed by changing values of R3, R4 or C1. When T1 senses light, T2 stops conducting. As a result, reset pin 4 of IC1 is held high. This makes the multivibrator to function and an alarm to sound in loudspeaker LS1. An actual-size, single-side PCB of the light-sensitive fire alarm is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on a PCB, enclose it in a suitable plastic case. Make a small hole in the plastic case for the photo transistor (L14F1) so that fire flame is detected/sensed easily through it. T1 sensor should not face directly towards normal light sources like sunlight,...