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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.


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This timer circuit provides an output cycle which is on for one minute and off for five-minutes.

The 7555 timer (IC1) produces a positive pulse about once every 60s to clock a 4017 divider (IC2). The ‘0’ output of IC2 is high for 60 seconds and drives Q1 and the relay during this period. On the next clock signal from IC1, the ‘1’ output goes high and the ‘0’ output goes low and the relay is off.

The ‘2’, ‘3’ and ‘4’ outputs successively go high after each clock pulse from IC1. On the sixth pulse, the ‘5’ output goes high, resetting the 4017 counter and the ‘0’ output goes high again.

IC1 is connected as an astable oscillator with the 100µF capacitor charged and discharged via a 470kΩ resistor. The threshold and trigger inputs at pin 6 and 2 monitor the capacitor voltage.

When the capacitor charges to 2/3rds the supply, the pin 3 output goes low to discharge the capacitor. When the capacitor discharges to 1/3rd the supply, the pin 3 output goes high to charge the capacitor. So the pin 3 output goes high (12V) and low (0V) repeatedly as it charges and discharges the capacitor.

The pin 3 output of IC1 clocks the 4017 divider via a 1kΩ resistor and 100pF capacitor to slow down the rise time of the 7555 timer output. This prevents multiple clocking of the 4017 counter at the positive edge of the clock signal.

The ‘5’ output from IC2 connects to the reset input via a 1N4148 diode (D1) and 10kΩ resistor. The 56pF capacitor provides a reset delay while this capacitor charges up via a high output from the ‘5’ output. The resulting high signal on the reset input of IC2 causes the ‘5’ output to go low again. The reset signal remains high until the capacitor discharges via the 10kΩ resistor. This ensures a sufficient reset pulse for IC2.

John Clarke,

External USB supply
for iPOD touch

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This USB supply is capable of feeding an iPOD or other device which require the D+ and D- signals in order to accept a charge. It comprises a low dropout regulator (REG1) to provide the 5V supply and two resistive dividers to provide the D+ and D- lines.

It needs to be wired to a suitable USB socket.

Jeff Teasdale,
Christchurch, NZ. ($30)

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