Help Designing a Light Activated Electronic Circuit

Try this website: "Phidgets

They have a wide assortment of these types of things, particularly for robotics. Maybe something there will work, but you may need to compromise on some of your requirements.

If this type of stuff interests you, nothing beats getting an education in basic electronics. You can teach yourself if you hunt around on the internet or you can even grab a course or two at your local community college.

Also, join some forums in the topics of your interests.

Seeming to have a hard $30 limit and declaring it to be a home project seems a little odd.

I would suggest looking thru Arduino and Raspberry Pi projects on the internet. I suspect that you may need to copy part of one project and part of another project to get what you need.

Having a $30 hard limit, having already consumed part of it on servo motors and considering using a $35 Raspberry Pi board seems like a bit of a mathematical/financial problem.

You can learn a lot from posted Raspberry Pi projects but the Pi is probably not a good choice for you. In order to avoid corrupting the file system (and OS) the Pi needs a clean shut down (typically with a halt command). There are projects on the internet to build circuits and pull a Pi pin down to safely shut down the Pi but that is adding even more complexity and cost to your project. As far as I know you can drop power on an Arduino all day long and never be at risk of corrupting the system. Thus, of the two boards you listed the Arduino is probably your better choice.

There are many other small boards available. In the past I have purchased several different items from http://www.microcontrollershop.com. Many other sources exist with Digikey and Mouser being common parts sources with some boards. Most if not all will require a little more effort on your part but a board with a microcontroller and a field of holes & pads for prototyping might hit your price point and allow you the mechanical advantage of having everything on one board.

Personally, I think a microcontroller is overkill and unnecessarily complex. I'm running out of time as my wife wants me home (now) but the logic you describe could easily be implemented in good old 4000 series CMOS. You would just need to figure out the circuits for the light sensor and the moisture sensor. We do a leak detector circuit using just one comparator, a few resistors, and two strips of foil.

Horowitz's Art of Electronics might be an excellent place to start if you can't find a washing machine leak alarm circuit on some BB somewhere. Same with a photocell sensor circuit.

I'll check in again tomorrow.

Good luck with your quest.

Here is a circuit that detects the change in resistance across two electrodes due to wetness. When the resistance across the two inputs decreases to about 5Kohm or lower, the comparator will switch. The one meg in the feedback loop gives you the hysteresis. This is designed for leak detection in pressure vessels in sea water but when we test this circuit, a drop of ordinary tap water will trigger the circuit. Changing the 22.1K resistor will 'tune' the circuit for different trigger thresholds. You can save power by increasing the two 10K resistors but you can't go too high or the input bias currents will mess with your circuit. This is where bench experimentation will show you how much you can tweak this circuit. The 4.99K and 1K resistors on the output were just for current limiting and loading to prevent EMI from causing false positives. Given that you are power constrained, I would recommend a 100K output resistor or even 1M as input to a CMOS gate. That won't drain much power but will keep the gate from floating away.

Any old op amp will do but since you are battery operated, I would look for one with low power characteristics. The supply voltage doesn't matter so long as your op amp will work with 9V.

Good luck with your quest (and have fun).

http://www.electronics-tutorials.ws/io/io_4.html

If you modify the light level sensing circuit in the above link, all sensing you want can be achieved.

Gajanan Phadte

Light sensors; try searching "ambient light sensor" Osram make several with prices typically UK £ .08 to 1.2 each. Feed the output of the sensor into one input of a comparator chip. Feed a variable resistor into the other input. By varying the value of the resistor you can adjust the trigger point of the comparator.

Rain sensors; I made a few of these for automatically closing conservatory roof windows when it rained. Make a one sided PCB with a single narrow track that is as long as possible given the size of the PCB. I made mine in the form of a double spiral loop like an electric cooker hob element about 3" diameter. (They were visible to the public so they needed to look aesthetic to passers by). Plate the Track in gold. Fix a heater to the rear of the PCB with epoxy glue (I used four 3W wire wound porcelain enclosed resistors as the heater) Mount the PCB so that rain can fall on the surface, with individual droplets shorting parts of the track so the the resistance changes. Use a second comparator chip to compare the PCB resistance against a variable preset and trigger when it rains. Use the output of the comparator to switch on the heater so the the droplets of water are evaporated from the tracks. While it continues to rain new droplets will replace those that are evaporated. Then the rain stops the comparator will reset once all the residual droplets have evaporated.