A reliable light sensing circuit?

It's been a few years since I did any significant electronics in my shed/ workshop. But I'm going to have to get stuck in again because I need to redesign my light sensing circuits. I've used LDRs in basic voltage divider circuits (with hysteresis) for the last decade or two for garden and porch lights 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 can adjust the switchover voltage with the preset, but even that soon becomes impossible.

To keep work to a minimum, I'm wondering if there is any photodiode-based circuit that I could simply substitute for the LDR? Failing that some relatively simple changes? Or some other modern but easily accessible component I could use?

Does anyone else have similar problems? If so, have you found any practical solutions?

Any suggestions would be much appreciated please.

Terry, East Grinstead, UK

 

[Jasen Betts]

On 2014-02-01, terrypingm@gmail.com <terrypingm@gmail.com> wrote:

It's been a few years since I did any significant electronics in my shed/ workshop. But I'm going to have to get stuck in again because I need to redesign my light sensing circuits. I've used LDRs in basic voltage divider circuits (with hysteresis) for the last decade or two for garden and porch lights 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 can adjust the switchover voltage with the preset, but even that soon becomes impossible.

To keep work to a minimum, I'm wondering if there is any
photodiode-based circuit that I could simply substitute for the LDR?
Failing that some relatively simple changes? Or some other modern but
easily accessible component I could use?

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.

--
For a good time: install ntp

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[default]

On Sat, 1 Feb 2014 09:22:36 -0800 (PST), terrypingm@gmail.com wrote:

It's been a few years since I did any significant electronics in my shed/ workshop. But I'm going to have to get stuck in again because I need to redesign my light sensing circuits. I've used LDRs in basic voltage divider circuits (with hysteresis) for the last decade or two for garden and porch lights 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 can adjust the switchover voltage with the preset, but even that soon becomes impossible.

To keep work to a minimum, I'm wondering if there is any photodiode-based circuit that I could simply substitute for the LDR? Failing that some relatively simple changes? Or some other modern but easily accessible component I could use?

Does anyone else have similar problems? If so, have you found any practical solutions?

Any suggestions would be much appreciated please.

Terry, East Grinstead, UK

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

 

[George Herold]

On Saturday, February 1, 2014 12:22:36 PM UTC-5, terry...@gmail.com wrote:

It's been a few years since I did any significant electronics in my shed/ workshop. But I'm going to have to get stuck in again because I need to redesign my light sensing circuits. I've used LDRs in basic voltage divider circuits (with hysteresis) for the last decade or two for garden and porch lights 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 can adjust the switchover voltage with the preset, but even that soon becomes impossible.



To keep work to a minimum, I'm wondering if there is any photodiode-based circuit that I could simply substitute for the LDR? Failing that some relatively simple changes? Or some other modern but easily accessible component I could use?



Does anyone else have similar problems? If so, have you found any practical solutions?



Any suggestions would be much appreciated please.



Terry, East Grinstead, UK

Besides the photo-transistor you could just use a photodiode, reversed biased 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 cheap PD on my bench with 100k ohm gives about a volt in the room lights.) I don'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 1 Meg would be 9uA max... a 9V battery would last a while.)

George H.

 

[Bill Bowden]

On Saturday, February 1, 2014 9:22:36 AM UTC-8, terry...@gmail.com wrote:

It's been a few years since I did any significant electronics in my shed/ workshop. But I'm going to have to get stuck in again because I need to redesign my light sensing circuits. I've used LDRs in basic voltage divider circuits (with hysteresis) for the last decade or two for garden and porch lights 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 can adjust the switchover voltage with the preset, but even that soon becomes impossible.



To keep work to a minimum, I'm wondering if there is any photodiode-based circuit that I could simply substitute for the LDR? Failing that some relatively simple changes? Or some other modern but easily accessible component I could use?



Does anyone else have similar problems? If so, have you found any practical solutions?



Any suggestions would be much appreciated please.



Terry, East Grinstead, UK

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