SparkFun Electronics Forum

My question is:
Is this circuit safe for the Arduino?

Background:
This is my first real Arduino and 555 timer project. I don't want to blow up my Arduino. The 555 will be in bi-stable mode. The idea is to use an LDR to turn on the output of the 555, which turns on a transistor which powers up the Arduino. When the Arduino gets its job done it will set a pin low to turn off the output of the 555, shutting down the Arduino. I want to power the system with a trusty 9v battery, so I am concerned about the voltage at the pin on the Arduino that will turn off the 555 output.

The relevant part of the circuit is here:

555-Pin4
|
9v--10k-+-10K--GND
|
Arduino Pin12

Between the 2 resistors there should be 4.5v and a max of .9mA, discounting the 555, which is what I am concerned about. Will grounding pin 4 of the 555 through the Arduino draw additional current from the 555 and damage it?

What Arduino?

The scheme should work. The reset pin goes (internally) to the base of a PNP transistor and so won't be forcing any voltage onto the pin. I've read that the reset pin is TTL compatible and so any TTL logic 1 voltage should keep the 555 out of reset, any voltage < 0.8v will reset it. Current draw from the pin should be < 0.1 mA. If you wanted to, you could use a 5v Zener in place of the "lower" resistor in the divider.

Any reason for using a 555 vs a comparator (LMx39 series) ? What stops the LDR from immediately retriggering the 555 and thus the Arduino ?

Great! I was hoping that to be the case. I'm using the 555 because I have several of them 'just sitting there' and have never used one. I don't have the LMx39 to play with.

The idea is sort of like when the mailman opens the mailbox once a day to put your mail in, the LDR will turn the system on so the Arduino can do its thing. Then the box gets closed for 24 hours. Sometime after the box gets closed, the Arduino will finish its job and shut things down, to preserve the battery.

That ascii art drawing doesn't make sense. Try to make a simple picture with paint or whatever and upload it in your next reply here in the forum. It looks like Arduino pin 12 is connected to 9 volt. That's gonna hurt it at some point.

Valen wrote:That ascii art drawing doesn't make sense.

No it doesn't but from the verbal description pins 4 and 12 are meant to be tied to the center of the voltage divider.

Or so I thought ...

Yes the plus-junction in the ascii-wires gives it away too. I just wanted to vent my anti-ascii-art rant. It almost always get's messed up in modern forum viewing.

Gandy909: Please use the code tags in your message to show it, it uses a fixed width typesetting:

Valen wrote: I just wanted to vent my anti-ascii-art rant. It almost always get's messed up in modern forum viewing.

Agreed. Seems all the forums now eliminate "unneeded" spaces and screw-up ASCII "art". Given "paint" is included on any system it's not really a loss at depicting things.

That said really complicated ASCII art, that works, is like Nixie tubes ... a blast from the past ... that might be anachronistic but is appreciated by us anachronisms from the past. :dance:

Ack. I see what I did there. Yes, the vertical lines were supposed to connect at the + sign. First post blunder... I'll take notice of the code /code blocks, or use paint next time.

Here is a crappy drawing of what I was trying to accomplish, and so far, it seems to be working, except for the arduino part. I have placed a switch in where the arduino data pin would be for testing.

Won't that floating pin 6 make the flipflop behave unpredictable?

I googled up bi-stable 555 schematics and it seemed like the examples were a lot unconnected as well as a lot grounded...wasn't sure which was best. Advice welcome...