Question

and then turns on solar cell that (10) You want to make a light detector that detects when the Sun has risen a water pump for a hydroponic greenhouse system. The light detector uses outputs just over 2.5 V when it is in sunlight strong enough for th The below circuit using an opamp (with no feedback loop) is used: e water pump to be required. +12V 12V +12V Pump R3 R2 -12V Solar Cell (i) What value does R2 need to be so that 2.5V is fed into the inverting terminal of the opamp What property of the opamp allows you to use voltage division to find what R2 is in (i) above? (ii) iii) Explain, using the characteristics of an opamp, how the opamp outputs +12V when the voltage from the solar cell is just over +2.5 volts and it outputs-12V when the voltage f the solar cell is less than 2.5 volts

Answer 1

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V_a = 2.5 v therefore current is I = 12 - 2.5/1k = 9.5 mA Now IR_2 = 2.5 v R_2 = 2.5 v/9.5 mA = 263 psi for op-amp the input independence is assume ab infinite . thus no- current will be flow between input termination thus current through 1k and R_2 will same we know v_out = A_0 (v_1 - v) A_0 open loop gain v_+ potential at non- inverting terminal v_- potential of inverting terminal Now for op amp A_0 rightarrow infinity thub if v_+ > v- then V_out will goes to + infinity but maximum operating voltage is plusminus 12v. thus output will at 12v and similarly for v_- > V + output will be - 12v thus when voltage from solar cell is just over + 2.5 v then from fig it's output will be - 12v similarly the other case.