The datasheet is generic for a range of comparators and Figure 17 shows how to use the pull-up resistor for the open-collector type. There's a good article on TI's LM311 datasheet which includes the following explanation of hysteresis. That's probably caused by the missing pull-up resistor and maybe the incorrect divider rations. My problem is that the op Amp is saturated to rail voltage no matter what the input differential voltage is. I suspect you have the divider the wrong way around. If R1 is 130 Ω and R2 is 49 kΩ then the division ratio will be \$ \frac \$ so there will be very little attenuation. I have a voltage divider network on the inputs to reduce gain to 379 = (49300/130) * (.015). When you connect the emitter to GND you need a pull-up resistor on the collector. A way of visualising the transistor output. Simulate this circuit – Schematic created using CircuitLabįigure 2. Pins 1 and 4 are grounded, pin 8 is 16.5 volt supply and pin 7 is the output. The comparator output will be either at V or 0 V so this should switch your MOSFET cleanly. The output of the Op Amp comparator will turn on a Mosfet that triggers at about 3.8V to power a relay to the drive motor. I am trying to use a change in sensor voltage of about 0.015 V (between pins 2 and 3) to operate the Op-Amp comparator with a sensor voltage range of about 2.5 to 3.5 volts. A comparator is designed somewhat differently to an op-amp but their schematic symbols are often the same or very similar. Thinking about your question today I suspect that your choice of chip may be appropriate but your description of it as an op-amp is the problem and confusing your readers. Even if you were using it as a comparitor your output voltage would fail to swing high due to this omission. While you have correctly grounded the EMIT OUT, pin 1, you have ommitted the required pull-up resistor on COL OUT, pin 7. Note also that its output is an open collector arrangement. Any other comments, ideas and help will be appreciated.Īs explained in the comments, the LM311 is a comparitor, not an op-amp.Is it possible that the "sensor cells" I am using do not have the current output to drive the Schmidt trigger?.Also there is a desired hysteresis effect on the Mosfet varying the output voltage using the pot. If I put a pot on the rail voltage to vary the output to the Mosfet, the relay picks up as intended when output voltage exceeds 3.8 volts. The output of the Op Amp will turn on a Mosfet that triggers at about 3.8V to power a relay to the drive motor. I am trying to use a change in sensor voltage of about 0.015v (between pins 2 and 3) to operate the Op-Amp with a sensor voltage range of about 2.5 to 3.5 volts. Expected cycle time would be about 15 seconds "on" every 10 /- minutes. I think this to be a very sluggish type system where frequencies or other gremlins will be subdued. I intend to use two smaller solar cells as my sun "position sensor" by having them at an angle to each other so as the sun moves West, the output voltage will change with the motor driving the sensor to null. I want to drive a 12v DC motor that will rotate an array of small solar cells to track the sun across the sky.
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