# Using a switch detect IC with negative voltage

I am designing a circuit where a negative voltage signal can have multiple levels, eg -12, -8, -6 depending on the circuit state.

I am looking at several switch detect ICs such as TI's TIC12400 that seems to work with -40V up to +60V on its inputs.

How do I set up a switch detect IC such as TI's TIC12400 or NXP's CD1020 or CD1030 to generate an interrupt whenever my signal changes its voltage, eg from -12 to -8 or from -8 to -6? Is this even possible without additional op-amps/comparators?

CD1020 datasheet: https://www.nxp.com/docs/en/data-sheet/CD1020.pdf

• You can convert this to +ve voltages with a series and pull-up R to 5V Feb 1, 2021 at 13:35
• Why not use a couple of resistors and feed the restored signal into an ADC input. Feb 1, 2021 at 13:36
• I have about 18 signals to monitor, would rather receive an interrupt on a pin and read a register over SPI, if at all possible? Feb 1, 2021 at 13:38
• If that's a requirement then it should be in the specifications in your question - not buried down here. Feb 1, 2021 at 13:46
• For any design you must define all the inputs and outputs with functions and tolerances Feb 1, 2021 at 22:26

I have about 18 signals to monitor, would rather receive an interrupt on a pin and read a register over SPI, if at all possible?

Then I would choose a suitably small (but quick) MCU capable of controlling a multiplexer (to scan the "restored" inputs) so that an analogue signal can be fed to an ADC input on said MCU.

To restore the raw input signal to a positive value you can use two resistors. If Vcc is 5 volts and your lowest negative voltage is 12 volts then use a 5 kΩ resistor and a 12 kΩ in series between Vcc and your input signal. When the input signal is -12 volts, the junction of the two resistors will be at 0 volts. As the input signal rises to -8 volts, the junction will be about +1.18 volts. When the input is -6 volts, the junction will be +1.76 volts.

You might even be able to get a multiplexer that can work with the raw negative inputs and use just one potential divider on the output. You might need to supply a negative supply rail of 12 volts to achieve this of course.

Then scan around all the 18 restored DC voltages (with the multiplexer) looking for a voltage change and generate an interrupt to your main circuit when you detect one.

5 lines control the multiplexer, 1 line for the interrupt and 3 lines for an SPI/IIC interface.

• Thank you Andy. So it's not possible to use the TIC12400 or similar to do this? Feb 1, 2021 at 14:26
• Didn't you say this: TIC12400 that seems to work with -40V up to +60V on its inputs Feb 1, 2021 at 14:30
• Yes that's what I read in their datasheet. Right now I am using a bunch of op-amps, transistors, resistors and capacitors, a handful of components per signal. I am looking to simplify my design and was curious whether such an IC can help, if I understand at all what it it does and how it works. Feb 1, 2021 at 14:38
• Well, I've proposed a simpler hardware way but it does need some code writing. Feb 1, 2021 at 14:45
• Thank you Andy, much appreciated! Feb 1, 2021 at 14:49