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This is a simple circuit which dims the LED according to the light recieved by the LDR or simply a light switch.

light swithc

Can anybody explain how two BJTs opearete in the circuit, or simply how the system works?

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Assuming everything is biased reasonably, ...

More current through LDR1 produces more current into the base of Q1.

That produces more current into the collector of Q1, which would normally mean more current through R1.

But, the voltage at the collector of Q1 is effectively pinned by the LED D2. So what actually happens is that more current into the Q1 collector has to come from diverting current that would otherwise have gone into D2, and the voltage there only changes very slightly.

Since there's less current flowing through D2, the light output is dimmed.

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  • \$\begingroup\$ So, why there is a relay, and a diode? Diode is reverse biased to the collector. Does the current goes through the coil? \$\endgroup\$ – Orkun Dec 29 '15 at 3:51
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    \$\begingroup\$ The relay is to control whatever load is attached to it --- that isn't shown in the schematic. Q2 controls the relay. D1 is a freewheeling diode to protect Q2 from inductive kickback in case the current through the relay coil is interrupted (for example by breaking the connection to the 5 V supply). \$\endgroup\$ – The Photon Dec 29 '15 at 5:05

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