# LM311 Op Amp as a differential amplifier to drive a solar tracking motor

I want to drive a 12v DC motor that will rotate an array of small solar cells to track the sun across the sky. 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 think this to be a very sluggish type system where frequencies or other gremlins will be subdued. Expected cycle time would be about 15 seconds "on" every 10 +/- minutes.

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. 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.

Pins 1 and 4 are grounded, pin 8 is 16.5 volt supply and pin 7 is the output. I have a voltage divider network on the inputs to reduce gain to 379 = (49300/130) * (.015). Pins 5 and 6 are not used.

My problem is that the op Amp is saturated to rail voltage no matter what the input differential voltage is. 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. Also there is a desired hysteresis effect on the Mosfet varying the output voltage using the pot.

1. Is it possible that the "sensor cells" I am using do not have the current output to drive the Schmidt trigger?
2. Any other comments, ideas and help will be appreciated.

• It is difficult to see what you see without a schematic. Please update your question and use the schematic option in the editor to add that information. Feb 21 '18 at 1:27
• LM311 is a comparator. The gain is over 100,000. Rethink your choice. Feb 21 '18 at 3:12
• Please show your schematic unless it will be difficult to discuss this matter.
– CNA
Feb 21 '18 at 3:20
• 9 hours is long enough to post a schematic. Voting to close as unclear. Feb 21 '18 at 11:13

As explained in the comments, the LM311 is a comparitor, not an op-amp.

Figure 1. The LM311 internals.

Note also that its output is an open collector arrangement. While you have correctly grounded the EMIT OUT, pin 1, you have ommitted the required pull-up resistor on COL OUT, pin 7. Even if you were using it as a comparitor your output voltage would fail to swing high due to this omission.

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. A comparator is designed somewhat differently to an op-amp but their schematic symbols are often the same or very similar. Let's see ...

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. 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.

OK. The comparator output will be either at V+ or 0 V so this should switch your MOSFET cleanly.

Pins 1 [EMIT OUT] and 4 [V-] are grounded, pin 8 [V+] is 16.5 volt supply and pin 7 is the output.

Yes, but pin 7 is open collector.

simulate this circuit – Schematic created using CircuitLab

Figure 2. A way of visualising the transistor output.

When you connect the emitter to GND you need a pull-up resistor on the collector.

I have a voltage divider network on the inputs to reduce gain to 379 = (49300/130) * (.015). Pins 5 and 6 are not used.

If R1 is 130 Ω and R2 is 49 kΩ then the division ratio will be $\frac {49k}{49k + 130}$ so there will be very little attenuation. I suspect you have the divider the wrong way around.

My problem is that the op Amp is saturated to rail voltage no matter what the input differential voltage is.

That's probably caused by the missing pull-up resistor and maybe the incorrect divider rations.

There's a good article on TI's LM311 datasheet which includes the following explanation of hysteresis.

Figure 3. Hysteresis setup.

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. Read the preceding paragraph to the figure.

• Thanks for the comment. I do not fully understand since i am a mechanical engineer dabbling in the EE world. it seems to me I would want a pull down resistor to drive the pin 7 output to zero unless the sensor voltage differential over came that state. How do correct my thinking and how to determine the resistor value? Feb 21 '18 at 18:04
• It has been pointed out twice that you are using the wrong chip. You need an op-amp, not a comparitor. Feb 21 '18 at 18:06
• ok I thought I was using a differential comparitor. What would be the correct chip? Feb 21 '18 at 18:08
• Would the LM 392-N chip work and which side, the comparator or op-amp side would be appropriate? Sorry I seem to be confused on terminology since I want to compare two voltages and generate an output decision. Feb 21 '18 at 18:33
• See the update. Feb 22 '18 at 23:29