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Circuit Notebook

(1)Curve Trace Adaptor; (2)Logic Probe With Sound; (3)7.2V Battery Replacement For Camcorders; (4)Stroboscope Uses White LEDs

Curve tracer adaptor

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This unit employs a dual trace oscilloscope with X-Y function as a display to test and demonstrate the action of circuits and components such as transistors, diodes, zener diodes, and terminated and unterminated transformers.

A low frequency sinewave (ie 10Hz - 1kHz) is applied to op amp IC2a via potentiometer VR1 to set the "X" and "Y" levels for the X-Y display on the scope. The output of IC2a is applied to the X input via R4 and IC2b and also to Probe 1 via the contacts of relay 1. IC2b provides a low impedance drive for the X input and also isolates the X input cable capacitance from probe 1.

The current flowing into the probes develops a voltage across R4 which is processed by IC2d and applied to the CRO Y input to represent current.

The scope display thus represents an X-Y graph where voltage across a circuit under test is displayed on the X axis (horizontal) and the current though it displayed on the Y axis (vertical). With a calibrated scope this equates to 1mA/V.

IC1 and a relay are included to enable two probes to be used and comparisons made between a known good device and a faulty one. The relay should be a low capacitance reed type.

By using the scope’s X and Y gain controls, the sinewave applied to the device under test should be adjustable from a few millivolts up to 24V peak-peak to get a very useable display. Thus, the unit can be used on voltage sensitive devices and at the other end of the scale apply enough voltage to check the operation of, say, a 10V zener diode.

Note that all devices should be tested in the unpowered condition. If used for in-circuit tests, the effects of circuit components will need to be taken into account.

Shielded coax leads should be used for the X and Y inputs and the probe leads should have zero resistance. Normal scope probes should not be used as these usually have significant built-in resistance which will interfere with measurements.

R. Rayner,
Willow Vale, NSW.

Logic probe with sound

This logic probe can be selected to operate on TTL or CMOS logic levels, depending on switch S1. A string of resistors associated with switch S1 sets the threshold levels for a window comparator comprising IC1a and IC1b. Depending on whether the level applied to the probe is high or low, the window comparator turns on LED1 (high) or LED2 (low). The 1.2M and 680k resistors set the probe signal to a midrange value when the probe is open-circuit, thereby preventing either LED from being lit.

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If a pulse signal is present, the output of IC1a will toggle the clock input of flipflop IC2a. This drives LED3 which either lights for each pulse or continuously, depending on the setting of switch S2.

Finally, the outputs of IC1a & IC1b are connected by diodes D5 & D6 to the base of transistor Q1 which is connected to the Reset input of flipflop IC2b. This has a piezo sounder (not buzzer) connected between its Q and Q-bar outputs so that it produces a sound which echoes the input pulse signal.

Tom Hughes,
North Canterbury, NZ. ($40)

7.2V battery replacement for camcorders

This circuit lets an external 12V SLA battery power a camcorder which normally has an inbuilt 7.2V battery. Such batteries can now be very difficult or expensive to obtain for earlier model camcorders.

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In essence, the circuit is a standard LM317 adjustable regulator with resistors R1 & R2 set to provide 7.2V (depending on the accuracy of the 1.25V internal reference). If the resulting output voltage is low, it can be increased by reducing the 130 resistor and vice versa.

The circuit can be assembled on to the Eliminator PC board, as featured in the May 1992 issue (48 x 61mm, code 04104921) or the simple DC power supply PC board, featured in the March 2004 issue (36.8 x 68.6mm, code 04103041). The regulator should be fitted with a flag heatsink.

Note that the circuit should be disconnected from the battery when not in use, otherwise its quiescent current (from the LED and regulator) will flatten the SLA battery.


Stroboscope uses white LEDs

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This stroboscope circuit uses 16 high-brightness white LEDs in a torch housing and it provides a signal output to a frequency counter to provide a rev counter display.

IC1 is 555 astable multivibrator and it provides a signal to IC2, a 4046 phase lock loop. IC2 and the two 4017 Johnson decade counters, IC3 & IC4, make up a frequency multiplier with a factor of 60 (IC3 divides by 10 while IC4 divides by six). The multiplied frequency is taken from the VCO (voltage controlled oscillator) output of IC2 at pin 4 and this becomes the signal to drive the frequency counter. Its output reading is the speed of the shaft being measured in RPM.

A narrow positive-going pulse train to turn on Q1 and the LEDs is obtained from pin 3 of IC4. This has the advantage of giving a much sharper marker line (on the shaft) illumination.

The unit can be powered from a 12V 500mA plugpack or a suitable battery.

(Editorial note: at switching frequencies above 100Hz (6000 RPM) the persistence of the phosphor of the white LEDs will make the circuit ineffective. To run the circuit at much higher frequencies, substitute LEDs without phosphors; eg, red, green or yellow or a mixture of these).

K. J. Benic,
Forestville, NSW. ($40)

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