U1 (NE555) needs a regulated supply voltage to supply the internal voltage divider references that control the input voltage comparators connected to pins 2 and 6. VR1, a 7.5v 1W zener diode along with R9 dropping resistor is simple and cheap. R9 resistance is chosen so that the zener voltage will be maintained down to about 10v input supply or below. C3 filters the regulated voltage. Should the supply voltage be reversed, the zener diode will clamp the voltage at -0.7v.
The monitored battery voltage input is fed to two voltage dividers, R1, R2 and R3 for the OFF adjustment and R4, R5 and R6 for the ON adjustment. The fixed resistors are chosen so that the pots cover the expected adjustment range (2-3 volts for R2 and 4-6 volts for R5) and make the adjustment easier.
R7 and C1 provide some time delay for the ON adjustment when the monitored voltage abruptly changes. Similarly R8 and C2 provide a time delay for the OFF setpoint for abrupt changes in the monitored voltage. These components have little effect on gradual changes in the monitored voltage but the time delay from abrupt changes depends on the amount of change and how far below the OFF setpoint the lower voltage is. The OFF time delay is about 5 times the ON delay. Since any leakage current in capacitors C1 and C2 will upset the setpoints, they are chosen for minimum leakage.
C1 and C2 capacitors in the time delay circuit also serve another purpose. Conventionally these capacitors would be connected to ground but by connecting them between the input and VCC (U1 regulated supply voltage) the time delay effect is the same and there is an added bonus. When the circuit is powered up, pins 2 and 6 will be forced high momentarily by the charging of C1 and C2 capacitors. This guarantees that the circuit will power up in the "ON" position which is essential for proper operation. If the capacitors were connected to ground, it would power up in the OFF position which would be very undesirable.
U1 (NE555) needs a regulated supply voltage to supply the internal voltage divider references that control the input voltage comparators connected to pins 2 and 6. VR1, a 7.5v 1W zener diode along with R9 dropping resistor is simple and cheap. R9 resistance is chosen so that the zener voltage will be maintained down to about 10v input supply or below. C3 filters the regulated voltage. Should the supply voltage be reversed, the zener diode will clamp the voltage at -0.7v.
The monitored battery voltage input is fed to two voltage dividers, R1, R2 and R3 for the OFF adjustment and R4, R5 and R6 for the ON adjustment. The fixed resistors are chosen so that the pots cover the expected adjustment range (2-3 volts for R2 and 4-6 volts for R5) and make the adjustment easier.
R7 and C1 provide some time delay for the ON adjustment when the monitored voltage abruptly changes. Similarly R8 and C2 provide a time delay for the OFF setpoint for abrupt changes in the monitored voltage. These components have little effect on gradual changes in the monitored voltage but the time delay from abrupt changes depends on the amount of change and how far below the OFF setpoint the lower voltage is. The OFF time delay is about 5 times the ON delay. Since any leakage current in capacitors C1 and C2 will upset the setpoints, they are chosen for minimum leakage.
C1 and C2 capacitors in the time delay circuit also serve another purpose. Conventionally these capacitors would be connected to ground but by connecting them between the input and VCC (U1 regulated supply voltage) the time delay effect is the same and there is an added bonus. When the circuit is powered up, pins 2 and 6 will be forced high momentarily by the charging of C1 and C2 capacitors. This guarantees that the circuit will power up in the "ON" position which is essential for proper operation. If the capacitors were connected to ground, it would power up in the OFF position which would be very undesirable.
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