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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Mains timer for battery chargers

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We all have mains rechargeable tools and devices but many don’t have smart chargers. It is too easy to put a drill battery on charge only to return, sometimes days later, to find a hot and probably overcharged battery. Mechanical timers (with the little plastic keys) can be used but they are fiddly and inconvenient.

This simple timer using a PIC­AXE08M micro is the answer. It switches power to a GPO (mains power socket), either during or at the end of a timing cycle, and has four timing periods which can be easily selected. The program allows a reset of the timer with any change in position of switch S1 or S2 during the timing period and S2 will reset the PICAXE-08M after the timing period is complete.

Diodes D1-D4 set up the logic via S1 to inputs P3 and P4 for the four timing periods of 1, 2, 3 or 4 hours duration. S2 is a SPDT centre-off switch. According to the program, in the centre-off position, ADC input P2 is at 5V which resets the timer.

In the “Off Time” position, P2 will be 2.5V, the 5V being divided by the two 10kΩ resistors. In this setting, relay RLY1 will not be turned on until the timing period has ended. This function could be used to delay turning on a slow cooker.

Conversely, with S2 in the “On Time” position, pin 2 will be 0V and relay RLY1 will operate during the timing period. This supplies 230V to the charger and then switches it off.

LED1 is a flashing type with low current drain, so no current-limiting resistor is required. LED2, across the relay coil, is a GPO mains on indicator.

You can change any of the four timing periods to a value between one second and over 18 hours by first calculating the number of seconds required. This then becomes the new value of Word1 (W1) in the program for the desired switch position. The set time is approximate and varies by a few seconds due to the internal clock speed and the time the PICAXE takes to execute the program. However, this is insignificant for the purpose of this project.

The software, GPOTimer­ACD.bas, can be downloaded from the SILICON CHIP website.

Paul Walsh,
Montmorency, Vic. ($80)

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