[Sanghera]

Hi Al,
Thanks for all the help. I don't know if I have found the circuit that you are talking about on the web yet. I did find this though: http://www.selfhelpandmore.com/howdo...lays/index.htm
I'm not sure if this is what you meant. It's near the bottom of the page.
Would you be able to give a website or a drawing? That would be much appreciated.
Thanks.

[Al_The_Man]

I have sent this in a PM it may help
http://www.seattlerobotics.org/encod...97/ladder1.htm
Also in the realm of this type of circuit control there is a device that is the basis of small PLC called a 'Smart Relay' One manuf. is Siemens Logo it is a device you can program to duplicate the hard wired logic of physical relays, it has 6 inputs for limits, push buttons etc and 4 relay outputs. It has latches, timers & a real-time clock.
It may be overkill for your case but it is worth looking into, there are alot of identical smart relays made by Omron, Idec and others, some are expandable.
Al.

[Sanghera]

Thanks Al, I looked at those and will have to further read into this latching circuit. Seems pretty simple though. Does it operate as this: push and let go, it goes on and stays on, push it and let go again and it goes off and stays off?
Thanks.

[Al_The_Man]

The initial circuit shown in the site I posted are of the type you push to operate and it latch's in untill another switch or PB in the string un-latches.
If you want to simulate flip-flops etc , the smart relay is the cleanest way to go as it has a selection of logic functions.
Al.

[gar]

060828-1024 EST USA

Sanghera:

Consider a system as follows which is similar to what I previously described.

There is a mechanical stop at each end of travel, a permanent magnet DC motor to move a carriage, a limit switch at each end of travel that is made while the carriage is closer than X inches from the respective stop, a double pole double throw relay for motor direction control, a constant voltage DC supply to drive the motor (maybe includes overload current limiting), a series resistor to limit motor current when either limit switch is made, a toggle switch to control the motor direction relay.

The idea here is that the motor runs full speed between the two limit switches with over load current protection in the power source. When a limit switch is made the series current limiting resistor is between the motor and power source. Made in this case means the carriage is in the X inch region of the mechanical stop. Made does not define whether the limit switch is open or closed in the made state. The motor will always be energized anywhere in its travel, including against a mechanical stop. The current limiting resistor will determine the holding torque against the mechanical stop, prevent overheating the motor in stall, and provide some deceleration before hitting the mechanical stop. At the mechanical stop there is torque from the motor present and thus force against the stop.

If the two limit switches are normally closed when not made, and the two are connected in series, and the series connection of the two switches is in parallel with the current limiting resistor, then when neither limit switch is made the motor runs full speed in the direction determine by the DPDT relay position and this is controlled by a toggle switch.

When either one or both limit switches are made then the motor is current limited by the series resistor.

One toggle switch position causes the motor to run in one direction and the other position causes the opposite direction. When leaving a mechanical stop the speed will be low until the respective limit is not made.

Other logic can added in place of the toggle switch to make a flip-flop action and momentary actuation. Also logic can be added that sends the motor fast when leaving the mechanical stop.

The aboved described technique is something you could easily build and in the future change its function.

.

[Sanghera]

Thank you very much. This is a very useful circuit in order to keep things in position at the end of mechanical stops. How do you determine to corect current limiting resistor so that your motor does not burn out. Does this add a lot of wear to the motor?
As for control, I was trying to make it control with one single push button. I'm not sure yet if Al's circuit is right for me as I want one button to control. I think I may go with a flip-flop. I want to try to find a circuit with no ic, that acts like a flip flop.
Thanks.

[Sanghera]

I found this one too:
http://www.edn.com/contents/images/102804di.pdf

[MaxHeadRoom]

Sanghera, This is a relay based flip flop I have used for similar functions,
The fourth contacts on RLB is used to control power relays for the forward/reverse of the motor, NC one direction NO the other, this circuit will also have the overtravel limits in it of course.
When the PB is pressed RLB will energize and de-energize when pushed the second time. Other relays may need different values of R & C .
Al.