Wrong voltage in alarm circuit

I am building 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 switch2 I have 12V like expected.

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

by snote
January 25, 2019

there may be some short circuit (high resistance) from top contact of speaker to the negative of battery.

by wangwg88
February 01, 2019
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