Read update at the end first.
I was searching for some regenerative receiver schematics and looked at some other circuits. Google says similar images, but it doesn’t know anything about the purpose of the image, it just knows that it has lines connecting different symbols, and even then I’m not sure if it even knows that. It may be just some mathematical algorithm. Someday I should find a bloody murder photo and see what other photos it thinks are similar.
One of the images it showed was a soldering iron shutoff timer. The specs called for several tens of minutes delay before shutting off. The schematic was somewhat complicated, it used a CD4060 chip that has an RC oscillator and divider to give long delays. I thought, why bother? The oscillator is not very accurate because it uses R and C, and even a 555 timer chip can be just as accurate. I could use a low leakage tantalum capacitor and a megohm resistor and get several minutes of delay. I know this because I built a timer using a 555, to control the exposure time of a photo enlarger. And that was when they didn’t have CMOS 555 chips. Today, a CMOS 555 draws almost no current from the RC timer circuit. So the 555 is a simple, cheap solution.
Later I got to thinking about how it could be done mechanically. People are not impressed by a box with electronic parts that does something. But when they see a mechanical device that they can understand, then they are impressed. Wow, this is a mechanical genius! Remember those Halloween boxes? You turned on a switch, and a second later a trap door opens up and a hand slides out and turns off the switch, and goes back inside! Cool! People actually bought these simple little things.
All sorts of ideas rushed through my head. I thought about a simple motor with a gearhead that turned slowly, with a spool on the shaft and a few feet of thread. After many minutes when the thread was all wound up, it would pull the switch off. I scrapped a microwave oven and recovered the turntable motor, which has a speed of a few RPM. If I could get a slower motor, I wouldn’t need that much string.
What has a slower motor? How about a light timer, to turn the lights on at the same time every day? Then bingo! It hit me that this would work perfectly for the timer. Just set the pins for an on time of a half hour, and plug the soldering iron into it. But the problem is that it turns on at the same time every day. What I have to do is put a relay between the timer and the wall power, so that when the timer turns off, it also turns off its own power. In order to start it, I would have to turn the dial around until the timer’s switch turns on. Then after a half hour, it shuts down everything including itself.
I guess this is the simplest and cheapest way to build a timer, and it’s mechanical but totally unimpressive to others. These electromechanical timers are only a few dollars at the discount stores. A 120 VAC relay might be bought for several dollars surplus. If the contacts are rated for 15 amps or more, then it could be used for a clothes iron or other appliance like a coffee pot.
I’m going to look online for something that might do this without having to build one.
Update – I found just what I was looking for. I’m glad I shopped around. Amazon and others wanted $20 or more, I got it on eBay for $11 and free shipping. Search for FD60MC.
https://www.amazon.com/Simple-Touch-original-Shut-Off-Setting/dp/B002KKCVZK
http://m.outletpc.com/vc8478-2-prong-12-hour-appliance-shutdown-timer.html
If these links are not working, then do a search for appliance shutdown timer.
