accelerometercapacitorswitches
I'm planning out a very simple circuit using an LED, capacitor, switch and a battery.
The Idea is that the LED is off until something "bumps into" the circuit. At this point it should light up and slowly dim out.
I could use a microcontroller, but it would be nicer to be able to solve it with just a large capacitor and something that can trigger it.
Is there such a thing as a switch that triggers (like a reed?) on impact?
I'm grateful for any other ideas or pointers on the circuit!
Most modern MCUs like the Atmel AVR (used in Arduino) have the ability to sleep in a very low power state (< 1 uA) and wake up from a variety of sources including low-power timer driven by 32 kHz crystal or pin change interrupts. Using a pin change interrupt or a low level triggered interrupt can wake up the AVR microcontroller from deep power down. Then the state can be saved in a program variable. See http://www.atmel.com/avr
An alternative hardware-only approach to a latching switch is the RS latch built from two 2 input NOR or NAND gates with output fed back to one of the 2 inputs. Whenever the output goes high, the feedback holds it in that state until the other input is pulled high to reset the latch.
Atmelfaebrian
You are making this way too complicated. You don't need a opamp and a logic gate to perform your very simple logic. The on/off switch can simply be wired in series with the supply and act like a normal on/off switch. The inversion you want from the momentary button can be accomplished by a single transistor:
Q1 acts like a low side switch, with R2 providing it's base current and thereby keeping it on. When SW1 is pressed, it shorts the base to ground, which turns off Q1. The LED and buzzer are wired in parallel, not in series as you had them. It is unlikely both need the same amount of current. The LED will have about 10mA thru it when on, which lights a ordinary LED bright enough for most purposes.
Nearly the full 12V will be applied to the buzzer. You didn't say what voltage the buzzer was rated at, so I picked 12V just for example. That's a common voltage for buzzers, but they are certainly available in other voltages too. Some buzzers can appear inductive from the outside circuit point of view, so D2 protects the circuit from the inductive kickback when the buzzer is turned off. Buzzers can also make a lot of high frequency electrical noise. C1 is there to shunt most of that locally to reduce the radio interference and other bad effects from this.
Best Answer
There is a form of accelerometer that is commonly seen on kids toys, is super cheap, and works reasonably well when calibration isn't critical. It's made from a spring and a metal post...
Imagine this: start with a simple spring like what you'd find in a ball-point pen. Inside the spring is a simple metal post (or a piece of solid-core wire without insulation). The post and spring are mounted together on one end and the other end flops in the breeze. Normally, the spring and post do not make electrical contact.
When this is struck (or, what it's mounted to is struck) the spring with deflect and contact the post, thus making electrical contact. It only responds to force perpendicular to the post, so you might need a second one to get "3d response". Vary the spring and post to change the sensitivity.
Here's a photo of one I just whipped up in the lab:
Ignore the other stuff on the PCB. I took a stiff piece of wire (not stiff enough, IMHO) and soldered it down. Then I took a spring from a pen, unwound a little bit of it and soldered it down. Then bend whatever you need to in order to get the wire centered on the spring.