Bad Bug Spray Baffles Engineer

My wife purchased a bottle of bug spray that had an innovative automatic, trigger-controlled sprayer. When it didn't work, she assumed the cheap batteries supplied with the sprayer were dead. New batteries did not solve the problem, so she handed it over to me for a solution. I double-checked the voltage on the new AA batteries. They were both over 1.5 volts. After disassembling the handle, I started to check for continuity. There was continuity across both ends of the battery terminals, the motor, and the switch. I even took apart the switch to verify good contact. Now why wasn't this working?

I then connected the batteries directly to the motor terminals and the motor turned just fine. Now what? I already checked the exterior of the battery terminals, so I checked from inside the battery holder. When I connected to the posts (with the switch closed), there was good contact. I then used a bolt to simulate the battery pushing against the spring (for the negative battery post). Voila! The circuit would go from open to closed, depending on how hard I was pushing.

I then drilled out the terminal post and punched it out of the circuit and the plastic case. The negative terminal was a double spring -- the large spring pushed against the battery and the smaller spring pushed against the terminal post. When the large spring was pushed, the smaller spring would open up and lose contact with the terminal post. I then soldered the connection between the smaller spring and the post and reinserted it back into the plastic case. After re-soldering the post to the circuitry (and reattaching the wire to the motor terminal, which broke during my handling of the components), I reassembled the handle and gave it a test run. It worked as advertised. Bugs beware!

— Josh Boatwright is a senior development engineer in Ohio.

Tell us your experience in solving a knotty engineering problem. Send stories to Jennifer Campbell for Sherlock Ohms.

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