I came across a rather intriguing gadget way back in my GPO telephones days..
You wire two lamps in series, red lead to one lamp, black to the other. Then you connect the mid point of the two lamps, to a probe. Assuming 12v lamps, you connect to the battery + and negative, at which both lamps show half lit. Connect probe to a +ve and one lights up fully, connect probe to -ve and the other lamps lights up. You just mark the lamps up to indicate + and -.
At GPo Telephones it used to just plug into the exchange racks 50v bus. For use on a car it can plug into a lighter socket. Instead of light bulbs, LED's could be used.
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Harry
How To's and items I offer for free, or just to cover the cost of my expenses...
http://www.the75andztclub.co.uk/foru...40#post1764540
Fix a poor handbrake; DIY ABS diagnostic unit; Loan of the spanner needed to change the CDT belts; free OBD diagnostics +MAF; Correct Bosch MAF cheap; DVB-T install in an ex-hi-line system; DD install with a HK amp; FBH servicing.
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