need help with light/dark sensor

i want to make a dark sensitive sensor to light up 10 5mm blue leds--my supply is 31 volts so i need to know what resistors i need and what kind of relay

another question- i know it probaly can't be done is can i run 10 leds at 3.0 voltage drop-28millamps throught a 12v light/dark circut

Plenty of light detector circuit using a comparator such as the LM311 but replace the single LED with two groups of five connected in parallel.

You'll need to either reduce the current to 25mA (the LM311 can only handle 50mA) or use a booster transistor.

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The above circuits can be improved by adding a 470k resistor from the output to the + input.