KEY-RING CALCULATOR CONVERSION TO WINDER COUNTER

by

Tony Elliot

KEY-RING CALCULATOR CONVERSION TO K & P WINDER COUNTER

The increasing interest in the indoor flying of rubber-powered models has led some modellers to fit various forms of electronic counting devices to their winders to indicate the number of turns of the rubber motor. These usually consist of cheap, but large and clumsy, calculators which have been modified by the addition of a micro-switch wired across the 'equals' key, and operated by a cam on the winder handle, although a few people have utilised the much smaller and neater, but more expensive, electronic cycle 'computers'. However, several stores sell very neat little 'key-ring calculators' for a mere 99p. (less than the cost of a replacement battery from the local camera shop!), which can be easily modified to do the same purpose.

Of several (4) types tried to date, by far the neatest is the Texet 4RN1, which was originally stocked by W. H. Smiths, but they have since dropped this in favour of another type (no. M1) which, although cheaper than the 4RN1, is larger and less convenient, having a sliding keypad cover instead of a flip-top one, which some people may find awkward to use. Also, the via- holes are tiny, and the battery appears to be glued in. However, Wilkinson's stores and Office World still stock the 4RN1, and other stores may also have them. Also, Woolworths have just started (March, 2002) selling another type called a Clip-Calc (designed like a Bulldog clip), made by Texet, also at 99p, but in a slightly larger case, and is without a keypad cover. Furthermore, the battery is not easily changed; not too easy in the 4RN1 either, but is possible. The p.c.b. is slightly different - just use the hole to the left of the battery, and the one in the right hand corner, both on the lower edge (p.c.b. number C600).
Fig. 1 P.C.B. Layouts and Wiring Modifications
Fig.2 Brackets and Mounting
The modification consists of locating suitable connection points to each side of the 'equals' key, soldering two thin wires to them, and bringing them out to a micro-switch fitted to the winder. This operation requires the use of a soldering iron having a fine pencil bit of no more than 1½ mm. diameter max., and should only be tackled by competent persons. To make matters more awkward, the via-hole pads do not 'take' solder too readily. Good quality double-sided p.c.b.s use plated-through via-holes (so called because a track on one side of the board continues on the other side via a conductive hole), but some of these 'cheapo' units have p.c.b.s which utilise what appears to be a conductive epoxy-like material, and which may need clearing with a pin or fine drill to accept the wire - not the M1, which is OK. My method is to carefully scrape the epoxy off half of the pad, ahead of the wire when folded over, down to the printed circuit pad - check with a magnifier! - and then solder the wire to the cleaned half of the pad, leaving the epoxy under the wire to continue the circuit through to the other side of the board

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Required

Key-ring calculator - Wilkinson's, Office World (Texet 4RN1 - preferred type), W. H. Smiths (M1), Woolworths (Clip-Calc)
V4 micro-switch with roller lever - Maplins, code no. GW 70M; 99p - ask for it at the
components counter, or FP 43W (£1.49 - pre-packaged, usually on the 'Switches' rack)
Short length of 7/02 or 16/0.1 wire (max. size), e.g. a piece of servo lead
Small piece of sheet metal - Zintec or similar (for bracket)
Two small self-tap screws 12 mm. long (to replace those removed from the winder)
Washers (to space the bracket from the winder body):


Method

Make a bracket to the drawing - fig. 2. Remove the top two screws from the winder body and secure the bracket to the winder using equivalent gauge 12 mm. long screws - fit washers between the bracket and winder to take account of the winder mounting boss. Alternatively, use a 4BA screw through the mounting boss - could use a simple strip metal bracket.
Unscrew and remove the calculator back cover, and discard the key-ring.
Carefully abrade (see above) the via-hole pads - fig. 1. Strip 2 mm. (no more) of insulation from two pieces of wire, push into the holes in the p.c.b., bend the wire flat against the board, to withdraw some of the bared wire to prevent the ends from fouling the keypads, and solder. Nb. a small pencil-bit or 1½ mm. (max.) bit electronics type soldering iron is required.
Cut or file notches in the rear cover to take the two lead-out wires. Cut off a p.c.b. support in the back cover which interferes with one of the lead-out wires, and reassemble.
Roughen the back cover (to remove any paint and provide a good 'key'), and glue the calculator to the bracket with 5-minute epoxy or a servo pad, ensuring clearance of the handle.
Glue fillets to the winder handle in line with the micro-switch roller, and temporarily fit the micro-switch to the bracket with double-sided tape - ensure that your normal winding direction passes over the micro-switch body before hitting the roller. Trim the wires to length and solder to the micro-switch 'C' and 'N. O.' contacts (the most widely spaced pair) - fig. 1. Adjust until reliable operation is obtained, mark the position on the bracket, and permanently fix the micro- switch with cyano or a servo pad. If it is required to wind both ways (e.g. to wind normal props. used in pusher mode), a longer 'lead' will be required in the reverse direction to allow for the slack in the switch lever. [Nb. if the 'trailing edge' of the handle web is left unmodified, the switch roller will 'lock' into it, thus preventing winding the wrong way, and also locking the handle when wound up!]:
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Job done!
To use, press 'ON', enter the winder ratio, then '+', turn the handle back to the roller, hook up your rubber motor, and get winding! To clear the last reading, press the 'ON' key again, and repeat. Note that there is no 'OFF' key - the calculator will auto-switch off after about 6 mins.
With a little ingenuity, this unit can readily be adapted for use with larger, heavy-duty drill type winders as used by outdoor rubber flyers, with the micro-switch being operated either by the handle, or directly from the chuck - just make sure that the micro-switch is not 'made' when starting or resetting the calculator, as this will cause it to 'lock up'. If you can count the number of teeth on the gears, you can use the calculator to work out the gear ratio! Some modellers have used a reed relay operated by a magnet on the handle in place of the micro-switch.
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