A reliable light sensing circuit?

a phototransistor will probably work better as an LDR substitute (in DC circuits) the sensitivity of 3-lead phototransistors can be reduced, if needed, by adding a resistor from base to emitter.

I've been using photo transistors (because they are small, cheap, and don't fail in damp environments). A single mosfet, 10 meg resistor, and photo transistor makes a dawn to dusk switch for low voltage DC lights.

Thanks both, I'll experiment along those lines.

Terry, East Grinstead, UK

workshop. But I'm going to have to get stuck in again because I need to re design my light sensing circuits. I've used LDRs in basic voltage divider c ircuits (with hysteresis) for the last decade or two for garden and porch l ights etc. But they need attention with annoying frequency, because of LDR deterioration. The ratio of R(dark)/R(light) drops too low. For a while I c an adjust the switchover voltage with the preset, but even that soon become s impossible.

circuit that I could simply substitute for the LDR? Failing that some rela tively simple changes? Or some other modern but easily accessible component I could use?

al solutions?

Besides the photo-transistor you could just use a photodiode, reversed bias ed to maybe 10 volts and a current sense resistor. The resistor size will depend on the photodiode area and light level. (I've got a ~5mm X 8mm chea p PD on my bench with 100k ohm gives about a volt in the room lights.) I d on't know what your power supply is, but if it's a battery you may want to have a smaller PD and a bigger resistor (to limit current flow.) (9V and

George H.

workshop. But I'm going to have to get stuck in again because I need to re design my light sensing circuits. I've used LDRs in basic voltage divider c ircuits (with hysteresis) for the last decade or two for garden and porch l ights etc. But they need attention with annoying frequency, because of LDR deterioration. The ratio of R(dark)/R(light) drops too low. For a while I c an adjust the switchover voltage with the preset, but even that soon become s impossible.

circuit that I could simply substitute for the LDR? Failing that some rela tively simple changes? Or some other modern but easily accessible component I could use?

al solutions?

I've found LDRs deteriorate if any moisture gets into the package. Last one I replaced I encased in epoxy and haven't had any more problems.

-Bill

Thanks both.

I'm going to try Bill's epoxy approach in just one more replacement. If that still fails I'll redesign for photo-transistor or photodiode.

Terry, East Grinstead, UK