SparkFun Forums

[motocrawler]

I am building a small/miniature electrical cylinder which will be used in scale models, the dimensions of the cylinder(s) are .5" diameter by 3" stroke. A small micro gear motor in the cylinder will be rotating a lead screw which will then extend the piston/ram, the gear motor will be proportionally controlled via a continuous rotation RC servo board (cheap form of a speed controller). My issue is that I don't know how or what I can use as a limit switch to stop the ram once it has reached its stroke, my obvious constraint is size and I want it to still look like a ram. Could I use a membrane pot on the inside wall of the cylinder? Any thoughts ideas etc would be very much appreciated. Thanks

[jremington]

If an external switch will do, you can buy very small microswitches, e.g. http://www.allproducts.com/ee/switronic ... h_ssm.html

Similar switches can be salvaged from junked floppy/CD/DVD drives or cassette recorders. Just about any device that detects a change of physical position of a part uses either a switch or photointerruptor.

[fll-freak]

Small hall effect sensor comes to mind as well. Would it be possible to design in the switch? Could a metal endcap of a plastic ram/piston make contact with a metal cylinder to make the switch?

[motocrawler]

Wow these small hall effect sensors seem great! I am just confused how I would be able to retract the cylinder once it has reached its stroke. Correct me if I am wrong but once the stroke is reached the sensor will cut the power to the gearmotor, but with the power cut how can I get it to reverse/retract with out somehow introducing another magnetic field to switch the sensor? Could the magnetic field be created by the signal wire of the rc servo board?

[fll-freak]

In modern days, the limit switch does not normally interrupt the power, but rather it just sends a signal back to a micro that stops the motion. The micro can then implement the logic that would allow the motor to turn the other way when up against a stop.

In the old days, with fancy wiring, a mechanical implementation was performed that did pass the power of the motor through the limit switch. If the power was too much for a switch, a relay was used. Now unless your motor is tiny, I suspect that the power from the motor could not be switched by the Hall effect sensor directly. If a micro is not in your future (a small PIC would be enough), then you will need some FETS to replace the relays and you will need o implement some clever wiring. I would Google limit switches and try to find an example of an old fashioned 'mechanical" implementation.

[Mee_n_Mac]

To be more specific ... what controls the motor (beyond the speed controller) ? How does it get told to run/stop and in what direction ? Is this supposed to be like your windshield wipers, just a momentary push on a switch and it completes a full (or in your case half) cycle ?