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Circuit Notebook

(1) Low-Cost Dual Digital Dice; (2) Maximum/Minimum Voltage Indicator; (3) Halogen Lamp Dimmer With Soft Start; (4) Correction - 100V Line Connection For The SC480 Amplifier.

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.

Low-cost dual digital dice

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This simple dual digital dice is based on three low-cost ICs, a few transistors and a handful of LEDs. IC1a & IC1b operate as an oscillator with a frequency of about 4kHz and this clocks IC2. The frequency of oscillation is not critical - it simply needs to be high enough to prevent cheating.

IC2 and IC3 are 4516 binary counters, configured to count in binary from 1-6. A power-on reset is not required here since, if the initial state is outside the correct range, the counters will count into the correct range after a few clock pulses.

Let's first consider how IC2 operates. When the counter reaches "7" (ie, 111), the AND gate formed by diodes D1 & D2 and the 47kΩ resistor applies a high to the PE pin (pin 1). This presets the counter to 1 (ie, 001) and so PE goes low again. The counter then increments in the normal manner until it reaches "7" again.

Counter IC3 operates in the same manner except that the clock signal is derived from IC2's O3 output.

The counter outputs (O1, O2 & O3) drive NPN transistors Q1-Q6 and these in turn drive the LEDs (ie, the LEDs indicate the states of the counters). Normally, the counters are incrementing continuously and the LEDs all appear to be lit. However, when pushbutton switch S1 is pressed, pin 6 of IC1c goes low and pin 9 of IC1d pulls the Ci input of IC2 high, thus stopping the counters.

Finally, toggle switch S2 allows the user to choose between having two dice operating simultaneously or just one.

Len Cox,
Forest Hill, Vic. ($40)

Maximum/minimum voltage indicator

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This circuit indicates which of three voltages in the range from about about -4V to about +4V - at A, B and C - is the highest by lighting one of three indicator LEDs. Alternatively, it can be wired to indicate the lowest of three voltages or to indicate both the highest and lowest voltages.

Op amps IC1a, IC1b & IC1c are wired as comparators, while the three indicator LEDs and their series 1kΩ current limiting resistors are strung across the op amp outputs to implement the appropriate logic functions. For example, LED A will light only when pin 8 of IC1c is low (ie, A > B) and pin 7 of IC1b is high (ie, A > C).

Similarly, LED B will light only when pin 8 of IC1c is high (ie, B > A) and pin 1 of IC1a is low (ie, B > C). LED C works in similar fashion if the voltage at C is the highest.

Note that if all the LEDs and their parallel 1N4148 diodes are reversed, the circuit will indicate the lowest of the three input voltages. And if each 1N4148 diode is replaced by a LED, the circuit will indicate both the highest and lowest inputs.

Andrew Partridge,
Kuranda, Qld. ($30)

Halogen lamp dimmer with soft start

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Most dimmers use pulse width modulation (PWM) to control the amount of power that is delivered to the lamp. Those that come bundled with a switch faceplate control the firing angle of a Triac on the 240V mains side. These work fine with resistive loads but may not be suitable for inductive loads such as low-voltage halogen lamp transformers.

This circuit also employs PWM but it switches at a high frequency (22kHz) on the low-voltage side of the lamp transformer. This high frequency also simplifies EMI filtering. Furthermore, because this circuit is isolated from the mains by the transformer, it is relatively safe to build and install.

IC1 is a standard 555 astable oscillator with a high duty cycle. It produces a narrow negative-going pulse at its pin 3 output approximately every 45μs (ie, the frequency of oscillation is about 22kHz). These pulses trigger IC2, another 555 timer, this time wired as a variable monostable.

IC2's pin 3 output is normally low which means that its internal discharge transistor is on and the 1nF capacitor on pins 6 & 7 is discharged. However, when the monostable is triggered (by IC1), its output goes high, the internal discharge transistor turns off and the 1nF capacitor charges via VR1 & VR2 until it reaches 2/3Vcc. At this point, the output at pin 3 switches low again.

Each time pin 3 of IC2 goes high, it turns on power Mosfet transistor Q1 which in turn switches on the lamp.

Potentiometer VR2 is used to control the time it takes the 1nF capacitor to charge to the threshold voltage and thus sets the width of the output pulses. At maximum resistance, the pulse width is 55ms. This is longer that the 45ms period of oscillator IC1, and so IC2's pin 3 output is high for 100% of the time and the lamp operates with maximum brightness.

Now consider what happens if the monostable's period is shorter than the astable's. In this case, each time IC1's pin 3 output goes low, pin 7 of IC1 also goes low and discharges IC2's 1nF timing capacitor via D3. This retriggers the monostable.

As a result, IC2 is triggered at a 22kHz rate and produces variable width pulses depending on the setting of VR2. It's output in turn pulses Q1 to control the lamp brightness.

D2 isolates IC1's timing circuitry from IC2's. VR1 is used to set the minimum lamp brightness when VR2 is at minimum resistance. If this control is not required, VR1 can be replaced with a 1.8kΩ resistor.

The 220μF capacitor on pin 5 of IC2 provides a soft-start facility to prolong lamp life. Initially, when power is first applied, the 220μF capacitor is discharged and this lowers the threshold voltage (which is normally 2/3Vcc). That in turn results in shorter pulses at the output. As the 220μF capacitor charges, the threshold voltage gradually increases until the circuit operates "normally".

For the prototype, Q1 was a BUK553-60A, rated at 60V, 20A & 75W. Q1's maximum on-state resistance is 0.1Ω, so switching a 4A lamp load results in a maximum power dissipation of 1.6W. The bridge rectifier comes in at around 5W and so both should be mounted on suitable heatsinks. The power dissipation in the bridge rectifier can be reduced by using power Schottky diodes rated at 5A or more.

The output of 555 timer IC2 is capable of directly driving several Mosfets (up to four in tests). Note, that if the Mosfet is going to be some distance from the 555, it will be necessary to buffer it.

Power for the control circuitry is derived from 3-terminal regulator REG1 which produces an 8V rail. This in turn is fed from the output of the bridge rectifier via diode D1

Dennis Chuah,
Waitakere City, NZ. ($50)

Correction - 100V line connection for SC480 amplifier

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The circuit showing how to connect a 100V line transformer top the SC480 module in last month's issue was incorrect. It showed a 1N5404 diode connected from -40V to 0V. It should have connected to the amplifier output instead. The circuit is presented correctly here.

SILICON CHIP

Contibute and win!
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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: starting next month, the best contribution will win one of these superb Peak Atlas LCR Meters - valued at around $195.00.

So don't keep that brilliant circuit secret any more: send it to SILICON CHIP and you could be a winner!

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