Joule Thief Door Lock Indicator.wmv
This is a simple device I made for my Mom, who is having memory problems and can’t remember if she locked her front door or not. She can see this from where she sleeps and if she wakes up and sees the red LED, it means her door is locked and she can go back to sleep.
Just a basic JT circuit using a 1″ ferrite bead instead of a toroid, 22 ga. bifilar windings, 2N3904 transistor and a 1k resistor. I modded a simple reed switch which is activated by 2 neo mags I hotglued to the lock knob. I used a very small red LED because she leaves this on almost 24 hours/day because her door is locked almost all the time. So far, in a month, I have only had to swap “dead” batteries once. Pretty good run time. For more info: Overunitydotcom.