[SOLVED] help with light/dimmer circuit

[wanie84]

i'm trying to combine these two circuits
a. light/dark sensor
b. dimmer

but having problem to figure out how to control the dimmer using ldr in the 1st circuit
and also i cant figure out why my relay doesnt work with 12v source.
can someone help me..
sorry my basic knowledge in electrical is very low.

 

[betwixt]

The circuits work in different ways and cannot be simply combined.

Circuit 1 controls the LED brightnes by altering the amount of time the LED is on to the time it is off within a fixed time interval. The interval is so short that the eye can't see the LEDs flickering. Making the on time longer and off time shorter makes the LED appear brighter and vice versa.

Circuit 2 uses a comparator to control the relay, it works by comparing the voltage you set at the wiper of P1 with a voltage created by the LDR and R1. As the illumination varies, the LDR changes resistance and the voltage rises or falls. So P1 just sets the trip point at which the relay turns on. The other components ensure it can't operate too quickly and make the relay 'chatter' if P1 is set very close to the LDR voltage. A better method is to use hysteresis but this circuit doesn't use it.

So one is a rapidly pulsing circuit, the other is a slow voltage comparing circuit, quite different to each other.

If you want to try to use the LDR, this is where to start: Use circuit 1 with the 555 timer. Remove P1 completely and replace it with a fixed 10K resistor from C1 to D2, then connect the LDR between C1 and D1. So effectively you are simulating P1 with the LDR and a fixed resistor. If the brightness changes the wrong way, it dims when it should be getting brighter, swap the 10K resistor and LDR over.

Brian.