Hi guys,

I'm making a garden solar light, which contains the usual basic components - solar panel, rechargeable battery, bright LED, control circuit, etc.

However, my design is different from common solar lights... once darkness falls I want the LED to switch on for a specific amount of time and then switch off (unlike typical solar lights that keep the LED powered until the battery runs down).

For the timer I will use a photoresistor to trigger a monostable circuit whose output will switch on the LED. The monostable will likely be a low power 555 timer, and the photoresistor circuit will use a low power op-amp to give a distinct output when the appropriate darkness level is reached.

My query is regarding how the system should function, rather than about the circuit's operation. So, I am concerned that the timer could be incorrectly triggered: e.g. a leaf or a bird covers the photoresistor for a few moments during daylight hours - this would cause the LED to incorrectly switch on during daylight hours. A possible solution to this is to pause the timer circuit while the photoresistor indicates daylight - this would allow the timer to be triggered at any time (day or night), but the timer will only actively count down while the photoresistor is in darkness. However, this creates a new problem - if the timer duration is longer than the number of hours of darkness: e.g. the timer is set for the LED to be on for 8 hours, but there are only 6 hours of darkness. In this scenario, the light would be on for 6 hours on the first night, the timer would then be paused for a day, and the following night the LED would only be on for 2 hours as the timer counts down its remaining time (total 8 hours).

So, my question is this, how do I protect against false triggering of the timer during the day (when something temporarily covers the photoresistor)?

Many thanks!

Andy_C_

 

As you said, make the timer so it only counts when it stays dark.
When it becomes light, reset the counter.

 

Use two photo-resistors.

 

Maybe using 2 photoresistors apart / in different orientation can protect against false triggering.

Edit too late ;-)

 

A leaf or bird should not trigger an LDR. Ambient light has to go very dark for the LDR to become a high resistance, of course depending on the model of the LDR. You should include a variable setting in your circuit in order to select the dark threshold of the trigger.

Once daylight appears, reset the timer.

 

Use a photo-transistor on pin 4 of the 555 timer, to turn it off in daytime.

 

Hi guys, thanks for your replies...

I guess I'm trying to envisage scenarios that could disrupt the correct functioning of the system, and then design the system to handle those disruptions.

So the problem lies in the fact that there isn't always an inviolable transition from day to night, and from night to day.

I think that a combination of your suggestions would work best: two LDRs placed to face in different directions, to appropriately guage the ambient light level. And, reset the timer if daylight comes before the end of the timer period.

Thanks again for your input, much appreciated!

Andy_C_