Solenoid assumption fail.

So, a while ago I was doing this steampunk clockwork mechanism for eurosteamcon 2013

The thing is, as it was all lasercut, I didn’t want to fiddle with a real escapement mechanism. I had not the time, nor willingness, to spend weeks tweaking such a mechanism.

So?

I decided to make a simple solenoid driven escapement. I always save miniature solenoids from everywhere (who does not? XD ) So I had the ideal one around my bit box, from a cheapass single-use analog camera:

I would just use a 2N3904 and a 555 to make it tick.

The actuator stops a simple lever in the last reduction stage, allowing for a complete revolution in every tick. (reduction was 60/10 – 30/10 – 30/10 – 30 (162 to 1)

Did I say simple?, that was the idea, yes…

Some of you might already have notticed what went wrong. For those who don’t, look at the whire gauge. I completely failed to nottice that that coil was going to take a lot of oomph, so imagine my face when I had everything set up, and I could not make it go.

When I did realize, I made a simple measurement, and saw that at 1,5V, the coil required 1,36A peak current to energize (less than that, and the plunger didn’t had any kind of authority, so it would not move under load), way beyond the 200mA of the 2N3904.

And I didn’t had any fet’s around, or a power transistor…nothing, nada…I didn’t even had smaller whire to re-reel the coil for less consumption, nor the time (I had to deliver the mechanism in less than 2 hours!)

What did I do in the end? Just a simple magnetic flux brake:

That copper disc you see, has a neodimium magnet underneath. As the force induced is directly proportional to the speed at what the disk wants to rotate, when it tries to go fast, magnetic induction brakes it, and when the mechanism is nearly unwound and rotating slowly, it has no effect.