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Circuit Notebook

(1) Signal Meter For Satellite Reciever; (2) Car Battery Failure Detector; (3) Switch Timer For Bathroom Light; (4)Model Theatre Lighting Dimmer; (5)Fully Adjustable Power Supply; (6)4-wire stepper Motor Driver.

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.

Signal meter for weather satellite receiver

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The VHF Weather Satellite Receiver described in our December 2003 issue was provided with just a simple LED indicator to show received signal strength, to keep the size and cost low. However, you can connect an external digital panel meter, if you want a more accurate signal strength indicator.

Only minor changes are required to the receiver circuit, assuming that you will be using a digital panel meter with an input sensitivity of 200mV FSR (full-scale reading) and with an input resistance of 10MΩ or more.

The existing 390kΩ resistor connected between pin 7 of IC1 and earth is changed to 620kΩ (R1). Two resistors (R2 and R3) are added in series, so that they form a 51:1 voltage divider across R1. The output from the divider is taken to a 2.5mm jack socket, which is mounted in a convenient position on the receiver’s rear panel.

When the receiver is operating, the voltage across R1 varies between about +0.26V and +5V, according to the received signal strength. As a result the output voltage provided at the meter jack also varies, from about +5mV up to about 100mV. This means that the panel meter will give a signal strength reading that will vary between ‘5’ (no signal) up to ‘100’ (maximum signal).


Car battery failure detector

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A car battery deteriorates in use and its life seldom exceeds four years. When new, its voltage may drop to only 2V while cranking
the engine. As the battery ages, its internal impedance increases and so the voltage drop while cranking also increases, until ultimately the drop is high enough to prevent the engine from starting.

This gradual increase in voltage drop while cranking can be used as an early warning of looming battery failure and so this circuit triggers an alarm when the battery voltage drops to 8V during cranking.

The circuit is based on a battery voltage indicator published in the September 1995 issue of "Electronics Australia" and available from Jaycar as a kit (Cat. KA-1778).

IC1 is a precision 2.5V device used as the reference for two comparators based on IC2, an LM358 dual op amp. IC2a monitors the voltage from trimpot VR1 and normally its output at pin 1 will be low while the output of IC2b will be high and LED1 will be green.

When pin 2 of IC2a falls below pin 3, its output at pin 1 will go high to drive the red section of LED1 to indicate a fault. At the same time, IC2b inverts the signal from pin 1 and its output at pin 7 goes low and turns off the green section of LED1 to indicate a fault.

Since the battery voltage drop occurs momentarily while cranking, a more permanent indication of the fault is provided by flashing LED2. When IC2a’s output goes high momentarily, the SCR is latched and LED2 flashes and can only be deactivated by pressing pushbutton S1.

Victor Erdstein,
Highett, Vic. ($25)

Switch timer for bathroom light

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This 9-minute timer switch can be used to control the light in a toilet or bathroom.

The timer is started by pushing S1 and stopped by pushing S1 again. If you forget to turn it off, the controlled light will go off after nine minutes. If you need the light on continuously non-stop, you need to press S1 (turn on) and then S2 (cancellation of timer) within 9 minutes and in this case the light will be on until you switch it off with S1.

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IC1 is a is 4013 dual flipflop. Flipflop IC1a is toggled on and off by switch S1 and it controls the relay which is switched by FET Q2.

IC1a controls IC1b which is connected as an RS flipflop to enable or disable IC2, a 4060 oscillator/divider. This has its timing interval set by the components at its pins 9, 10 & 11.

The relay should have 250VAC mains-rated contacts and these are connected in parallel with an existing wall switch.

Rasim Kucalovic
Liverpool, NSW.

Model theatre lighting dimmer

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This circuit is the basis for the dimmers in a model theatre lighting system which uses torch globes as the light source. The circuit is based around a 555 timer (IC1), driving a Triac.

All dimmers share the one power supply and zero-crossing detector. As it will only work if there is a common AC/DC return path, it has a simple DC supply circuit consisting of one 1N4004 diode and one 4700μF capacitor.

Transistors Q1-Q3 comprise a zero-crossing detector whose output is inverted into a negative-going pulse by Q4. This pulse is fed to the trigger input (pin 2) of the 555 IC which then starts its timing period at the beginning of each mains half cycle.

The length of this period is set by a 220nF capacitor, a 1kΩ resistor and trimpots VR1 and VR2. The output of IC1 at pin 3 is then fed to transistor Q5 which inverts this signal to trigger the Triac via a 100Ω resistor.

When the timing period is short, the Triac is turned on early in each half cycle and the lights are bright. Conversely, when the timing period is longer, the lights are dim or turned off. The main dimmer control is potentiometer VR1.

Trimpot VR2 is used to set the range of VR1. With VR1 set fully clockwise (ie, maximum resistance), trimpot VR2 is adjusted until the lights are just turned off. The lights should then be able to be faded over the full range by the control potentiometer.

Barry Freeman,
Morphett Vale, SA. ($40)

Fully adjustable power supply

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Based on a National Semiconductor application note, this circuit uses an LM317 3-terminal regulator (REG1), chosen because of its built-in over-current and over-temperature protection. Its output is boosted up to just over 5A by the MJ2955 transistor (Q1).

The output voltage is varied by adjusting the voltage on REG1’s ADJ terminal using VR1 (a 10kΩ potentiometer), via the 270Ω resistor.

Adjustable current limiting is provided by op amp IC1, used as a comparator, which monitors the voltage across the 0.1Ω current sensing resistors. Once this voltage exceeds a level set by potentiometer VR2, then its output goes low, dragging down the adjust pin of REG1 and thus the output voltage. LED1 illuminates when current limiting is occurring.

The 10kΩ voltage adjust potentiometer (VR1) has one side connected to -5V instead of 0V so that the output voltage can be varied down to 0V instead of 1.2V (normal limit of an LM317). Trimpot VR3 is adjusted to set the minimum output voltage to +100mV or so.

Note that because the -5V rail is used as a reference, it should be regulated using an LM7905 or similar.

The LM317 3-terminal regulator and Q1 should be mounted on the same heatsink to take advantage of REG1’s thermal control.

Philip Chugg,
Rocherlea, Tas. ($40)

4-wire stepper motor driver

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This circuit enables the 6-wire stepper motor driver circuit from the May 2002 issue of SILICON CHIP to drive a 4-wire stepper motor, which is widely available in scrapped Epson or
Cannon inkjet printers. A 4-wire stepper motor requires bi-directional current drive which is provided by adding four P-channel IRF9530 Mosfets to the original circuit to form two H-bridge driver circuits. The original IRFZ44Ns can be replaced with IRF530s.

Note that the input voltage applied to the circuit should be about 12V. If a higher voltage is required, a separate driver for each Mosfet may be needed.

Paul Chen,
Eastwood, NSW. ($35)

Contribute and Win!

As you can see, we pay good money for each of the "Circuit Notebook" contributions published in SILICON CHIP. But now there’s an even better reason to send in your circuit idea: each month, the best contribution published will win a superb Peak Atlas LCR Meter valued at $195.00.

So don’t keep that brilliant circuit secret any more: sketch it out, write a brief description and send it to SILICON CHIP and you could be a winner!

You can either email your idea to or post is to PO Box 139, Collaroy, NSW 2097.

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