Relay/solenoid control circuit help

I'm cobbling together a limit switch/automatic carriage stop for my lathe, I have most of it planned out but stuck on one issue. I have the mechanical parts built: limit switches (two, to handle both directions), linkage rod to activate them, adjustable stops on the linkage (one on each side of the carriage), and a 12V solenoid with a flex cable going to the half-nut lever.

The issue is, I need the solenoid to be powered for only some incredibly short period of time (1/4 second or less), regardless of the state of the limit switch. A timed pulse, instead of just on/off via the switches. Is there a clever bit that can be put in with the solenoid's relay (two relays? one N/O to activate and another N/C to turn it off?) without resorting to an additional microswitch that trips when the solenoid pulls in? That would be functional, but clumsy and bulky and complicated.

Liking the concept but can’t offer a lot towards a solution.

Being frugal, when I need a decent solenoid for a project I pull one from an old 8-Track tape player. 12VDC coils and a decent amount of pull plus tapped holes for mounting. Most will function at 6V-24V in momentary use with enough A behind the V.

Getting harder to find dead 8-Track players these days but dirt cheap when you do.


Electrically, what you are describing is called a ‘one shot’ or to get really fancy, a monostable multivibrator. A google search for a circuit using 555 timer should provide you with a bunch of examples. You will need a signal to trigger the one shot to fire, which sounds like it should be coming from your switch.

The 555 can be ‘programmed’ with a resistor/capacitor circuit from milli-second to several second pulses. I’m on my phone now so I may screw up this link to an Instructable describing their usage. 555’s are ubiquitous and cheap (50 cents a piece) or you can get a pre-built circuit board with pots to adjust timing for $2.50 (says one of the links in my Google search).

Yes, 555 timer/relay boards look like exactly what I need. Relay onboard will simplify building the thing, too (I was going to use some 2-channel relay boards I already had in the pile). Thanks!

I already have the solenoid, a massive overkill thing (that's just how I roll...) meant for a car remote trunk release. Tested with 9V/2A transformer, it'll break your finger if you aren't careful. 12V/1A should be more than enough to kick the lever out of gear.

Just for fun, this automatic stop will be dual-mode, as well. In turning mode, the limit switches will use the solenoid to disengage the half-nuts and the spindle motor stays running. When switched over to threading mode, the limit switches trip the motor controller's inhibit line instead, and stops the motor (but leaves the half-nuts engaged). This means a relief groove isn't needed when cutting a thread, at low speeds the spindle will stop at the same spot within about 1/8th turn (already tested).