Solenoid selection

A project log for GPS (or not) chime clock

A 5 note chiming clock that gets time from either GPS or WiFi/NTP (via Raspberry Pi)

Nick SayerNick Sayer 02/22/2020 at 18:230 Comments

This is a bit of a retrospective log, since the project is basically finished at this point.

I didn't give a whole lot of thought to solenoid selection. I just brought up Adafruit and searched. There I found a 12 volt model with a spring return and just went with it.

One issue with bells and chimes is that you want to strike them with force, but you want the hammer to rebound after striking - that is, you don't want the hammer to dwell after the strike. So you really want to actuate the solenoid until the hammer is just about to strike the chime, let the momentum finish the stroke and let the rebound and spring return carry the hammer back.

Most school bells, in fact, have a switch in line with the solenoid coil power, and the switch is turned on when the hammer is back. The solenoid is thus energized long enough to move the hammer forward, which breaks the contact. The magnetic field and momentum are enough to have the hammer strike, but then return, whereupon the switch is closed and the cycle repeats.

I don't have the luxury of any feedback in the current design, so it's a matter of trial and error to get a pulse long enough for a proper throw, but short enough to let the hammer rebound and not continue to be driven forward.

For my mechanical design, that appears to be around 25-30 ms or so.

Unfortunately, these solenoids have a distinctive click-clack noise to them. I haven't really addressed that as of yet. One hope is that simply by adding a cover (my wife wants me to hide all the ugly wiring anyway) the clicking will be muffled somewhat while the chimes will still sound, as they'll extend out of the bottom of the box into free air.