Switching on and off at different voltage while maintaining states

Question

I am designing my own circuit to switch on / off a 9V motor via solar panel. I am connecting 2 sets of 3.7V 18650 batteries in series, that means the circuit runs on 7.4V. I am also using a generic battery charger, so the battery protection part is solved.

My question is, how do I design a circuit (without a microcontroller) so that

1. It switches ON from OFF state only if the voltage is above 7V
2. It switches OFF from ON state only if the voltage is below 6V
3. When it's switched ON due to (1) above, it'll hold it's ON state until (2) happens
4. When it's switched OFF due to (2) above, it'll hold it's OFF state until (1) happens

When you combine (1) - (4), it means I want a kind of "latching" behavior where, when it's charging, it'll charge until at least 7V before switching on the motor, and when it's discharging (motor running,) it'll discharge until 6V at which point we'll stop the motor so that it can go back to charging mode again.

As mentioned earlier, I am not planning to use a microcontroller here. I have BJTs, MOSFETs and comparators (LM393) on hand, so I thought I would go try to go fully analog.

Answer 1

You can use two comparators and one SR flop to achieve this function. Make a 5v power supply from battery using some 5v LDO to supply comparators and for 2.5v reference also(2x 10k devider). R3 and R6 make from 22k pots for precise setting of upper and lower limits.

SR flop holds the motor state until battery voltage mets some boundary (upper or lower).

Red is battery voltage, green is low level comparator output, blue is high level comparator output.

Edit:

If you dont have an oportunity to buy a latching IC you can build one from BJTs and connect it according circuit below:

Blue is battery voltage, green is motor current.

Answer 2

Here's a lttle circuit to do that, using the components you have:

^{simulate this circuit – Schematic created using CircuitLab}

A simulation shows this output (orange), given a rising and falling input voltage (blue):

R3 provides positive feedback, for about 1V hysteresis. R1, R2 and VR1 set a potential at some point between the thresholds, to be modulated by R3. Since the values necessary to obtain the required switching thresholds depend on D1's breakdown voltage, and other component tolerances, VR1 is there to be able to fine tune the center point.

In this CircuitLab simulation, there's no LM393, and I've actually used an LM358 instead. Using a real LM393 comparator will result in slightly different thresholds, due to its open collector output, but VR1 will allow for center correction, and changing R3 will permit you modify the voltage interval between switching thresholds.

Q1 is switched on or off by the comparator output, to connect or disconnect the output load from the input voltage source, V1, which represents your battery of course.

The load RL is just there to provide a current sink for Q1 in the simulation, representing whatever you intend to connect to the output. Don't put it in the final design.

Answer 3

Check out this 555 timer low-voltage disconnect.

The relevant notes you need to see are:

When the voltage on the trigger input is less than 1/3 of Vcc, the NE555 output pin 3 goes and stays HIGH.
When the voltage on the threshold input is greater than 2/3 of Vcc, the NE555 output pin 3 goes and stays LOW.

To repeat:
threshold pin HI, output LO
trigger pin LO, output HI

So the function is inverted, but inverting the output pin is not so hard. (use the inverting output of a relay, for example).
By replacing R1/R2 and R3/R4 with pots, you can fine-tune the trigger voltages, and the circuit will stay in its previous state while the voltage remains in between.

One thing you may have to be careful of is that the voltage of the circuit should not vary wildly, or it may cause the timer circuit to oscillate. But I suspect it should work fine. You WILL need a solid voltage reference, however, like a 3.3v or 5v regulator or similar. Do not try to run it from the battery directly as its trigger and threshold voltages will swing as the battery voltage changes.

Answer 4

I answered before you edited your question, so I had to rewrite my answer.

I did a search for "low voltage cutoff with hysteresis" and found this circuit:

EDIT: link to the article.

It has two adjustments, one for turn on voltage, and one for turn off voltage. It uses a standard IC, the 555.