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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.

Adding a restart to the modified Flexitimer

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The modified Flexitimer circuit featured in the Circuit Notebook pages of the April 2009 issue was designed to start timing at power up and then stop after the timeout period. To repeat the cycle, the power would be switched off and on again to reset counter IC2.

This add-on circuit, highlighted in pink, allows timing to be started with a pushbutton switch.

When power is first applied to the whole circuit, the 10µF capacitor associated with switch S2 is discharged and transistor Q2 is off. This allows the reset at pin 11 of IC2 to be initially pulled high at power up via a 1µF capacitor to the 12V rail. This resets IC2 and allows the timing cycle to start.

After timeout, the circuit is quiescent until switch S2 is pressed. This pulls the negative side of the 10µF capacitor low and transistor Q2 is biased on via current through the 2.2kΩ resistor. Q2 then pulls pin 11 of IC2 high, resetting it so that the timing cycle can restart. Q2 then switches off when the 10µF capacitor charges to 12V.

When S2 is released, the negative side of the capacitor is pulled high via the 10kΩ resistor and the positive side of the charged capacitor is clamped by diode D4 to the +12V supply rail. This discharges the capacitor, ready for another restart.

John Clarke,
SILICON CHIP.

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