Proximity Sensor
"Magic Wire" is so called because of detecting proximity to antenna.
THE MAGIC WIRE
As shown in the diagram, the input tube is a type 6R7 duo-diode triode. The triode section forms the oscillator, in conjunction with the coil L1 which is center-tapped to the cathode. When the triode section is oscillating, the r.f. voltage developed from cathode to ground is impressed on the diode section, causing current to flow through R2 and making the diode plates negative with respect to ground. The control grid of the 25L6 power tube is connected to the diode plates of the 6R7 and consequently a negative bias is placed on the grid which reduces its plate current to a very low value. As soon as the triode ceases to oscillate, there is no longer any r.f. voltage applied to the diodes, the voltage drops and the 25L6 draws high plate current, causing the relay to operate.
It will be noted that no rectifier tube or filler circuit is required in this design, yet the instrument functions on either a.c. or d.c. On a.c., the 6R7 oscillations and the 25L6 draws plate current only on the positive half-cycles. This principle effects a considerable saving in construction cost and in the size of the instrument.
After the parts required have been obtained, the first step in building the unit is to make the chassis, which consists simply of a piece of 16-gauge aluminum or steel bent and drilled in accordance with the plan shown. The front panel, which is included with the standard 6 by 6 cabinet, is drilled and a hole and grommet are placed in the rear panel. The oscillator coil is made by winding 100 turns of No.28 d.c.c. wire on a one-inch bakelite tube 3-1/4 inches long. A tap is brought out at the center of the winding. When the winding has been completed, the entire coil is dipped in a hot half-and-half mixture of beeswax and paraffin to keep the winding in place and exclude moisture. The sensitivity of the outfit is largely dependent upon the efficiency of the coil, so it should be carefully made. C1 is mounted on a small piece of 1/8-inch bakelite, because it must be insulated from the panel.
Wire the chassis first, starting with the heater circuits. Do not connect in the power cord until all wiring has been completed. The shield of the 25L6 is connected to its cathode, the shield of the 6E7 to the heater terminal which goes directly to the line. When all the main wiring has been completed, bring the power cord through the rear panel hole, and solder the three terminals to the terminal strip. The antenna wire is brought in through a rubber-grommeted hole in the top of the cabinet and connected to the stator or plate terminal of C1. A knot in the wire will relieve any strain on this connection. Stranded wire is preferred for the antenna.
The capacitances of C1 and C2 are largely dependent upon the length of antenna wire desired. If only 4 or 5 feet are required, C2 may be omitted. On the other hand, if the wire exceeds 15 feet, C2 will have to be larger than the value given. If the capacitance of C1 were made large (say 150 mmf. or more), C2 could of course be omitted but then the adjustment would become too critical.
The relay employed is a 3,000-ohm plug-in type of standard manufacture. It is a double-pole model and will handle a non-inductive load of 100 watts. It is somewhat more sensitive than is required and any other good relay of 1,000 ohms or more resistance should be suitable. The capacitor, C4, is shunted across the relay coil to prevent chattering. It may be advisable, in some cases, to put a 0.1 mf. paper capacitor across the relay contacts to stop sparking on heavy loads. It is better practice, however, to use a separate power relay when operating any but light loads.
In operation, the antenna wire is strung out well away from grounded metal objects and a 110-volt lamp is plugged into the outlet on the panel. When the tubes have heated, the lamp should light when the antenna wire is touched. If it lights without touching the wire, C2 should be screwed down until the lamp goes out. These adjustments should be made with C1 about one-half mashed. The panel may then screwed in on the cabinet and final adjustment made. This is done by gradually adjusting the vernier knob of the dial until the light remains lit when adjusting but goes out when the hand is removed from the dial. This may be carried to a point where the light will flash as soon as one approaches within 3 feet of the wire or instrument. It is better not to aim for such sensitivity, though, since it will vary somewhat with line voltage. A good, practical and stable point is about six to fifteen minutes or so for the instrument to acquire a stable point of operation owing to its sensitivity.
PARTS REQUIRED
C1 - Midget variable capacitor, 60 mmf. (see text)
C2 - Trimmer capacitor, 35 mmf. or more (see text)
C3 - Tubular paper capacitor, 0.05 mf. or more, 200 v.
C4 - Electrolytic capacitor, 10 mf., 100 V.
R1 - Carbon resistor, 5 meg, 1 watt
R2 - Carbon resistor, 1 meg., 1 watt
R4 - Wire-wound resistor, 5,000 ohms, 10 watts
R5 - Wire-wound resistor, 10,000 ohms, 10 watts
1 -- Steel cabinet 6x6x6 inches, front & back panels removable
1 -- Piece 16-gauge aluminum, for chassis 5-1/2 x 7-3/4 inches
1 -- Piece bakelite tubing, 1 inch diameter., 3-1/2 inches long
1 -- Piece bakelite, 1'1/2 x 1-1/2, 1/8 inch thick for C1
2 -- Octal wafer sockets, 1-1/2 inches center for mounting holes
1 -- 5-prong wafer socket, 1-1/2 inches center for mounting holes
1 -- Relay, Utah type RAC-110, 3,000 ohm
1 -- 6R7 metal tube
1 -- 25L6 metal tube
1 -- Kurz-Kasch vernier dial, small
1 -- Resistor line cord, 280 ohms (R3)
1 -- Single outlet receptacle
Miscellaneous screws, nuts, mounting bracket, and grommets.
