Grow light
 

Jerry Komor     

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Heart of the circuit is LM383 a low power low offset comparator and some passive parts. A non-inverting pin is fed from a divider comprised of LDR tied to ground and a 10Kohm pull up resistor. An inverting input is fed from a 10Kohm trim pot. This trim pot sets reference or darkness level that the output will go high when the LDR resistance go high when it gets past the dark threshold. Output of the comparator is then fed into the gate of the N-channel FET transistor through 10Kohm resistor. However as the 393 is open-collector a pull up resistor is required from positive rail to gate in order to turn it on. Since the FET transistors operate more in voltage state rather then needing current to drive them a large 1Mohm resistor is used for the pull up. I'm using ILR540 FET transistor with works with 0-5 logic levels however any other IRF series transistors like the IRF540 or IRF740 should work as they gate needs less then 10v to drive them into saturation.
Once the FET transistor is turned on it switches negative rail to a constant current driver and in turn drives the grow LED.


 

   

Bill of materials
1  LM393 low power dual comparator
2  10Kohm resistors
1  1Mohm resistor
1  LDR  Light Dependant Resistor
1  10Kohm trim potensiometer
1  1N4001 diode  as reverse polarity protection for the comparator (optional)
1  IRL540  FET transistor
1  10w  constant current 12v LED driver
1  10w  full spectrum grow LED  with heat sink
1  power jack  (optional)
1  5x7cm prototyping perf board