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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.

Lithium-polymer peak charger

This circuit was developed to charge the Lithium-Polymer cells used in a model aircraft.

Lithium-Polymer cells are incredibly lightweight compared to Nicad battery packs of the same voltage and amp-hour rating. Their only drawback is that they require a rigid charge and discharge regime to achieve maximum life. The most important points of note are as follows:

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(1). They should be charged using a constant-current, constant-voltage method, which stops the charge once the current has dropped to about the C/10 rate. For example, for an 800mAh pack, charging should be terminated once the current falls to approximately 80mA.

(2). They should never be discharged below 3V per cell otherwise they will be permanently damaged.

(3). They should not be charged or discharged above their rated current otherwise an explosion and fire can result!

To initiate charging, the momentary "Start" button (switch S1) is pressed, closing the relay contacts and connecting the battery pack to the output of REG1. The circuit will then charge two 800mAh cells in series at a constant current of 600mAh until they reach a peak terminal voltage of 4.2V per cell (nominal terminal voltage for these cells is 3.7V).

REG1 and transistor Q2 form a current-limited voltage regulator. When the charge current exceeds about 600mA, the voltage developed across R7 turns on Q2, which in turn pulls the adjust terminal of REG1 towards ground. This shunts the voltage adjustment resistance chain formed by VR2 and R3, thereby limiting the output to 600mA.

When the battery voltage reaches about 8.4V, the regulator limits any further voltage increase, as set by VR2. The charge current will then slowly decrease as the cells reach full capacity. As a result, the voltage across R7 also falls, until the bias voltage on the base of Q1 is too small to keep it in conduction. When Q1 turns off, the relay also turns off, isolating the fully charged battery.

The charger is set up as follows:

(1). Connect it to 12V DC and place a digital voltmeter between the output of REG1 and the negative output for the battery pack. Adjust VR2 for a reading of 8.4V.

(2). Adjust VR1 so that the voltage on the base of Q1 is at maximum.

(3). Place an ammeter in series with the battery to be charged and press the "Start" button. The output current will shoot up to around 600mA, then slowly decrease over the next one to two hours.

Once it falls to around 80mA (or whatever the C/10 rate is for your cells), slowly turn VR1 until the relay switches off and indicator LED goes out.

The circuit should now charge your battery packs to within 97% of their rated capacity.

Finally, note that in most cases, REG1 will need to be fitted with a heatsink.

Wayne Robjent,

Tuart Hill, WA. ($50)

Efficient fan speed controller

A partial solution to quietening noisy PCs can be to reduce the speed of internal cooling fans. Low-cost fan speed controllers are available, but they often employ inefficient, heat-generating linear regulators and contain no temperature feedback mechanism.

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This idea makes use of a readily available, cheap in-car mobile phone charger. The majority of these use common circuitry and require only minor modifications to operate as efficient fan speed controllers complete with temperature feedback.

Most in-car chargers are based on the well-known MC34063 DC-DC switchmode IC. When used for charging mobile phones, the open-circuit output voltage is typically set to between 7V and 9V. This is achieved with a simple voltage divider across the output, the centre point of which connects to the feedback input (pin 5) of the MC34063.

To make the output voltage var-iable with air temperature, first replace the upper resistor of the divider with a 4.7kΩ resistor in series with a 4.7kΩ trimpot. The lower half of the divider is then replaced with a 470Ω resistor in series with a 500Ω NTC thermistor. These values are only a guide and can be varied to suit different thermistor and fan types. Note that component lead length should be minimised to avoid introducing noise into the feedback circuitry.

Getting the correct fan starting voltage is a matter of trial and error. The values shown on the circuit give a starting voltage of about 6.8V at room temperature but trimpot VR1 can be used to raise this voltage as necessary. The output can then rise to about 10V if the interior temperature rises sufficiently.

The 4.7kΩ resistor could be reduced to 3.9kΩ and VR1 adjusted to give a lower starting voltage if the fan speed is still too high at 7V.

After running for one hour or so, the fan voltage as set by the interior case temperature thermistor on my PC settled at 7.4V.

