Three Hour Timer

Manufacturers of
cordless drills generally recommend a battery charging time of three
hours. Once the charging time is up the battery must be disconnected
from the charger: if you forget to do this there is a danger of
overcharging the battery. This circuit, which sits between the charger
circuit and its battery socket, prevents that possibility: the contact
of relay Re1 interrupts the charging current when the three hours are
up. Ten LEDs show the remaining charging time
in steps of 20 minutes. The timer is reset each time power is applied
and it is then ready for a new cycle. When power is applied IC3 is reset
via C4 and R5. When the charging time has elapsed, Q9 (pin 11) goes
high, which turns the relay on and interrupts the charging current.


Circuit diagram

Since Q9 is connected to the active-low EN (enable) input, the
counter will now remain in this state. The 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 charging time of 3 hours 7minutes. The
greater the resistance of P1, the shorter the charging time. The timing
of the circuit is not particularly precise, but its accuracy is entirely
adequate for the job. When adjusting the charging time it is worth
noting that the first clock cycle after the circuit is turned on (from Q0
to Q1) is longer than the subsequent ones. This is because initially
capacitor C3 has to be charged to around half the supply voltage.

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