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I have a small microcontroller (VoCore, runs Linux, has wifi and GPIOs) and i want to use it to hook into another system (basically a doorbell) to make the state of the doorbell-buzzer available over the network.

The system I want to hook into runs on 8V DC, there is a buzzer in it which normally doesn't get any power, if somebody rings the doorbell it gets 8V. My board runs on 5V with a 3.3V GPIO logic level, so i have a DC-DC converter which transforms the 8V to 5V, that the things i want to connect already share a common ground-level.

I am unsure of how I can hook into / "read" the state of the buzzer-line. It should still work as it does now. I had the following 2 ideas:

  • First I thought that I'll have to use a relay which is connected in parallel to the buzzer and is thereby switched with the buzzer, then simply connect a GPIO from my microcontroller to the output side of the relay and connect it to V+ from the microcontroller via some resistor. Basically like swithing a relay with the microcontroller only that the relay is put in "backwards".
  • When I looked around for parts I saw that logic level converters (e.g. for TTL lines) are often cheaper as relays and would probably have some other advantages because of the lack of moving parts. If I can find one which provides 8V to 3V (or simply uses reference voltage lines), I would assume that it can be used instead of a relay to read the state of the buzzer.

So my question is: Is my second assumption true? Can logic level converters be used like relays for "detecting" voltage on some line? Or am I on the wrong track from the start - how is detecting voltage on some line with different voltage than my microcontroller typically done?

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2 Answers 2

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how is detecting voltage on some line with different voltage than my microcontroller typically done?

Normally people use an opto-coupler; an LED and photodiode in close proximity sealed into one package: -

enter image description here

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  • \$\begingroup\$ thank you, totally forgot about those (I only occasionally work with electronics in my spare time). I'll definitely go and learn / play around with them. Would logic level converters also work here (even though it would be overkill)? \$\endgroup\$
    – NoUsername
    Apr 25, 2015 at 22:25
  • \$\begingroup\$ @NoUsername It is difficult to say without a circuit and knowledge of the variations in the 8V supply - remember logic level converters are non-isolating and this gives me concern in your application. \$\endgroup\$
    – Andy aka
    Apr 26, 2015 at 8:39
  • \$\begingroup\$ The OP has stated that he/she is using a DC-DC converter to produce the 5V needed to power their MCU board. It is highly likely that the DC-DC converter is not an isolating type in which case the buzzer/bell system GND will be shared with the MCU board GND. In such case there is not too much reason to use an opto-coupler for isolation. In fact the voltage divider circuit that I have in my answer would be just as effective although if there is concern that the 8V signal is very noisy a capacitor can be added across the R2 resistor of the divider to low pass filter the signal. \$\endgroup\$
    – Michael Karas
    Apr 27, 2015 at 1:36
  • \$\begingroup\$ @MichaelKaras you may indeed be correct. \$\endgroup\$
    – Andy aka
    Apr 27, 2015 at 7:15
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Since the I/O pin input to your microcontroller is pretty high impedance you could simply use a voltage divider as follows to convert the 8V buzzer signal to a 3.3V level suitable for the MCU.

enter image description here

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