Suitable chargers are available from Oatley Electronics, Cat. No. 2D0074. They’re currently listed at $5 for two, which is less than the price of the MC34063 ICs alone! Data on the MC34063 can be downloaded from and a useful development aid is to be found at

Finally, note that not all chargers have an output filter capacitor installed. Typically, this is a 220µF 10V or 16V electrolytic type. To save a few cents, the manufacturers sometimes leave this component out, relying on the mobile’s battery to perform the filtering task. If this component is missing from your charger’s PC board, it should be installed before the supply is used.

Brad Sheargold,

Collaroy, NSW. ($35)

Simple white noise generator

This two-transistor white noise generator has a surprising feature – about 30dB more noise than the more traditional designs.

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Q1 and Q2 can be any small-signal transistors with a beta of up to 400. The reverse-biased emitter-base junction of Q1 provides the noise source, which is fed into the base of Q2. Q2 forms a simple amplifier with a gain of 45dB. The improved output level is due mainly to the inclusion of C1, which provides a low-impedance AC source to the noise source while not disturbing the DC bias of Q1.

The low amount of feedback also makes this circuit very resistant to oscillations and tolerant to circuit layout. Unfortunately, the truism of "no such thing as free lunch" also applies; C1 makes the circuit very sensitive to power supply ripple.

David Eather,

Camp Hill, Qld. ($25)

PICAXE-based toy traffic lights with battery saver

With help from a PICAXE-08 micro, this trivial circuit is all that’s needed to build a nifty set of LED toy traffic lights. The circuit and associated program also demonstrate a technique to achieve long battery life in circuits that must periodically monitor a port input. (Program available to download at the bottom of the article).

Pushbutton switch S1 functions as both a power switch and a user input. When the unit is off, it can be turned on by pressing and holding S1 until all the LEDs come on (this may take up to two seconds). While the unit is operating, pressing the button briefly reduces the "red" period from 30 seconds to 3 seconds. Alternatively, pressing and holding S1 for 3 seconds turns the unit off. It also switches itself off automatically if S1 is not pressed for 5 minutes.

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These functions are implemented in the program as follows:

When the unit is "off", the program alternates between polling S1 and SLEEPing for 2 seconds. When the unit is "on", the program polls S1 continually to determine the amount of time it is pressed or not pressed. If the preset time limit is exceeded for either condition, the unit goes into the "off" state.

Of particular interest is the method used to terminate the 10kΩ pull-down resistor for the switch input. It is usual to connect one end of this resistor to ground, so that pressing the switch changes the normally-low input on pin 3 (leg 4) to a high. However, with one end of the resistor grounded, the circuit would draw about 0.5mA while S1 is pressed. If the switch is continually held down, such as might happen when the unit is tossed into a toy box, the battery would eventually run flat.

To eliminate current drain, the resistor is connected to pin 4 (leg 3) of the PICAXE instead of ground. This pin is programmed as an output and set low while S1 is being polled. At other times, the pin is set high so that no current will flow regardless of whether S1 is pressed or not.

This technique can also be used to reduce the standing current consumption when analog input devices such as LDRs must be periodically sampled.

The unit draws about 9uA in the off state, even if S1 is pressed, so the batteries should last for their shelf life.

Andrew Partridge,

Kuranda, Qld.

12V halogen dimmer

I use a 12V 20W halogen lamp (MR16) and a 4.2Ah SLA battery for my bike light system. The battery has only limited life at this power rating, so I designed this cheap light dimmer to reduce the battery drain and allow for longer rides at night.

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Based on a simple 555 timer circuit and Mosfet switch Q1, it works by pulse-width modulating the 12V supply to the lamp. The 555 (IC1) is wired as a free-running oscillator, with two different mark/space ratios selectable via a 2-pole, 5-position rotary switch (S1).

The third switch position bypasses the electronic circuitry and connects the lamp directly to battery negative. This gives three power levels of about 7W, 13W and 20W.

A logic-level IRL530N Mosfet with a drain-source "on" resistance of only 0.1Ω ensures low losses and eliminates the need for a heatsink. An STP30NE06L Mosfet (Jaycar
Cat. ZT-2271) would also be suitable.

Mike Dennis,

Canberra, ACT. ($30)


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