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# Low Voltage disconnects for Solar charged Battery

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#### rotorgoat

##### New Member
I'm trying to find a suitable DIY controller for my solar slowpump (Dankoff 1322-12) so that I don't have to control it manually. I've devised a relay circuit to turn it off and on but I need two IC type circuits to operate the relays to shut the pump off at 11.5V and not allow it to turn back on until 14.4V (after full battery recharge). I can buy one from NewMar (LVD 12-30) for \$165. but would like to put it together at a lower cost if possible. I've seen some circuits in other areas of this site but they seem to be limited to less than 10V.

Thanks Mike, It's taking me awhile to navigate this site but I'm improving. Looks like there was a lot of "adjusting" to get the desired result. I'm not sure why his input voltage was so high, i.e. 230V to charge batteries? So far from getting re-acquainted with basic circuitry it looks like resistance values must be coupled with the proper load (wattage) values to create the proper circuit parameters.

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If the 555 is fed from a well-regulated fixed voltage Vdd, like 9.00V, the trip points of the internal comparators will be at 3V (Trig pin at 1/3 of 9V) and 6V (Thresh pin at 2/3 of 9v), respectively. The output of the 555 goes high when the Trig pin goes below 1/3Vdd, and goes low when the Thresh pin goes above 2/3Vdd.

To accomplish your specific trip points, you will have to set up two voltage dividers (total of four resistors or four resistors and two pots) such that when your battery voltage is 11.5V, the first voltage divider puts 3V onto the Trig pin. This will cause the 555 output pin to go high and turn off the pump. The second voltage divider should put 9V on the Thresh pin when the battery voltage reaches 14.4V, causing the 555 output to go low, and turning on the pump.

I'll work out the first one: Vin = 11.5V. Vout=3V. Say we use a 100K pot for the voltage divider. So R1+R2=100k. The general equation for a voltage divider is Vout = Vin * R2/(R1+R2).

Rearranging: R2 = (R1+R2) * Vout/Vin = 100K * 3/11.5 = 26K, so a 100K pot with the wiper set 26K from the bottom end would do it. To make it easier to adjust, I would use a 20K pot with a fixed 11K resistor in the ground leg, and a fixed 68K resistor in the high leg to the battery. Note that the sum of 11K + 20K + 68K adds to 100K, our starting assumption.
This way, the 20K pot ends up being set near its center position, thereby making it easier to adjust.

I'll let you work out the other divider.

The original posting referred to above talks about how to invert the output signal and switch a relay from the 555 output pin.

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Thanks Mike, I'm leaving PHX today to retrieve my "camp" before the snow claims it. I'm going to start constructing the circuit when I return mid-month. I will do some studying so as to better understand all of this. Keith

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