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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Updated Aug 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This is the circuit of a £1 screwdriver voltage tester...

ROLSTON's ORIGINAL VOLTMETER SCREWDRIVER
If you have not seen one of these, they have a small LCD panel in the flattened black handle.
_________________________________
/ ~~~---...___
│ -- -- ┌──────────┐ __ ~~~---...
│ |()|<│ |()|<│ │ │ ( ) ├─────┐
│ -- -- └──────────┘ ~~ ___...--┘~~~~~~
\ ___...---~~~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

CIRCUIT
______________________________________
│com L C D │
│ 220V 110V 55V 36V 12V ~ │
DIRECT └-┬-----┬-----┬-----┬-----┬-----┬----┬-┘<- Conductive Mats
TEST │ │ │ │ │ │ │
██─22M──22M─┴─M92─┴─M91─┴─1M2─┴─2M7─┴─10M─┴────)─────┬─=======>SCREWDRIVER
│ NEON
██─────────────────────────────────────────────┴─2M7─┘
INDUCTIVE
BREAK TEST

DIRECT TEST
When you touch this button (earth) & prod the testing voltage, it indicates if the voltage is greater than the displayed graphics. This occurs when any of the LCD sections sees greater than about 1V. As the LCD current required for any of the display sections is only the odd pA very high value resistors can be used to make up the multiplier chain. This means the return circuit can safely be made though ones finger to earth.

INDUCTIVE BREAK TEST
A neon is also fitted for a Inductive Break-point Test. (what ever that is? To do with Fluorecent lamps?) It lights when voltages are above 80V, above 2V you just see the ~ symbol on the LCD.

N.B. with this circuit, this test is not too safe, as current COULD flow through the LCD to you! So I don't recommended you use the neon option as a mains tester!


G8MNY's MODIFIED VERSION
To open the unit to improve the calibration & safety etc, pop out the back panel cover & ...

1/ Change one of the 22M to a 10M (100V) to increase the sensitivity on DC.
2/ Then add a 270pF (20v) cap to reduce the sensitivity on AC.
3/ Add a 220k (250V) into the inductive break test circuit.

______________________________________
│com L C D │
│ 220V 110V 55V 36V 12V ~ │
DIRECT └-┬-----┬-----┬-----┬-----┬-----┬----┬-┘<- Conductive Mat
TEST │ │ │ │ │ │ │
██─22M──10M─┬┴─M92─┴─M91─┴─1M2─┴─2M7─┴─10M─┴┬───)─────┬─=======>SCREWDRIVER
└────────────┤├─────────────────┘ │ NEON
270pF │ │
██─────────220k─────────────────────────────────┴─2M7─┘
INDUCTIVE
BREAK TEST

Now test, & you should find the accuracy better & from DC-50Hz. And there is
no danger of electrocution if the neon indicator is used on Mains.


Why Don't U send an interesting bul?

73 De John G8MNY @ GB7CIP

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