We need to run a circuit from a 24v DC battery via an inverter through to a light source. We want to fit a light sensor in so the light comes on when dark. We know it needs to be DC output, but what kind of light sensor do we need and does anyone know where we can get one?

 

Do you want to turn on the inverter when it gets dark?
Then you need a light sensor like a light-dependent-resistor or a photo-transistor, an amplifying transistor or opamp and a high-current switch like a Mosfet.

Then you need somebody to analyse how much current it will be switching and to design the circuit.

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Uncle $crooge

 

Well, for the light detection side you can you a simple LDR, Photodiode or Phototransistor.

See diagram below for the arrangement you'll need. My circuit gives a high output when it's dark, and a low output when it's light. If you need it the other way around, then swap the positions of R1 and LDR1. You should follow the circuit with a comparator. The comparator will then output high when the output from the potential divider exceeds the reference voltage on the inverting pin of the comparator. If you want that the other way around, then put the reference voltage on the non-inverting pin instead. Adjusting the reference voltage will change the point at which the comparator says it's dark (or light, depending on your arrangement).

Alternatively, you can get away without a comparator if you want - use a variable resistor in place of R1 to set the sensitivity of the circuit. A comparator is advantageous because it gives a definitive switch between the light and dark conditions which is better for controlling the rest of your circuit. You'll want to add some hysteresis though, otherwise you'll get rapid switching between states when the condition is on the border between light / dark




Brian

Attached Images

LDR.gif (6.0 KB, 26 views)