A newbie need's pointing in the right direction

[bentbarrel]

Afternoon everyone,
Im trying to build a circuit using LDR's and im struggling at the minute. What im trying to do is build a circuit using two LDR's one for day light and one for night and when these are 'active' then a 12vdc relay is to be operated for approx 5-10 seconds and then wait until the next LDR is active and continue the cycle.
I look forward to your replies.

[pebe]

What do you mean by 'active'? An LDR will change its resitance between daylight and darkness, so why do you need two?

Can you give a bit more detail about how you are going to use them?

[bentbarrel]

Hi, sorry for the long delay, i had to go abroad with work.
What im after is to build a circuit which is activated by an LDR at dawn, upon activation a relay is operated for approx 5-6 seconds, then ideally when the light levels fall again (dusk) the relay is again operated for 5-6 seconds.
My experience with electronic's is growing daily but still isnt great. Thanks

[pebe]

I can probably give you a circuit. Can you tell me the resistance of the LDR, measured at the light level you want the operation to begin?

[bitsandpieces]

place the ldr in series with another resistor running from vcc to gnd and tap the centre.LDR above resistor will increase power to relay as light increases and inversely LDR below resistor will decrease power to relay as light increases,you mat have to experiment a bit to get the result you want but thats what its all about!

[pebe]

Here is a circuit that should do what you want.

The XOR gates are two of a package of four in the CD4070. The first one is used as a level trigger and the second as the pulse generator. Both circuits use the feature of an XOR, whereby if the two inputs are the same, then the output is low but is they are at different levels then the output is high.

The resistance of the LDR at the switchover light level is assumed to be about 800ohms. With R1 set to that resistance, pin 5 will be about half the supply rail. During daylight, the resistance of the LDR will be lower so pin 5 is high. As pins 5 and 6 are at different levels, pin 4 will be high. Pin 1 will be high and C1 is charged so pin 2 will equal pin 1. Pin 3 will be low and the relay will be off.

As it gets darker the voltage on pin 5 will drop, until at about 6volts pin 5 changes from high to low. Both input pins will then be low and pin4 will go low. As it does so, current will flow through R2 pulling pin 5 slightly lower. This to give a Schmitt trigger effect to stop the slowly varying input going high again with noise.

Pin 1 will go low but pin2 will still be high until C1 has discharged through R3 down to about 6V. During that time pin3 will go high turning on the relay. The relay will be on for about 6secs.

When the LDR starts to increase in resistance at dawn, the reverse action will take place and the relay will again be switched on for about 6secs.

C1 should be a low leakage electrolytic or preferably a tantalum. If pin 4 oscillates due to noise at changeover, decrease R2 as necessary. You may need to go down to 22K.

When setting R1 for the changeover level, measure the voltage on pin4 – because relay changeover will have a delay.

I hope that all make sense.