Why is my outside dusc to dawn light buzzing

You will need an LED Bulb that states 'Dimmable'. They have different circuitry and a little more expensive.

https://www.google.com/search?q=dimmable+led+spotlights

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It has light censer unit that switches it on and off.

Most likely the transformer in the light. It could also be the photo cell controlling the on and off switch. Good luck

It sounds like you have a dawn to dusk photo cell instead of a dusk to dawn. Would need to be replaced.

The dawn to dusk feature is activated by photoelectric eye that senses when it gets dark.
If you put timer in a dark room, it will turn ON ... let's say to operate Christmas tree after you leave a room for the evening.
But if room stays dark, the timer will not sense when to turn off unless you also set timer to turn off 1-9 hours after ON.
That being said, a Christmas tree might make too much light for the timer's photoelectric eye.
Make sure photoelectric eye is turned away from direct light.

Here is how it works. The push button repeats itself in following sequence (must plug in).
F 1 2 3 4 5 6 7 8 9 A d 1. 2. 3. 4. 5. 6. 7. 8. 9.

F: timer is off.
1 2 3 4 5 6 7 8 9: from dusk to dawn count down number of hours repeated every day. That's most useful to control outdoor lightings.
A: from dusk to dawn count down number of hours which is randomly selected by this timer. Which is usually for indoor use to turn lights on and off for number of hours. The randomly selected hours is a security feature so that lights on (on from dust, but off randomly) time shows no pattern to outsiders.
d: just on from dusk and off at dawn. It is also used to control outside lights but just a little wasteful in my opinion.
1. 2. 3. 4. 5. 6. 7. 8. 9.: meaning count down number of hours starts from the time when the button is pushed so that you can have a countdown timer at any time.

Here is how it works. The push button repeats itself in following sequence (must plug in). F 1 2 3 4 5 6 7 8 9 A d 1. 2. 3. 4. 5. 6. 7. 8. 9.
F: timer is off. 1 2 3 4 5 6 7 8 9: from dusk to dawn count down number of hours repeated every day. That's most useful to control outdoor lightings. A: from dusk to dawn count down number of hours which is randomly selected by this timer. Which is usually for indoor use to turn lights on and off for number of hours. The randomly selected hours is a security feature so that lights on (I believe the on time is within the range of 1 hour from dusk set by the timer) and off (it is randomly set, you can see the display blinking showing the selected countdown hours) shows no pattern to outsiders. d: just on from dusk and off at dawn. It is also used to control outside lights but just a little wasteful in my opinion. 1. 2. 3. 4. 5. 6. 7. 8. 9.: meaning count down number of hours starts from the time when the button is pushed so that you can have a countdown timer at any time.

Here is how it works. The push button repeats itself in following sequence (must plug in).
F 1 2 3 4 5 6 7 8 9 A d 1. 2. 3. 4. 5. 6. 7. 8. 9.

F: timer is off.
1 2 3 4 5 6 7 8 9: from dusk to dawn count down number of hours repeated every day. That's most useful to control outdoor lightings.
A: from dusk to dawn count down number of hours which is randomly selected by this timer. Which is usually for indoor use to turn lights on and off for number of hours. The randomly selected hours is a security feature so that lights on (on from dust, but off randomly) time shows no pattern to outsiders.
d: just on from dusk and off at dawn. It is also used to control outside lights but just a little wasteful in my opinion.
1. 2. 3. 4. 5. 6. 7. 8. 9.: meaning count down number of hours starts from the time when the button is pushed so that you can have a countdown timer at any time.