I am working on a little light sensor, it is a very simple project to detect when a light comes on in a rack cabinet at work.
(This way I can hear it from the bottom of the stairs and don't have to run all the way up just to check one dang light all the time).
I have designed it out of parts I have knocking about.
I am not sure what value the LDR is though, it is unlabelled, but I just used trial and error to find a working voltage divider combination.
The LDR measures over 200MΩ in the dark and 0Ω in the light...
The breadboard version is working great so far on a 9V battery!
It uses 14mA when idle and 39mA when the buzzer and LED come on! (practically nothing).
Next is to put it in a case and make a power supply.
I have an old faulty mains battery charger which I would like to use as a project case. It is a wall-wart type device that has been sitting in my box of broken junk for ages.
It already has 3 see-through plastic windows in the front of the casing which I can use for the LDR and 2 indicator LEDs and the hole from the original wire can be used to let the sound out from the buzzer.
I think this would be a great casing as once it is assembled it will be impossible to actually touch any of the components unless you unplug it and unscrew the casing! so pretty safe (safer than some of the LED light bulbs I have seen).
So I have settled on this as the casing as a winner!
Next I need to supply the circuit with 9V. I would like to use a capacitor dropper supply for this for cheapness and simplicity.
So I re-used my design from my LED light bulb project, however this is still experimental and there are a few bits I would like people's opinions on:
C1 (Class X2 dropper cap) - I need to pick a value but when I calculated this the same as I did for my LED project I ended up having to use the next value up. is 470nF enough?
C2 - is 47µF large enough here to smooth the ripple?
ZD1 - Is a 9.1V 0.5W zener going to be sufficient here? (1N4739A is the one I have)
R3 - I thought this would be good as a load resistor? Is this even required?