which of these two automatic light circuits more efficient ?!

Thread Starter


Joined Apr 3, 2018
Hi everyone,

i want to control the outdoor lights ,i found the following two circuits :

Circuit 1:

Circuit 2:


which circuit more efficient ?

i built a circuit using transistor and it has a problem at a certain light ,the relay state fluctuating !!


regards :)

Vytas Klyvis

Joined Dec 5, 2016
The problem with your transistor circuit is that it has no hysterisis. Meaning that if the voltage swings around the transistor turn on voltage, the relay will oscillate.

Circuit 1 has no hysterisis but it only periodically checks the voltage on the trigger pin depending on the value of r1 and c2. So the fluctuating will be less if any.

Circuit 2 has hysterisis, so when it turns on, it takes a lower voltage then it did to turn on to turn off again. Stopping the fluctuations.

Now as for the efficiency. The most can be gained by selecting a low power relay (ssr would be best I think) as this would probably be your biggest power consumer.
Don;t even bother with photo-detectors. Use an astronomic timer. These timers can, for instance turn on 1/2 hr after dawn and off at 11:30 and compensate for DST and have battery backup.

I use this for a hall light at home and it works super. It might have a glitch when DST changes for a day or so.

I believe PIR combined with photo-detection can also work. I had that set up at home but it annoyed mom. The use was in a living room to provide a "night light" after the autonomic timer expired. My mom used a walker and would "get caught" in the dining room with it being dark to get out.

I just put a night light on the wireless reciever, but it still bugged mom. It used electricity. (I know 7W know - less for 20 seconds a day). Old school. An autonomic timer would have worked to keep the light off. It would use power you cuuld not see.

The system was designed for the driveway as an audible annunciator or announce with a light, but not in a dark living room. The motion detector portion is battery operated.

I thought it worked really well. If you were sitting and watching TV, there would basically be no motion. The light would be on till 11:30. She needed help when she would be using the dining room table and the hall light went out.

Changed to astronomic type of timer. Uses lat and log to compute sunrise/sunset,
Last edited:


Joined Mar 14, 2008
A better, but more complex circuit, is to have the photodetector turn on the light with a timer to turn it off after the desired number of hours (unless you want to have the light on all night).
That way you don't have to adjust the time for the change in sunset time.


Joined Apr 5, 2008

What is the use of the light?
An other option to save power is a motion detector with a timer, that is only active during the sundown.

the idea is to use the astronomic timer to limit the possible hours of operation.

PIR motion detection seems to work the best. Forget ultrasonic.

It's usually the amount of light and a PIR motion detector.