- James P Hughes
Proximity Sensor
"Magic Wire" is so called because of detecting proximity to antenna.
THE MAGIC WIRE
As shown in the diagram, the input tube is a type 6R7 duo-diode triode. The triode section forms the oscillator, in conjunction with the coil L1 which is center-tapped to the cathode. When the triode section is oscillating, the r.f. voltage developed from cathode to ground is impressed on the diode section, causing current to flow through R2 and making the diode plates negative with respect to ground. The control grid of the 25L6 power tube is connected to the diode plates of the 6R7 and consequently a negative bias is placed on the grid which reduces its plate current to a very low value. As soon as the triode ceases to oscillate, there is no longer any r.f. voltage applied to the diodes, the voltage drops and the 25L6 draws high plate current, causing the relay to operate.
It will be noted that no rectifier tube or filler circuit is required in this design, yet the instrument functions on either a.c. or d.c. On a.c., the 6R7 oscillations and the 25L6 draws plate current only on the positive half-cycles. This principle effects a considerable saving in construction cost and in the size of the instrument.
After the parts required have been obtained, the first step in building the unit is to make the chassis, which consists simply of a piece of 16-gauge aluminum or steel bent and drilled in accordance with the plan shown. The front panel, which is included with the standard 6 by 6 cabinet, is drilled and a hole and grommet are placed in the rear panel. The oscillator coil is made by winding 100 turns of No.28 d.c.c. wire on a one-inch bakelite tube 3-1/4 inches long. A tap is brought out at the center of the winding. When the winding has been completed, the entire coil is dipped in a hot half-and-half mixture of beeswax and paraffin to keep the winding in place and exclude moisture. The sensitivity of the outfit is largely dependent upon the efficiency of the coil, so it should be carefully made. C1 is mounted on a small piece of 1/8-inch bakelite, because it must be insulated from the panel.
Wire the chassis first, starting with the heater circuits. Do not connect in the power cord until all wiring has been completed. The shield of the 25L6 is connected to its cathode, the shield of the 6E7 to the heater terminal which goes directly to the line. When all the main wiring has been completed, bring the power cord through the rear panel hole, and solder the three terminals to the terminal strip. The antenna wire is brought in through a rubber-grommeted hole in the top of the cabinet and connected to the stator or plate terminal of C1. A knot in the wire will relieve any strain on this connection. Stranded wire is preferred for the antenna.
The capacitances of C1 and C2 are largely dependent upon the length of antenna wire desired. If only 4 or 5 feet are required, C2 may be omitted. On the other hand, if the wire exceeds 15 feet, C2 will have to be larger than the value given. If the capacitance of C1 were made large (say 150 mmf. or more), C2 could of course be omitted but then the adjustment would become too critical.
The relay employed is a 3,000-ohm plug-in type of standard manufacture. It is a double-pole model and will handle a non-inductive load of 100 watts. It is somewhat more sensitive than is required and any other good relay of 1,000 ohms or more resistance should be suitable. The capacitor, C4, is shunted across the relay coil to prevent chattering. It may be advisable, in some cases, to put a 0.1 mf. paper capacitor across the relay contacts to stop sparking on heavy loads. It is better practice, however, to use a separate power relay when operating any but light loads.
In operation, the antenna wire is strung out well away from grounded metal objects and a 110-volt lamp is plugged into the outlet on the panel. When the tubes have heated, the lamp should light when the antenna wire is touched. If it lights without touching the wire, C2 should be screwed down until the lamp goes out. These adjustments should be made with C1 about one-half mashed. The panel may then screwed in on the cabinet and final adjustment made. This is done by gradually adjusting the vernier knob of the dial until the light remains lit when adjusting but goes out when the hand is removed from the dial. This may be carried to a point where the light will flash as soon as one approaches within 3 feet of the wire or instrument. It is better not to aim for such sensitivity, though, since it will vary somewhat with line voltage. A good, practical and stable point is about six to fifteen minutes or so for the instrument to acquire a stable point of operation owing to its sensitivity.
PARTS REQUIRED
C1 - Midget variable capacitor, 60 mmf. (see text)
C2 - Trimmer capacitor, 35 mmf. or more (see text)
C3 - Tubular paper capacitor, 0.05 mf. or more, 200 v.
C4 - Electrolytic capacitor, 10 mf., 100 V.
R1 - Carbon resistor, 5 meg, 1 watt
R2 - Carbon resistor, 1 meg., 1 watt
R4 - Wire-wound resistor, 5,000 ohms, 10 watts
R5 - Wire-wound resistor, 10,000 ohms, 10 watts
1 -- Steel cabinet 6x6x6 inches, front & back panels removable
1 -- Piece 16-gauge aluminum, for chassis 5-1/2 x 7-3/4 inches
1 -- Piece bakelite tubing, 1 inch diameter., 3-1/2 inches long
1 -- Piece bakelite, 1'1/2 x 1-1/2, 1/8 inch thick for C1
2 -- Octal wafer sockets, 1-1/2 inches center for mounting holes
1 -- 5-prong wafer socket, 1-1/2 inches center for mounting holes
1 -- Relay, Utah type RAC-110, 3,000 ohm
1 -- 6R7 metal tube
1 -- 25L6 metal tube
1 -- Kurz-Kasch vernier dial, small
1 -- Resistor line cord, 280 ohms (R3)
1 -- Single outlet receptacle
Miscellaneous screws, nuts, mounting bracket, and grommets.
- James P Hughes