Light Sensitive and Differential Temperature Switch

n 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. 
Fig. 2 Differential Temperature Switch Circuit Diagram:

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. 
Part List :
R1=LDR *see text 
R2-3=10Kohm 
R4=2.2Kohm 
R5-6-7-10=4.7Kohm 
R8-9=2.7Kohm 
RV1=22Kohm  pot. 
RV2=1Kohm  pot. 
D1-4=1N4001 
D3-4=1N4148 
Q1-2=BC214L 
IC1-2=LM741 
RL1-2=RELAY 12V >120 ohm