I recently purchased a Dusk-to-dawn outlet sensor (https://www.amazon.com/dp/B00002NB2X?ref_=pe_2640190_261118450_302_E_DDE_dt_1), but I'm wanting to convert this thing into a Dawn-to-Dusk device instead! Why you ask? I bought a plant light for our living room that I want to only be on when the sun is up, because the room all of our houseplants in is north facing, so there is very little light in there.

Seems like it should be a fairly easy 'mod'. Tinkering with it, it looks like this thing is using a Photocell Resistor to control the SPDT relay. When resistence is low, the relay 'opens', when it is high, it 'closes'. Someone on another forum said that you may want to replace the photocell resistor with a photocell transistor or photodiode? Does this sound like the right way to go?

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    \$\begingroup\$ Need a schematic please \$\endgroup\$ – Voltage Spike Oct 17 '19 at 20:14
  • \$\begingroup\$ Phototransistors conduct with (IR)light and stop conducting in "dark". \$\endgroup\$ – Aaron Oct 17 '19 at 20:17
  • \$\begingroup\$ Trace the schematic, then the answer should become obvious. \$\endgroup\$ – Bruce Abbott Oct 17 '19 at 20:21
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    \$\begingroup\$ I'm VTC this as a repair question because it has all the drawbacks of one even if you technically don't want to repair it. \$\endgroup\$ – pipe Oct 17 '19 at 20:22
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    \$\begingroup\$ get another dusk to dawn, and use it to control a nitelight that is tightly taped onto the first dusk to dawn. that will invert the on/off control to be dawn to dusk. \$\endgroup\$ – dandavis Oct 17 '19 at 21:37

A photocell transistor (aka optocoupler) or photodiode is not a perfect opposite of a photocell resistor (aka LDR). Going this route would require characterising the LDR and then carefully designing an optocoupler circuit to do the opposite. Sounds tricky.

Instead, I suggest leaving the detection circuit alone. It sounds like it is doing what you want - detecting the two switch points based on ambient light. All you really care is that the switch-off event becomes a switch-on event and vice-versa.

If that really is a SPDT relay, then it already has the function you need. A SPDT has a normally-open (NO) contact and a normally-closed (NC) contact. At switching time the relay will close one contact and open the other. The mains power will be connected to one of the contacts. All you need to do is connect it to the other, so every time the switching event occurs, the opposite switch action takes place.

For example, the current circuit might look like this:


simulate this circuit – Schematic created using CircuitLab

and all you need to do is change it to this:


simulate this circuit

Note the numbers in the schematics are not important, just the change in wiring on the relay.

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  • \$\begingroup\$ This totally makes sense and is exactly what I needed. Thank you!! I need to replace the S7-24-A with a S7-24-B or C and jumper a few cables. Really appreciate the second set of eyes, as well as the detailed explaination. Heath++ \$\endgroup\$ – dobbs Oct 18 '19 at 5:39

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