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I need a way to switch a -5V voltage supply to the Flexiforce sensor below. The circuit I am using is this:

enter image description here

I looked into using a 74HC4066, but it does not allow switching negative voltage. It can only switch between GND and VCC. I need to find an IC that has multiple switches and can switch negative voltage. If not, how could I incorporate a transistor circuit to solve this?

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  • \$\begingroup\$ You have a -1 V supply for the sensor, why not use it for the switch, too? They draw very little current. \$\endgroup\$
    – endolith
    Dec 9, 2015 at 21:33
  • \$\begingroup\$ Not sure what you mean by that. I need to use Arduino's digital outputs to control the -5V signal (it says -1V in the picture, but its actually -5V). So basically, if i toggle one of Arduino's digital outputs on, it should switch on that -5V signal and if i toggle it off, it should disconnect that -5V signal. Could you clarify what you mean? \$\endgroup\$
    – tabchas
    Dec 9, 2015 at 21:36
  • \$\begingroup\$ I thought you wanted to switch the signal on and off, not the power supply. Can you be specific about the outcome that you want? \$\endgroup\$
    – endolith
    Dec 9, 2015 at 21:49
  • \$\begingroup\$ I would like to control the -5V supply to the sensor (its labeled as Vt -1V on the picture). I would like to turn an Arduino pin HIGH, and that would connect the -5V to the pin of the sensor. When I turn the Arduino pin LOW, that disconnects the -5V to the pin of the sensor. \$\endgroup\$
    – tabchas
    Dec 9, 2015 at 21:51

3 Answers 3

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I am trying to hook up multiple of these sensors to one analog input on my Arduino, and would like to switch them on and off by controlling the -5V signal input (-1V in the picture) using a switch mechanism.

  • For good signal quality, it's probably a good idea to use the op-amp to drive the micro's ADC.
  • The circuit shown can be converted to a summing amplifier, with each of the sensors connected to the inverting input of the op-amp.
  • To do this circuit without needing a negative supply, connect the bottom of the sensor to ground instead of -1 V and connect the non-inverting input of the op-amp to a mid-voltage, like 2.5 V from a resistor divider. Your 0 point will be shifted, which you'll subtract in software.
  • To switch them out of the circuit, you could ground each sensor through its own transistor and only enable one transistor at a time. You could also use the 74HC4066 on the lines going into the summing amp, but that's a more complicated circuit.
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  • \$\begingroup\$ To your second bullet, aren't I looking measurement room by using a mid-voltage like 2.5V? It cuts off the measurement room by half correct (0-5V to 2.5-5V)? Would I have to subtract and scale that using an additional op amp (subtract 2.5V and amplify to 0-5V)? \$\endgroup\$
    – tabchas
    Dec 9, 2015 at 21:59
  • \$\begingroup\$ Also what do you mean that the op amp drives the micro's ADC? I The output of the op amp (Vout) connects to the ADC directly. Is that what you are referring to? \$\endgroup\$
    – tabchas
    Dec 9, 2015 at 22:03
  • \$\begingroup\$ You mean third bullet? Yes, it would reduce measurement range by half. You could increase it again with a second op-amp stage if you want, but it might be good enough as is, just subtract in software. \$\endgroup\$
    – endolith
    Dec 9, 2015 at 22:05
  • \$\begingroup\$ Yes it's generally better to drive micro ADCs with a low impedance source like an op-amp, instead of a high impedance source like a resistor divider. Why do you need to multiplex all analog sensors onto one ADC pin, by the way? Doesn't Arduino have multiple inputs multiplexed to the same ADC already? \$\endgroup\$
    – endolith
    Dec 9, 2015 at 22:07
  • \$\begingroup\$ If I do the positive voltage configuration of this, the other side of the sensor will be connected to GND rather than -5V. Can I use the 74HC4066 to switch this (when I output Arduino's pin HIGH, it connects the sensor to GND, and disconnect when I output Arduino's pin LOW)? \$\endgroup\$
    – tabchas
    Dec 9, 2015 at 22:11
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First, can you simplify your power supply configuration by using a non-inverting amplifier as shown below?

schematic

simulate this circuit – Schematic created using CircuitLab

Second: this doesn't allow virtual earth summing configuration but since you were going to need a pin per -V switch, and they're not required in this configuration, you can now connect each amplifier to its own analog input. Downside is that you need an opamp for each sensor.

Third option: use tristate pins to power each sensor. This time the example show the sensor in the V+ line.

schematic

simulate this circuit

Each of the 'BUF' outputs from your micro / Arduino / whatever is sequentially switched high while all of the others have their outputs floating. This way only one sensor is powered at a time and you don't need any negative voltages.

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That's just one circuit, which happens to produce a voltage inversely proportional to the resistance. There are many other possibilities, some of which don't require a voltage outside the output voltage range.

If this is going into a microcontroller, and in just about any application it would nowadays, the output doesn't have to be proportional to anything in particular. In a micro you can easily do the math or do the lookup to convert from whatever A/D reading you get to linear pressure.

The simplest option is just a resistor divider, with the other resistor in the middle of the range you care about coming out of the sensor. One side advantage of this is that the only thing you have to calibrate is the other resistor. The actual supply voltage cancels out. You will have highest resolution at that resistance, then lower at both ends of the range. This method actually measures from 0 to infinite resistance, but again, the resolution goes down at the ends of the range.

With today's A/Ds, I'd probably use the divider method and make up any loss of resolution with a higher resolution A/D. You can now get 12 bit A/Ds routinely in low cost micros. That's probably enough, but if not, a external delta-sigma A/D could certainly do it.

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  • \$\begingroup\$ The circuit above is recommended by the manufacturers of the sensor shown. It allows dynamic control of the force range which is what i would like to have. I am trying to hook up multiple of these sensors to one analog input on my Arduino, and would like to switch them on and off by controlling the -5V signal input (-1V in the picture) using a switch mechanism. Would you know how to do this? \$\endgroup\$
    – tabchas
    Dec 9, 2015 at 21:42
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    \$\begingroup\$ @tab: Manufacturers are free to recommend whatever they want without having to face the realities and tradeoffs in your system. It's your job as the engineer of your system to use your own brain, which includes evaluating the manufacturer's suggested circuit and using something else if that fits better. To your second point, why do you want to "switch" these sensors on and off? They take little power. Whatever extra power the arduino takes relative to a low power microcontroller design will swamp that anyway. \$\endgroup\$ Dec 9, 2015 at 21:46
  • \$\begingroup\$ Hmm, this is actually for a project (more of a learning exercise for me, im an EE student :). If you could help me out in coming up with a switch for this, that would be great! I understand what you mean in terms of power consumption, but am simply looking for a switch mechanism right now. \$\endgroup\$
    – tabchas
    Dec 9, 2015 at 21:49

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