Hello all, I worked up this circuit to test out an idea I had. Using a solar panel to charge up a capacitor which then supplies current to an LED when the voltage of the solar panel becomes low enough to forward bias a PNP transistor. Essentially, when there is low enough ambient light I want the LED on for a given time. I think I'm on the right track and I'd like the opinion of others on this as I am still learning. Tell me what you think, good or bad, thanks!

Also, for now, I'm using an SPDT switch to simulate a charged/not charged solar panel.

https://photos.app.goo.gl/x3GHV95ypmfebJ2C9

 

hi odm.
Posted your image here, will make it easier to access.
E

 

Solar garden lights use a rechargeable battery cell, not a capacitor.
A rechargeable battery voltage stays fairly high during most of a discharge so the LED will stay bright. A capacitor discharging has its voltage dropping quickly at first so the LED will be bright for a few seconds, dim then turn off even when the capacitor still has plenty of charge in it but its voltage has dropped too low to light the LED.
Many solar garden lights use a single AAA rechargeable battery cell and an IC that boosts its voltage to light a 4V LED and an electronic switch for charge/discharge. It lights the LED brightly all night long.
Your capacitor must be HUGE to light the LED for a short time.
Some solar garden lights cost only $1.00 so the solar panel and battery are free.

 

With the component values shown, assuming the solar panel generates a maximum of 10V the LED will be on for less than a minute. What is your 'given time'? What is the peak panel voltage?

 

With the component values shown, assuming the solar panel generates a maximum of 10V the LED will be on for less than a minute. What is your 'given time'? What is the peak panel voltage?

Thanks to both of you for the reply. This was just something I threw together quickly. I don't have any desired run time at the moment but I will look into the rechargeable batteries. My main concern was wether or not my use of the PNP transistor was correct. Thanks for the input!

 

My main concern was wether or not my use of the PNP transistor was correct.

It should work, but the switching point will be rather vague. Usually hysteresis is added to circuits like these, to provide a snap action; otherwise the LED will flicker erratically at dawn and dusk.