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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number.

Substitute for AC motor reversing switch

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This circuit was devised as a substitute for a failed AC motor switch. It can start the motor in forward or reverse mode, as required.

It is based around a pair of NAND gates which are configured as an RS (Reset/Set) latch, with one input of each NAND gate cross-connected to the other gate’s output. Hence, pin 12 of IC1d is the Set input while pin 9 is the Reset input. At power-up, the latch is reset because pin 9 of IC1c is initially held low by a 1µF capacitor. The capacitor charges up via a 47kΩ resistor and subsequently pulls pin 9 high.

Switch S1 is the start switch. Pressing it pulls pin 12 of IC1d low. This sets the latch and pin 11 goes high, turning on transistor Q2 and relay RLY2. This applies 230VAC to the A1 terminal of the motor and so the motor runs in the forward direction.

Gates IC1a & IC1b form the reset circuit. Switch S2 functions as the Stop/Reverse switch. Pressing switch S2 momentarily pulls pins 1 & 2 high, so pin 3 goes low and pulls pin 9 low, via diode D5 and a 100Ω resistor. This resets the latch and relay RLY2 is deactivated.

Pressing and holding switch S2 provides the reverse mode. As well as resetting the latch as before, it pulls pins 5 & 6 of IC1b high and this sets pin 4 low, turning on transistor Q1 and relay RLY1 which applies 230VAC to reverse the motor. Diodes D3 & D4 clamp the back-EMF voltage when RLY1 & RLY2 turn off.

Switch S2 has to be held down to maintain the motor in reverse mode. Releasing the switch allows pins 5 & 6 is go low after a delay and relay RLY1 is deactivated to turn the motor off.

By that time, pin 9 of IC1c will have also gone high, pulled up by the 47kΩ resistor. The motor can then be started again in the forward direction, by pushing switch S1.

LED1 indicates the forward mode while LED2 indicates the reverse mode.

Michael Azzopardi,
Deer Park, Vic.

Simple lithium-ion battery charger

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This circuit is for charging lithium-ion batteries from cell phones and cameras where the original chargers are not available. These batteries are normally charged to 4.2V and the charging current gradually falls as full charge is approached. In this design, the battery is charged from 4.3V via a 1Ω 5W resistor.

Comparator IC1 compares the terminal of the battery being charged with a 4.25V reference, as set by VR2. When the charging current falls below about 50mA, the charge is terminated. This may not achieve full charge but it’s close enough for most purposes. Charging is started by pressing S1. This connects one side of the battery to 0V and causes the comparator to turn on the relay, which holds the battery grounded until charging is switched off.

The input supply voltage is nominally 8-12V and is determined by the voltage required for the low power relay; it will also determine the degree of heatsinking required for the LM317 (REG1). The supply should be able to deliver around 2A.

The circuit employs an LM319 dual comparator which means that two batteries could easily be charged separately with an additional relay. LED1 indicates when charging is occurring.

The circuit is not intended as a fast charger (which may be an advantage with older batteries) and it may take several hours to charge a flat battery. Since these batteries should not be discharged below about 2.7V, care should be taken with any battery with a lower initial voltage.

There is no temperature monitoring of the batteries but this could be included by attaching a 10kΩ NTC (negative temperature coefficient) thermistor and connecting it in parallel with 470Ω trimpot VR1.

Finally, note that some lithium-ion should be charged to 3.6V (rather than 4.2V). VR1 and VR2 can be adjusted to suit these (ie, adjust VR1 for 3.7V at the output of REG1 and VR2 for 3.65V on pin 5 of IC1a).

Graham Jackman,
Oakleigh East, Vic. ($50)

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