So I have been working on this circuit that basically will activate an LED when the lux levels drop low enough. The time that the LED will remain active for is determined by VR4. I'm having two issues that I want to overcome, however. The first is that I want to make it so that LED "D6" will turn off after the time that I have chosen has passed (say 10 seconds, etc) and not turn back on again until the lux levels have risen above the threshold and then dropped below again. The second is that I want to remove LED "D5" from the circuit without altering functionality, as it is only on when lux levels are high, but the point of the circuit is that no LED is on when the lux levels get to a certain level or after the set time has passed.
For the D5 question, you can simply remove it. It will not have any effect on the balance of the circuit.
The monostable mode of the 555 timer normally requires that the trigger pulse be shorter than the timer length. If not, the 555 stays triggered prolonging the length of the output pulse beyond the programmed time.
A simply fix for this is to change your circuit as follows:
(Image credit to tcrosley on StackExchange 180786)
Remove R9 and R10 in your circuit, add the components shown, and place the emitter and collector of Q2 in place of the SW1 switch shown.
Your timer will then run for the programmed length and not retrigger until your LUX sensor turns off and again turns on the transistor.
If you can't design any LUX specs , then try this circuit which works with SHarp/Vishay/etc Photo Diodes PD ( not photo transistors) With suitable sunlight and aperture You can choose current for LED OFF and set R as PD current sink with 1.1V /250K = 4.4uA for twilight to turn on LED.
The threshold is typically 1/3 and 2/3 of Vdd for hysteresis
- e.g. 1.1V, 2.2V for Vdd = 3.3V
I chose a 74ALVC14 because it operates down to 3V easily and consumes almost no current when LED is OFF and can easily drive 20mA with its internal RdsOn shown in simulation as an added R.
- paste above into browser addressbar for JAVA simulator.
- Pot is just to simulate twilight