This project could be assembled on a Vero board but, I’ve made a custom PCB just for this project. And if you’re interested in ordering your own PCBs for this project. You can find the gerber files using this link Click here
PCB dimensions: 66x40mm
Hole pitch: 58x33mm
About this circuit:
What is a low volt disconnect? A low volt disconnect is used to prevent batteries from being over-discharged. Since this circuit operates a relay to connect/disconnect a load from a power source. It can also be used to switch between multiple power sources (such as a battery & power supply) which is what I’m using it for in my Dual power fume extractor project.
Keep in mind that when the relay is switched on, the circuit consumes about 60mA. That is not much power unless you plan on running it from a solar panel with a small battery. In which case I’d recommend modifying the circuit to utilize a MOSFET to replace the relay. It would drop power consumption down to around 10mA
I also want to credit the source of inspiration for this circuit Click here
Resistors R2 & R4:
Resistors R2 & R4 determine the Disconnect & Reconnect voltages. Which you can customize anywhere between 3.5v to 24 volts. But how do you go about choosing the correct resistor value?
Pins 2 & 6 of the 555 Timer measure the voltage through a pair of voltage dividers. And if the voltage is high enough, the timer will switch on the relay. And if the voltage drops below a certain value, the timer will switch off the relay.
Pin 2 determines the Disconnect or “Cut off” voltage, and Pin 6 determines the Reconnect voltage.
Let’s focus our attention to Pin 6. When pin 6 has 3.33volts or higher, the relay will switch on.
In order to figure out the correct resistor value for R2. First, you need to choose what voltage you desire the relay to switch on. This can be anywhere from 3.50 to 24 volts
I’ll leave a link to this handy voltage divider calculator which makes it super simple to calculate the resistance value of R2 https://ohmslawcalculator.com/voltage-divider-calculator
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In the “Voltage Source” box, enter your desired voltage you want the relay to switch on.
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In the “Resistance 1 (R1)” box, enter the resistance value of R1 (which is 10k ohms)
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Leave the “Resistance 2 (R2)” box empty
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Lastly in the “Output Voltage (Vout)” box, enter the desired output voltage (which for Pin 6 of the 555 timer is 3.33volts)
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Press calculate, and the calculator will now display the required resistance value of R2 in the “Resistance 2 (R2)” box.
Unless you get lucky, most of the time you’ll calculate a random resistance value (Such as 7924 Ohms) and you won’t find a resistor that perfectly matches your calculated value. So just select a resistor with the nearest value.
Before deciding on a disconnect voltage value for your application, it is important to note that this circuit requires a minimum hysteresis of 0.25volts to prevent unwanted oscillation (rapidly switching on & off repeatedly) For example if your Connect voltage is 10.0 volts, then the maximum disconnect voltage would be 9.75 volts. For high current loads that pull-down the source voltage, you may need to substantially increase the hysteresis to prevent unwanted switching.
Now you need to repeat the process for resistor R4.
Pin 2 of the Timer determines the cut off voltage (which switches off the relay). When the voltage on pin 2 drops below 1.66 volts, the relay will switch off
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In the “Voltage Source” box, enter your desired voltage you want the relay to switch on.
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In the “Resistance 1 (R1)” box, enter the resistance value of R1 (which is 10k ohms)
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Leave the “Resistance 2 (R2)” box empty
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Lastly in the “Output Voltage (Vout)” box, enter the desired output voltage (which for Pin 2 of the 555 timer is 1.66volts)
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Press calculate, and the calculator will now display the required resistance value of R4 in the “Resistance 2 (R2)” box.
Components:
C1 = 220uF 35v, D8.0mm_P3.50mm
C2, 3 = 10uF 16v D5.0mm_P2.50mm
D1 = 1N4001 THT Diode
D2 = 5mm THT LED (change R5 value to suit your LED's forward current if needed)
J1 = 2way Screw terminal P5.08mm
J2 = 3way Screw terminal P5.08mm
J3 = External LED can be connected to J3 if required
K1 = 5v Sugarcube style relay (Omron G5LE-1 Or Songle SRD-05VDC-SL-C Or similar)
R1, 3 = 10k Ohm, 1/2w, THT resistor
R2, 4 = User to choose value based on desired connect/disconnect voltages
R5 = 1k Ohm, 1/2w, THT resistor
U1 = LM7805 Voltage regulator, TO-220
U2 = NE555P Timer, DIP-8
