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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.

Regulator for solar battery charger

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Some solar regulators are complicated and tricky to set up, and may also have relay chatter. This version is easy to set up and works positively. The relay is a 12V 400-ohm coil type with 10A contacts (such as Dick Smith P-8017).

Q1 monitors the battery being charged via the 330kΩ and 10kΩ resistors feeding its base. While the battery voltage is below the threshold set by trimpot VR1, Q1 is off, Q2 is on and the relay is energised to pass current from the solar panel and diode D1. Q2 also lights LED1.

When the battery voltage rises above the threshold set by VR1, Q1 turns on, removing base current from Q2 which then turns off the relay. The 220μF capacitor at the base of Q1 prevents relay chatter.

Once the relay is open, a trickle charge can be maintained via a suitable incandescent lamp which bypasses the relay contacts.

The meters and changeover switch can be regarded as optional. To set the cutoff voltage (eg, 13.8V), apply 14-15V to the battery terminals and adjust trimpot VR1 until LED1 goes out at the desired voltage.

E. Sherman,
Kawhia, New Zealand. ($30)

Simple time switch for battery-operated projects

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The use of this simple timer will prevent batteries from going flat because you forgot to turn off the power. It uses a momentary pushbutton switch to apply power to the controlled device and then, after a delay of about eight minutes, power is automatically cut. Timing can be restarted at any time during the 8-minute interval.

The 4060 14-Stage Ripple Carry Binary Counter governs the ON timing duration with the R/C combination at its clock inputs, pins 9, 10 and 11, providing the rate of clock pulses. Q1 switches power in the positive line to the device to be controlled when IC1's Q14 output goes low. A high-brightness LED is included as an ON indicator but can be deleted if not required.

IC1 is permanently connected to the battery supply but draws just a few microamps of current when dormant. This current flows through a 100kΩ resistor and insufficient voltage is induced across the base-emitter junction of Q1 to turn it on. The Reset pin of the 4060 is held low by a 47kΩ resistor and as a result, output Q14 (pin 3) is set high. This also holds pin 11 high via D1, preventing the clock from pulsing.

When the pushbutton is pressed, two events take place. The Reset pin of IC1 now becomes momentarily high, resetting all Q outputs Low. With Q14 low, Q1 then turns on to power the external circuit. After about eight minutes, the count sets Q14 high again and Q1 turns off.

The timing duration can be varied by using a different capacitor value on pin 9 of IC1. Supply voltages from 5-15V are acceptable but the 3.3kΩ resistor should be tailored to provide about 5mA base current to Q1.

Colin Christensen,
Redcliffe, Qld. ($40)

Door monitor for a dog

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This circuit was produced to monitor the movement of an old dog which at times during the night must be let outside. A door monitor set low off the ground would have sufficed but it needed to discriminate against cats as well.

The first 555 pulses the infrared LED at 38kHz and the light output reflects off a car or truck reflector to be picked up by the infrared detector IC. When it picks up infrared, its output goes low to turn on the visible LED via the BC558 transistor. This LED serves both as a power on indicator and is used to set up the reflector.

The height of the transmitter and receiver (optically isolated but in the same box) was set a little below the shoulder height of the dog. To prevent false alarms from cats' tails occurring, an RC time delay was included, comprising the 1MΩ resistor and 47μF capacitor at pin 3 of the LM311 comparator. This comparator goes low to provide a clean trigger signal to the second 555 timer, which operates as a monostable to drive a flashing LED and/or a piezo buzzer for 10 seconds.

Paul Walsh,
Montmorency, Victoria.

Electric field proximity switch

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This little circuit does not pretend to compete with the Body Detector sensor featured in the October 2001 issue of SILICON CHIP but its sensitivity for so simple a circuit is very high. The circuit will respond to both RF and electrostatic fields.

The proximity switch behaves rather like the automatic door-opener at the entrance to many stores. A person approaching will activate the device and open the doors but if the person then stops and remains still, the doors will close.

With no moving objects within its range, the field strength varies slowly and has no effect on the device. But a large body such as a person entering its zone of operation results in large changes in the field strength.

Referring to the circuit diagram, the sensor is nothing more than a stiff piece of wire, insulated or bare, standing vertically like an antenna, approximately 15cm long. This is connected to the gate of FET Q1 which is connected as a source follower and operates as a high-impedance buffer with a low output impedance.

The source of Q1 drives the base of PNP transistor Q2 which charges the 10μF capacitor at its collector in response to voltage changes picked up by the antenna. Q2 also drives Darlington transistor Q3 via a 220kΩ resistor and this, in turn, drives the relay. The 220kΩ resistor and 10μF capacitor provide a delay so the relay stays on for up to 30 seconds.

In use, trimpot VR1 should be adjusted for maximum sensitivity by setting it at the point where the output just turns off. This is preferably done before installing the 10μF capacitor.

The circuit may be susceptible to certain nearby electrical appliances but not normally overhead lighting. In some situations, the unit can sense the presence of a person from two metres away but its performance depends on the clothes worn, the level of friction generated, the dryness of the air, etc. It reacts violently to polythene film.

In its standby state, it draws less than 1mA, depending on the setting of VR1.

J. A. Lee,
Old Reynella, SA. ($30)

White LED torch circuit uses flash parts

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This circuit is essentially the same as in the December 2000 article but has a current regulator to drive the white LED. In effect, the current regulator senses the LED current and adjusts the pulse width from the two-transistor multivibrator to vary for the charging of the inductor. The discharge time for the inductor remains constant.

Hence, with a fresh battery, the multivibrator runs fast and gradually slows as the battery runs down. This circuit will allow operation down to a battery voltage of 0.8V with a useful light output. With a battery voltage of 1.5V the LED current is approximately 20.5mA. A battery of 1.2V, gives a LED current of approximately 19mA. The efficiency of the circuit varies between 52% and 58%.

The main switching transistor is a 2SD965. It is available for free, along with the small inductor (a transformer with a 8.5mm x 8.5mm core) that is used in this circuit. These parts along with many other useful bits and pieces come from the electronic flash units that are fitted into cheap disposable cameras.

To get them, go to a 1-hour film processing shop and kindly ask for any disposed cameras from their rubbish. Most places are helpful. Sometimes you even get the alkaline battery which is still useful. Get several different cameras because the transistor and tiny inductor come out of different makes of camera.

WARNING: when taking these cameras apart, be aware that the flash reservoir capacitor may still be charged up to about 300V DC. This can be LETHAL. Be sure to discharge this capacitor before working on the unit.

The transformer comes apart very easily. Remove it from the flash unit. Unwrap the tape around the core and heat the core on the barrel of your soldering iron for a short time. The wax holding the core together will soften. Using rags to protect your fingers, gently open the core. While the core is still hot wipe off as much wax as you can.

Be gentle as the ferrite and former are delicate. Remove the windings from the former. Wind on 24 turns of 0.3mm or 0.4 mm enamelled copper wire. Wire size is not too critical. Reassemble the core onto the former and wrap with insulation tape.

When testing, use a 150Ω resistor as the load. The voltage across the load will be close to 3V. If operating the torch from a 3V battery, increase the current sense resistor to 27Ω.

Duncan Graham,
Hamilton, NZ. ($40)

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