Abel Sensor Construction Notes

Aidan Hedley August 1998

The sensors I have designed use a coil and capacitor resonant circuit together with two similar circuits on the bell to detect its position. The construction is rather more complex than optical sensors, but the sensor is far more reliable and easier to fi"

We have built six simulators to this design in the Durham & Newcastle Association and all have worked well without any attention for several years.

The reason for these comprehensive notes is that the design has evolved towards the easiest and quickest means of construction and also to give guidance to those who may not otherwise consider themselves able to tackle a project such as this.

Installation for Two Bells

The sensors are built in small plastic boxes 80 x 60 x 40 mm these are fitted under a frame member adjacent to the outside of the bell wheel. The fixing depends on the type of frame etc. but a wood or plastic bracket and adhesive tape would be adequate

Two small coils are mounted on small pieces of plastic sheet or plywood and secured to the wheel shrouding with a simple clamp.

Four core cable is used to supply power and send the signals from the sensors to the computer (P.C.) running ABEL. The two sensors can be connected together in a daisy chain arrangement to with the single cable clipped under the frame members to give a

The connections to the computer use a simple cable assembly to bring together the signals from the sensors and foot switches and also distribute DC power. This is either taken from the computer drive supply cable as 12 Volts out through an expansion sl

The parts required will cost about 80.00, additionally you will require a redundant 286/386/486 P.C. with a VGA monitor. If you cannot beg one, budget about another 100.00. A sound card is also needed, an ex-office P.C. is not likely to have one fit"

Parts required, all available from Maplin Electronics, Mail order, 01702-554000 or Fax 01702-554001. Also Maplin shops are in most cities.

Parts For Each Sensor

DescriptionMaplin Order CodeQuantity Component I D
Min Resistor 220RM220R1R1
Min Resistor 6K8M6K81R2
Min Resistor 1K2M1K21R3
Min Resistor 10KM10K1R4
Min Resistor 30KM30K1R5
Min Resistor 3KM3K1R6
Ver Preset 4K7UH15R1VR1
1% Polstyrene 2200BX60Q4C1,C2,CX,CY
Disc 0.01uF 50VBX00A2C3,C5
Tant 10uF 16V WW68Y1C4
100uF 16V Elect.AT40T1C6
CS 209AUH59P1IC1
NE 555NQH66W1IC2
ABS Box MB1LH20W1Box

For All Sensors

DescriptionMaplin Order CodeQuantity
Four Core Alarm CablePA77J100 Metres
Remote FootswitchKW02C2
Min Resistor 3KM3K2
D Range 9 Way SktRK61R1
9 Way HoodKE94C1
Strip Board 3962JP50E1
4 Pin DIN PlugHH26D1
4 Pin DIN Line SocketHH42V1
2.1 mm line SocketJK11M1
GrommetsJX64U1 pack
Pin 2145FL241 pack

Power Supply

Supply TypeDescriptionMaplin Order CodeQuantity
From Computer0.2" Power ConnectorJW64U1
 2.1 mm Line PlugHH60Q1
 47R 3W resistorW47R1
 Spacer M3 insulatedFS37S2
Separate9 Volt 400mA supplyMG 80B1


M6 x 100 Gutter bolts4
M6 Nyloc Nuts8
M6 Steel Nuts4
M6 Washers4
Rigid Plastic Sheet, Mains Cable, IEC plugs for Computer, Mains Plugs and Sockets

Tools Required

Sensor Circuit Diagram

Sensor Veroboard Layout

Track cuts: where indicated at VR1 and under IC1 and IC2 nine positions.

LED: Fitted on rear side of board on 8 mm lead length. Flat on flange (cathode) towards IC2.

Pin 2145: Fitted in 5 positions, coil, power and output connections."

Sensor Box Layout

Coil Construction

There are three coils required for each sensor. The coils are rectangular in shape and 60 x 30 mm O.D. with 30 turns of wire. To construct them you will need a simple jig.

Parts required:

Mark and drill 4 mm holes on 50 and 20 mm centres in the two pieces of softwood as shown. If possible use a drill stand to ensure the holes are straight.

Fit nails through the holes with the heads projecting 40 mm from the wood and glue in if necessary.

