transistor treashold switch

Im realy new to electronics. and transistors even more so.
what i was wondering. in the attached, would it work?
i couldnt find anything to resemble a LDR in eagle cad, but...
all the resistor values i havent decided on yet, but
R5 is my LDR, when that reaches a point i want T1 to turn on and in turn, activate T2.
problem is it has to be a very clean switch.
at the moment its activating a relay, but its going to activate a IC input, so i cant build up to the fully on status, (like slowly amplify the it)
anyhelp would be greatly appericated.
any comments also, as i said im new to electronics.
Cheers

I doubt your circuit will work. For starters, the collector of T1 is at a lower potential than its emitter.

If you want a clean signal, I'd suggest using a comparator such as the venerable LM393 and LM339 devices. Have a look at the "limit comparator" example in the datasheets.

If the problem is to just do this with transistors, have a look at this circuit: **broken link removed**

thanks for the reply, i've had a play with the LM339 in the past. so i've sort of looking at this way, what im looking at doing is, when activated the this is going to go to a input pin on another IC.
Jaycar's open tomorrow, so i'll go spend a bit of cash.

Gr3mlin said:

problem is it has to be a very clean switch.

Click to expand...

You need a schmitt trigger; you can do that with two transistors that share an emitter resistor.

To find the LDR trigger point, use the geometric average of the light resistance and the dark resistance.
E.g., for 10k to 500k, [(10x500)^0.5] = ~70k