At first, I thought that I could just use an inverting op-amp with a 1/5 Gain wired to the +5V pin on the arduino, but doesn’t an op-amp need a negative Vcc?
I need this voltage for a flexiforce pressure sensor.
Never actually built anything like this… But I’ve looked it up out of curiosity. Google charge pump negative voltage. Also there are various dc-dc converter ICs out there that can create negative voltage from + supply. Maybe use octopart to find some.
The easy circuit would be a diode voltage doubler/inverter.
But you don’t need it.
Bias the opamp + input with a voltage divider, rather than connecting it to ground. Easier and better than the example they show, which is optimized for the fewest components.
If you’re only building one of these, I think the best solution is a pre-packaged switcher, like the NME0505.
Tie input side to the 5V supply of the Arduino. Then tie the output + to GND and walla, you’ve got -5V on the GND output
Yes, its $10, but its done in 5 minutes
If you are going to build lots of them, then a voltage inverter is the way to go, National has a good selection of cheap ones, Linear Tech has higher performance ones.
Will the MAX202 provide the -1V? If so, how to I wire it to the Pressure Sensor and to the Ground and 5V pin on the Arduino?
Also, I’m really interested in the Biasing the OPAMP strategy. I have a 741n. Can I bias this and not worry about creating a negative one voltage? I have limited space on the breadboard
Again, thank y’all so much for your help so far. Please bare with me, I’m new to this!
Thanks elevator4, but the circuit didn’t work. We used your circuit with a feedback resister of 1K. The op-amp got really hot. When using the arduino software, the reading stayed constant no matter what pressure we applied to the pressure sensor.
The sensor is basically a variable resistor, if you can do the math you don’t really need the negative supply.
According to their user manual, no force is about 1M, max force about 50K. So all you really need to do is measure that resistance, which could be done by measuring the voltage of a resistor divider with the sensor and a fixed resistor.
With a 200K resistor connected to 5V and the sensor connected to GND, 0 force would be the 5V, and full force would be 1V. You’d have to calibrate the setup and try to match their transfer curve.
well, you certainly chose the godfather of all op-amps. Unfortunately the '741 requires at least 7-8V to operate, and its output won’t be able to swing between 0V and the maximum. It only adds more problems.
The LM324 works off a 5V supply and is able to swing to ground (but not to 5V; more like 4.3V or thereabouts) so I would stick with that one, or better still any CMOS rail-to-rail input and output (RRIO) like the MCP6001/2/4 etc.
– Assuming the op-amp in the picture is a '741, I believe you swapped the +5V and GND connections. That’s probably the reason why it gets hot. It may be fried by now.
– The 87.5K resistor between +5V and GND doesn’t do any good; this should be a pot as in the schematic, with the wiper connected to pin 3 (the + input).
– always bypass the power supply pins (GND and +5V) with a blocking cap, close to the chip. This prevents oscillations and glitches. Anything above 50nF works.
If you’re happy without an op-amp (connect one end of the pressure sensor to +5V; the other end to a resistor of about 100K; then the other end of the resistor to GND). Pick up the signal at the junction between the two; measure with the voltmeter and send to the arduino pin. However, the signal will have a narrower range (say from 0.5V with no pressure to 3.5V at max).