Mark the two pieces of wood to ensure correct alignment and fasten together with the elastic bands.

Push fit the dowel in the reel of enamelled wire to make a handle.

To wind the coils secure the end of the wire under the elastic band and holding the reel in one hand and the blocks of wood on their side in the other. Wind on 30 turns with a hand over hand action, starting and finishing on a short side. The turns sho"

By letting the dowel handle rotate through your fingers an even, light tension can be maintained.

Tie the coil using thread around in eight places secured with a drop of super glue.

Release the elastic bands and the former can be removed from inside the coil.

Pre-tin the lead ends using a hot soldering iron to melt the enamel.

Sensor Box Assembly

Having constructed the Veroboard circuits, clean flux from the board using Meths, inspect it carefully for shorts between tracks, and connect one of the coils and test using a 9V battery and activator. If operation is OK. It should operate over a 100 mm r"

Activator Assembly

Each activator consists of a coil and capacitor mounted on a piece of rigid plastic 3 mm thick and 70 x 40 mm. If this is hard to obtain then plywood will do.

Shorten the coil leads to 15 mm and pre-tin. Solder to the capacitor, take care as this type of capacitor melts!

Fix to the pre-drilled support using hot melt adhesive.

Sleeve with cycle inner tube 28 x 700C size to protect and give a non-slip surface.

Fit M6 gutter bolt with back-nut and washer. Lock nut with super glue.

Computer Connections

Using the 9 Way D type connector as a junction box is the easiest way to combine all the cables.

The two foot switches can be glued onto a piece of wood and labelled. Push out the hinge pins to fit the internal 3K resistors, remove the Co-axial wires and fit a single piece of four-core cable. daisy chained through.

Power supply

The sensors can be powered from the host computers drive supply connector using the simple circuit shown.

The connector is shown from the front, pin side and the 47 Ohm current limiting resistor can conveniently be mounted together with a cable outlet on an expansion slot blanking plate using insulated stand-offs.

The 9.1V Zener diodes in the sensors will regulate the supply.

Cable Harness Assembly


Failure of the sensors to work is usually due to a shorted track or error in the construction. If your design is different to the one suggested you may have to adjust R2, a lower value is less sensitive.

The sensors will be affected by metal i.e. frames but this will not be a problem as long as the active area of the coil is at least 150 mm clear.


Fit the sensors under the frame, a wooden bracket will be needed. The activators are clamped over the inside edge of the wheel see diagrams in the Abel documentation. With two sensors it is easiest to connect the two boxes in series with plenty of spare c"

With the sensors powered adjust VR1 so the activators operate at about twice the normal distance, which should be about 40 mm for reliable operation this can be done before fixing the sensors.

Set-up using the Abel menus and diagnostics.

The clamp design for the activator makes adjustment of striking straightforward

Cautions and Warnings

We have not yet needed a faculty for any of our installations, however we always make sure they are temporary. Use clamps and tape, not nails and screws.

Lightning Damage. Your tower would not need to be struck for damage to be possible. Voltages can be induced into the cables by nearby thunderstorms. Get into the habit of disconnecting the sensor connector and unplugging the computer from the mains whe

Multiple Sensors and Other Possible Snags

If you want the silent practice facility first add a serial card and then some minor design changes.

The PC power supply will only support three sensors. Use a three terminal regulator instead of the 47 Ohm resistor, or a separate PSU.

Each sensor uses about 12mA.

Using unscreened cable between activator and computer has not yet caused any problems with two bells connected. The price difference is the justification.

If this occurs in multiple bell installations reduce R6 to 300 Ohms and fit 2.7K series resistors at the computer end together with 1000 pF capacitors to form a low-pass filter.

The 555 Timer has an output pulse width of 0.33 seconds. This pulses twice in one rotation of the bell. You may have to reduce C4 with light bells to shorten this pulse.


While the information given in this document is given in good faith and every effort has been made to ensure accuracy, the author cannot be held responsible for any loss or damage consequential or otherwise from the construction and use of the circuits an"


All this hardware depends on Abel software written by Chris Hughes and Simon Feather.

Abel is available mail order on disc (please specify type) from Chris Hughes,

8 Daisy Bank Crescent
United Kingdom

Cost 15.00 (All profits to charity or bell restoration)