.jpg)
Cordless drill manufacturers usually recommend a charging time of three hours. After the charging time has elapsed, the battery must be disconnected from the charger: if you forget to do this, you risk overcharging the battery. This circuit, located between the charging circuit and its battery socket, avoids this possibility: the contact of the Re1 relay will cut off the charging current when the three hours are up. Ten LEDs indicate remaining charge time in 20-minute steps. The timer is reset every time the power is turned on and then it is ready for a new cycle. When power is applied, IC3 is reset through C4 and R5. When the charging time is up, Q9 (pin 11) goes high, triggering the relay and interrupting the charging current.
Since Q9 is connected to an active low (on) EN input, the counter will remain in this state. Charging time can be adjusted from about 2 hours 15 minutes to 4 hours 30 minutes using P1. The author set P1 to 30 kΩ, giving a charge time of 3 hours 7 minutes. The larger the resistance of P1 the shorter the charging time. The circuit's timing isn't particularly accurate, but its accuracy is quite consistent with current work. When setting the charging time, it should be noted that the first clock cycle after turning on the circuit (from Q0 to Q1) is longer than the following cycles. Indeed, capacitor C3 must initially be charged to about half of the supply voltage.