Homework Help: Electrons circuits

Posted by Jacky on Friday, April 4, 2008 at 11:26pm.

Hi everyone! I'm new in here.

Recently I bought a movement monitor which detects micro movement. If the sensor does not detect a movement in 20 seconds, the build-in speaker will beep until the sensor detects a movement. I'm using it for my project and I have a question about it. I want to intercept a wire from the speaker to a micro controller. The point is, in order to "power up" the micro controller, a 9V DC input is needed. Therefore I want to connect a wire from the speaker to the input of the micro controller.

The situation is something like this:
When the alarm is ON (speaker beeps), nearly 9V DC will go into the input of micro controller. When the alarm is OFF (no sound), there is nearly 0V DC going to the input.

As I measured the voltage across the speaker, I got roughly 0.9V to 1.5V AC. Is there any circuit i can build to satisfy this suitation?

Electrons circuits - bobpursley, Saturday, April 5, 2008 at 9:38am
rectify the speaker voltage with a diode followed by a capacitor in parallel with a large resistor (long RC time constant) to allow a discharge path. Then, that voltage will be a dc signal of about 1.3 volts dc, enough to sense.

Using this sense signal, you can operate any number of logic circuits to switch on/off power. A SCR or thyristor comes to mind.

You will be unsuccessful, I suspect trying to use speaker power to provide power to the next microchip, a separate power source is recommended
- Electrons circuits - Jacky, Saturday, April 5, 2008 at 12:36pm
  I am sorry for my unclear explanation.
  
  Actually the monitor is powered by a 9V 100mA adaptor. I am not sure if the voltage drop across the speaker is AC or DC because the voltage keep changing from 0.9V to 1.5V on my voltmeter. Current is around 3.8mA.
  
  The microcontroller is powered by a 12V 700mA adaptor. In order to send something out by the microcontroller, input with a logical HIGH (more than 7V) is need to perform the action. The microcontroller won't do anything if less than 7V going into the input.
  
  I tried using a non-inverting opamp with voltage gain of 6, supply voltages +V=+9V, -V=-9V by two separate batteries. I tested the circuit using a 1.5V battery, it gave me around 8V. So I think it's ok. When the alarm of the monitor is on, I supposed to get 1V*8=9V at the output of opamp. Instead I just received 2.5V to 3V. How come...?

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