Designing a Better Test Box for Fencing « My Site/Blog
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Designing a Better Test Box for Fencing

The idea behind this was born of part necessity and part boredom. As primarily an epeeist, I’ve had my fair share of faulty weapons and would end up having to use the connection detector on my multimeter to debug any problems. This was not the most practical approach. However, instead of shelling out $15 for a box with LEDs I decided to go and make my own.

The first thing I wanted to avoid was using two batteries. A regularly used tester would see maybe an hour of on time in a couple years and I really wanted to make the case as light and slim as possible. However, that left me with a different problem. The reason most testers use two batteries (AA) is because the general forward voltage for LEDs between 1.8 and 2.2V, just over the 1.2 or 1.5V you’d find in an NiMH or alkaline AA. There are a few ways to go about boosting the voltage to the necessary levels, but the approach I took was to use a circuit called a joule thief.

I found an easy-to-follow instructable and made a quick trip down to the EE lab to see if there were and toroid magnets which I could use to make the inductor. Lucky, there were. I only managed to get 7ish complete turns around however, it seems to work just fine:

IMG_9708I wanted to see how low the voltage could drop before the LED would become unusable so I hooked up the circuit to my bench supply. I slowly dropped the voltage to the point where the LED was no longer emitting any detectable light which was around 0.3V

DS2000Above is a capture at 0.3V in. The top waveform is from the toroid output to the base of the 3904 and the bottom waveform was of the LED. This makes it much easier to see the LED being powered by higher voltage pulses from the joule thief circuit. The capture below was taken at around 1.5V in and shows the difference in frequency relative to voltage (excuse the different time bases):

Newfile1iHaving solved the issue of the voltage supply à la joule thief, the test circuit is then a simple matter of wiring up three pins to connect to a body cord and then foil or epee. IMG_9712Above is a photo of the body cord that will be connected to the pins in the test box (the test box whose housing needs to be made). The middle pin (B) is connected to the collector of the 3904. The left pin (A) is then connected to a green LED whose cathode is connected to the emitter or the 3904, such that when the circuit is closed the LED will turn on (the weapon is working). The same is done for the right pin (C) with a second LED. Thus completes the wiring component of the test box, I’ll cover the housing in another post when the fab shop reopens.

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2 comments

  1. John Martin says:

    Do you know what the voltage running through a fencing wire is? volts, amps, mva? I am hoping you can tell me this. No forum has it.

    I have an idea and need a circuit designed if you are game.

    John Martin

    • c says:

      Hi John,

      The voltage running through the wire need only be enough to account for the forward voltage of the LEDs, likewise you would also need enough current to drive the LED when the circuit is closed. I think most commercial boxes use a 9V battery for simplicity but you could make a test box work across a wide range of voltages. Please feel free to contact me if there’s anything you’d like to discuss.

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