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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.

Solar battery protector prevents excessive discharge

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This circuit prevents the battery in a solar lighting system from being excessively discharged. It's for small systems with less than 100W of lighting, such as several fluorescent lights, although with a higher rated Mosfet at the output, it could switch larger loads.

The circuit has two comparators based on an LM393 dual op amp. One monitors the ambient light so that lamps cannot be turned on during the day. The second monitors the battery voltage, to prevent it from being excessively discharged.

IC1b monitors the ambient light by virtue of the light dependent resistor connected to its non-inverting input. When exposed to light, the resistance of the LDR is low and so the output at pin 7 is low.

IC1a monitors the battery voltage via a voltage divider connected to its non-inverting input. Its inverting input is connected to a reference voltage provided by ZD1. Trimpot VR1 is set so that when the battery is charged, the output at pin 1 is high and so Mosfet Q1 turns on to operate the lights.

The two comparator outputs are connected together in OR gate fashion, which is permissible because they are open-collector outputs. Therefore, if either comparator output is low (ie, the internal output transistor is on) then the Mosfet (Q1) is prevented from turning on.

In practice, VR1 would be set to turn off the Mosfet if the battery voltage falls below 12V.

The suggested LDR is a NORP12, a weather resistant type available from Farnell Electronic Components Pty Ltd.

Michael Moore,
Beecroft, NSW.

Radio controlled electronic flash

A radio controlled electronic flash is a useful item in any photographer's kit. Professionals use them all the time. For example, a wedding photographer would put one behind the bride to back-light her gown and veil. You don't want wires showing in a shot like that.

To build this control you will need an old R/C car (the simplest sort) in which the car runs in reverse at switch-on and goes ahead only when the remote is operated. They can be picked up cheaply as school fetes and garage sales. A typical car will run from 3V (two cells) and use 9V in the transmitter.

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Before proceeding, make sure that the electronics in the car are operating. It doesn't matter if the wheels are broken or the motor is dead. You need to gain access to the leads to the motor. Normally (ie, without the remote operating), one is positive with respect to the other. Label them accordingly. On pressing the remote button, the polarity of the motor leads should swap.

You will also need a flash extension cord you can cut into two sections.

At the transmitter, the camera end of the extension cord is fed into the case and soldered to the control button contacts, as shown in Fig.1. The contacts are in series with the battery supply, so if you don't want to open the transmitter, just cut one of the battery leads and connect the flash extension cord into the gap so created.

You will then need to tape down the remote button so that it is permanently operated (ie, closed).

All that needs to be done at the receiver end is to connect the normally negative motor lead to the gate circuit of an SCR, as shown in Fig.2, while the normally positive lead goes to the cathode of the SCR. Now, when the transmitter is operated by the camera's contacts, the lead polarity is
reversed and the SCR acts as a switch to fire a portable electronic flash via the other half of the flash extension cord.

The transmitter can be attached to the camera via a flash bracket or a screw into the tripod socket, depending on what is the most convenient arrangement.

The added components in the receiver can be mounted on Veroboard and housed in the space where the electric motor was. If appearance is a primary consideration, the receiver and the added components could be mounted in a standard jiffy box.

Finally, a note of caution: when connecting the flash end half of the extension cord to the SCR, make sure that it is the positive wire which goes to the anode of the SCR. Flash cords do not always have the centre wire connected to the centre pin of the plug. The centre pin of the lead on the flash unit will be positive and this must connect to the anode of the SCR via the lead connected to the R/C receiver.

A. J. Lowe,
Bardon, Qld. ($40)

Luminescent generator

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When spun rapidly between the fingers, a bipolar stepper motor will generate around 10VAC. If this is stepped up with a small 240V to 6-0-6V transformer in reverse (with series connected secondaries), a small bipolar stepper motor is capable of powering a standard 5cm by 6cm luminescent sheet at full brightness. These are designed to be powered from 20V to 200VAC (typically 115VAC), producing 1.5 candelas of light - which will dimly light the average room, or adequately light a camp table. They are manufactured by Seikosha (RS Components Cat. 267-8726).

The transformer should be a
small one (around 100mA or so), otherwise efficiency is compromised. The wires of the motor's two phases are usually paired white & yellow and red & blue. Just one of these phases is employed in the circuit. If a small bipolar stepper motor from a discarded 3.5-inch disk drive is used, the Luminescent Generator may be built into a very small enclosure. To sustain rapid, smooth spinning of the motor, a geared handle may be added.

Thomas Scarborough,
Cape Town, SA. ($30)

Neon flasher runs from 3V supply

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A neon indicator typically requires at least 70V to fire it and normally would not be contemplated in a battery circuit. However, this little switchmode circuit from the Linear Technology website ( steps up the 3V battery supply to around 95V or so, to drive a neon with ease.

The circuit has two parts: IC1 operating as step-up converter at around 75kHz and a diode pump, consisting of three 1N4148 diodes and associated .022μF capacitors. The 3.3MΩ resistor and the 0.68μF capacitor set the flashing rate to about once every two seconds.

The average DC level from the diode pump is set to about 95V by the 100MΩ feedback resistor to pin 8.

The circuit could also use an LT1111 (RS Components Cat 217-0448) which would run at about 20kHz so L1 could be reduced to 100mH and use a powdered iron toroid core from Neosid or Jaycar.


Isolation for PC boards in cars

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These two mounting methods were devised to protect PC boards from vibration when installed in the engine compartment of a car. They could also be used in other applications where vibration is a problem.

Method 1 involves rigidly mounting the PC board inside a diecast box and then mounting the box itself to provide vibration isolation. As shown, small grommets are installed in suitably sized holes in the sides of the box. The box is then secured to angle mounting brackets using M4 screws, washers and nylock nuts.

Method 2 involves mounting the diecast case onto the chassis of the car and then mounting the PC board as shown, using M3 screws, grommets, hollow spacers and nylock nuts. In this case, the grommets are fitted into suitably sized holes in the PC board itself. Once the nuts are tightened, the PC board should be able to move slightly, relative to the box.

If there is not enough space on the board to fit the grommets, then Method 1 is the way to do it.

David Boyes,
Gordon, ACT. ($35)

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