Ive made up a laser security sytem that uses a pnp and npn transistor. Everything is going well except for the LDR. When there is no light the led will stay off. And once there is light it will turn on. How do i reverse it? here is my diagram. Please help.
Looks like i found a solution for your question,,,
If you used the LDR in the feedback resistor chain of an opamp, then that ought to work as you require...
You'd need to pay close attention fixed resistance(10K) though...probably a bit of experimentation would be in order depending upon LDR behaviour,
You can also exchange LDR place with that 10k resistance,
Solution 2(WITH SINGLE BJT!):
The concept here is simple you connect 2 series resistance (1Pot to experiment) such the voltage drop is sufficient to turn on the transistor form the source at the same time allowing most of the current to flow through LED ,voltage at node 1 should be barely enough to turn on the transistor.
When LDR is on as you might be knowing it offers low resistance and current chooses path of least resistance hence transistor will be OFF so will be the LED,,
You might want to consider to add a base resistance but is optional i think so.
hope this helps
I am sorry i could'nt simulate it.. Consider pototiometer is the photoresistor and pnp transistor and pull down resistor that keeps the base low at night..led on.. In the day the photoresistor will have low resistance so the base will be high the led off.. Please simulated first to know if and what resistors you wil need with the led .. and if you need resistor in serie with photoresistor..