- intervalometer based on 555 timer
arduino-based home climate sensor network with web server (not from kit)
The LED light strips:
The LED light strip comes in two 16" pieces and is powered by a single 12v AC-DC adapter rated to 800mA. I measured 330mA for the combined strips and verified about 165mA with just one LED strip in operation.
The PIR motion sensors:
The PIR motion sensors (pictured here) are usually powered by 3xAAA and drive a package of 3 LEDs. When I removed the battery assembly and replaced it with an Arduino's 5v, I measured 63mA when the original 3xLED package was lit, and also saw a 1.7v drop across the LED pack's terminals. It also has a daylight sensor that shuts down the unit when there is too much ambient light.
The combined circuit:
This is the schematic I have so far, with just the main components. The general idea is that either of the PIR motion sensors can send a 5v signal to an OR logic gate, which will tickle two transistors that will allow 12v and 170mA into each LED strip. I haven't worked out all the currents to see if I need some additional components like resistors and diodes for protection, so the circuit may look a little bare right now. Before I get into that I'd like to confirm that I'm generally heading in the right direction.
The first big problem I have is how to reconcile the 5v circuit with the 12v circuit. I want to get rid of batteries, so I'll just have the 12v wall wart supplying the whole circuit. My first thought is to use a 7805 voltage regulator even though I have to drop 7v, because there won't be any significant load (2x PIR boards, 1x 74HC32, 2x 2N3904, and no LEDs) and it will be on a short duration timer.
The questions:
- Is the 7805 the right approach or is there a more typical/efficient way to get to 5v logic levels from a 12v source?
Do I need protection diodes on the ground side of the 5v stuff, because I have 5v and 12v potential meeting at a common ground?
If the PIR sensors provide a 5v output when triggered, is that all I need to care about as input to the OR gate, or do I need to worry about stuff response time, current, etc?
Can the single output from my 74HC32 activate both transistors? I'm not really sure how to work this out, I just know that the combined light strips in series (330mA) would overwhelm a single 2N3904 so I thought I should have two transistors and run the separated LED strips in parallel. I got a little overwhelmed trying to find a way to choose the right transistor for my application and instead worked backwards based on the few I types I had on hand. all I had were 2N3904, 3906 and some 2222s that I forgot I had. I was a little impatient and have a RadioShack across the street so I picked up a TIP31 power transistor, good for 3A (see datasheet: http://www.kynix.com/uploadfiles/pdf8827/TIP31A.pdf). I think I'll stock up on some logic level MOSFETs so I have those on hand in the future.