Automatically reversing a dc motor with two SPDT pressure switches and DPDT relays

Thread Starter

jaxmwana

Joined Mar 26, 2012
4
Hello everyone. I'm new here and a rookie when it comes to electronics. I'm currently working on a project that requires an object to move along a rail. When the object hits a pressure switch (SPDT) it reverse direction and once it hits another pressure switch at the start of the rail, it should start moving in the normal direction. Think of it as an object moving back and forth along a rail automatically.
Is it possible to do this with a couple of DTDP relays and two SPDT switches? if so how would you recommend me connecting them.

Note: the problem I'm faced with is that once the object hits a pressure switch, it bounces off and releases the switch.
I currently have: 12V, 2A DC motor. 12V, 5A 8 pin DPDT relays. 2 SPDT pressure change over switches.
I am willing to purchase new items depending what you guys suggest.
thank you
 
Last edited:

Bernard

Joined Aug 7, 2008
5,784
A simpler form requires a single DPDT relay but does not have an off position except for power off. Motor current does not flow thru pressure switches.
 

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Thread Starter

jaxmwana

Joined Mar 26, 2012
4
Cheers Bernard. i managed to come up with this circuit, using 2 push-release switches. (I need two separate switches for what i'm trying to achieve) Only fear I have with the circuit is whether the motor might damage the relay because of back emf. And I am a little sceptical with the constant switching without time delay of the motor. Would that damage the dc motor rated 12V, 2A?
 
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Bernard

Joined Aug 7, 2008
5,784
Unless there is some majic in the DPDT latching relay , looks like the load will oscillate over SW2, & do nothing if it ever hits SW1. With magnetically latched DPDT relay, I believe relay is latched with a pulse, & to release a reversed polarity pulse is needed, or a second coil is used.
To reduce motor hash, connect a resistor of 2X motor resistance in series with a .1μF across motor. Add as much resistance in series with motor as loss in power will allow, around 1 to 3 Ω, & relay contacts should be rated for locked roror current. Power rating of resistors will depend on shuttle cycle rate.
 

djsfantasi

Joined Apr 11, 2010
9,163
A simpler form requires a single DPDT relay but does not have an off position except for power off. Motor current does not flow thru pressure switches.
Nice one, Bernard! I was thinking of something similar - but it required a 3TDP or better relay (or two wired as one). I have a mental block when thinking of wiring a reversing switch for a DC motor from my model railroad days and their "best" practices. Your circuit turned it around (you can see what I mean, by looking at my schematic below). Also, while both recognizes that this can be implemented with a mix of NO and NC switches, your diode steering component is the icing on the cake!

The portion of the circuit in the upper red box is the reversing part. The lower red box uses another set of contacts to implement the latching part of my solution.
 

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Thread Starter

jaxmwana

Joined Mar 26, 2012
4
ah, that makes a lot of sense. I kind of forgot to state that both s1 and s2 are normally open. I'll try that and post my results possibly by the end of the week. I really appreciate the help
 

Leckyneil

Joined Sep 25, 2013
9
ah, that makes a lot of sense. I kind of forgot to state that both s1 and s2 are normally open. I'll try that and post my results possibly by the end of the week. I really appreciate the help

Did you find success in your project, and which circuit did you use? I have exactly the same project in mind, and am very worried about the sudden change from forward to reverse damaging the components. I'm confused about the last djsfantasi circuit as to which relay or relays are being used.
 
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