small alarm system - error?

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snote

Joined Jan 25, 2019
10
I have to build a small alarm system. Voltage is 12V. The speaker is a small piezo sirene. SW2 should be the sensor. SW1 the activator. I want to have a control LED to see if sensor is open or closed.

If I mesure the voltage at the sirene, it is -10v. If I close switch1 I mesure 3V. If I close switch1 and switch2 I have 12V like expected.

I am a bit confused about the negative high voltage with all switches open.
 

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wayneh

Joined Sep 9, 2010
17,496
I have to build a small alarm system. Voltage is 12V. The speaker is a small piezo sirene. SW2 should be the sensor. SW1 the activator. I want to have a control LED to see if sensor is open or closed.

If I mesure the voltage at the sirene, it is -10v. If I close switch1 I mesure 3V. If I close switch1 and switch2 I have 12V like expected.

I am a bit confused about the negative high voltage with all switches open.
How are you measuring voltages? It would be conventional to use battery-minus as the zero reference point. There shouldn't be any negative voltages in that case.
 

wayneh

Joined Sep 9, 2010
17,496
If I measure between minus and open switch2 I got -10,88V??
Take a picture of your meter and how you have it hooked up. Sounds like the leads are crossed or something. What do you see as the voltage of the battery? (The diagram shows the power source is just 1V.)
 

wayneh

Joined Sep 9, 2010
17,496
If I switch on Switch2 I read 12,0V, if I switch it off, -10,88V.
I don't understand how you can ever measure a negative voltage in that diagram without have the leads arranged improperly. The meter's negative lead should be connected to the power supply negative. There will then not be anything in the circuit with a negative voltage.

There's nothing wrong with a negative voltage and it's not absolutely necessary that the power supply negative be the reference, the problem is that a negative voltage merely indicates you're measuring in a way we don't understand. We can't take for granted the reference you are using. If you specify which nodes the meter's leads are attached to, that would resolve the confusion.
 
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