Hardwired Circuit

I know this is slightly of subject, but I need some help creating a hardwired circuit.

What I am trying to accomplish is cycling a single solenoid off and on using one pushbutton. In other words, press and hold the button once and the solenoid engages. Let off the button and then press and hold it again to disengage the solenoid.

I know this is an easy program to write in a PLC using latch/unlatch coils and a one-shot, but I need a simple way to hardwire one with minimal components. Does anyone have a clean little method for this? I guess the real problem I have is effectively creating the one-shot.

Thanks in advance.

Steve

The thread below dealt with this question using logic, as there are no one shots in the replies, it should be quite easy to convert this to relays

http://www.plctalk.net/qanda/showthread.php?s=&threadid=150

Paul

The thread quoted uses - like many other PLC baased single pushbutton alternators - the fact of the program scan for part of the logic. REAL WORLD relays, in effect, solve ALL the logic at the same time (within limits of contact closing times etc). I have had problems going back to a REAL RELAY circuit of any moderate complexity with my mind still in the PLC scan mode. This is not to offer a solution, just a caution.

Thanks for the lead. I knew I had seen this but could not remember just when....anyway, I don't think any of these schemes work in hardwired state. All of them seem to rely on sequencial logic in order to operate correctly.

It looks to me like all of these solutions would have the solenoid chatter on and off while the button is being pressed and then finishing by chance in one of the two states. Unfortunately the hardwired world will not submit to my desire to have it read top-to-bottom and left-to-right.

If I am missing something or reading these schematics wrong, I sure need some help getting my blinders off.

Thanks

I am curious as to why it has to be a pushbutton. Why not a standard lever switch. It does what you want and is self indicating. Is it that you always want it to be off at power up?

You are essentially trying to create an alternator circuit. This is pretty tough with relays (unless you use a time delay for one of them) because of the relay races that ensue.

Fortunately, Square D, Cutler Hammer, Time Mark, and goodness knows how many others, have two pump alternator relays available with everything canned in an octal or 11-pin socket relay. Check them out.

You need a "ratchet relay". Google will tell you where to find them.

This sounds like the toggle button on my night stand lamp. Press it once and the light is on. Press it again and the light is off. I think I have seen these buttons at Radio Shack. Will this work for you?

Sorry it took so long to get back to this..got called out of town at the last minute and just got back.

Rick/Godfrey - In truth, it is a foot switch. The application is to energize/de-energize a brake with the same foot switch in an industrial application. The least expensive industrial unit we have found (and hence most desirable) is a simple switch type.

Tom/Gerry - I am not yet familiar with the alternator or ratchet relays..sounds promising..I will look in to those today. Thanks.

And I think I have worked out a circuit that will work using four relays. I will attempt to post a schematic of it later this morning for anyone else is who is interested. I may not have to use it though once I learn about the alternator/ratchet relays.

Steve

An impulse relay may work out.

This is my first attempt to post code, so I hope you will forgive me with regards to its look....anyway, here goes:

Relay R1 directly controls the solenoid (Y0).
Relay R2 must be low in order for relay R1 to engergize
and energizing relay R3 will cause it to drop out.

  PB     R2     R3     R1

--| |----|/|----|/|----( )--

             +

  R1         +

--| |---------

Relay R2 is used to signal that the pushbutton has been pressed
and then released to energize the solenoid.

Relay R1 must be high in order for relay R2 to engergize.
Energizing relay R4 will cause it to drop out.

  PB     R1     R4     R2

--|/|----| |----|/|----( )--

             +

  R2         +

--| |---------

Relay R3 is used to drop out R1. Relay R2 remains energized
thereby preventing R0 from re-energizing while the pushbutton is pressed.

Relay R2 must be high in order for relay R3 to engergize.
This prevents R3 from energizing when the pushbutton is pressed the first time.

  PB     R2            R3

--| |----| |-----------( )--

Relay R4 is used to drop out R2.

Relay R1 must be low in order for relay R4 to engergize.
This prevents R4 from energizing when the pushbutton is release the first time.

  PB     R1            R4

--|/|----|/|-----------( )--

Solenoid is energized whenever relay R1 is.

  R1                   Y0

--| |------------------( )--

Keep in mind that this is hardwired code so the left-right top-bottom thought process does not work here.

If anybody sees any problems with this code, please let me know.

Thanks
Steve

Steve

This is a very interesting problem, that I have used many times to challenge people in training sessions.

I have many solutions to this using DC Voltage, AC Voltage, and PLC ladder. I have one posted at a web site that works with all three.

The down side to this solution for hard wire is that it requires a PB with 2 NO and 2 NC contacts, One relay with 3 NO contacts, and one relay with 1 NO and 1 NC contacts. You can reduce the 3 NO contacts to 2 NO on the first relay by putting the light/coil in paralell with the coil.

If you have only one contact on the PB, you may add a third relay, a DPDT to fill in for the PB contacts.

Enough of all of the variations, the posting is at...

http://www.geocities.com/plctech99/index.html

I have not actually hardwired and tested this circuit for race condition, but I have built and used a DC version that uses 3 SPDT relays and 2 diodes.

Hope this helps
Archie

IMO, by the time you mess with all of the relays, you're better off with a small programmable relay, (like the Klockner EASY). Program the basic ladder right there on the screen and be done with it.

-Phil

The ratchet relay solution:
- 1 PB or FtSw with 1 NO contact
- 1 ratchet relay with one contact

A ratchet relay can be thought of as an electro-mechanical flip-flop. When you energise the coil, the armature moves the pawl on a ratchet wheel, indexing it by one tooth. The wheel turns a cam which operates the contacts. For each increment of the wheel, the contacts change state.

Very simple.

Takoga,
Though this has been a nice exercise for everyone, re-reading your second most recent post got me thinking...

Since I'm not sure of the actual function of the foot switch, I can only go by your description of it's application...

First off, do you really need the brake to toggle state with each press of the foot switch? Could you possibly just use the foot switch to disengage (or engage) the brake only while pressed?

If you need it to toggle, you may want to revisit your actual cost savings of a "simple switch" (momentary) version vs. a slightly higher (if any) cost push-on/push-off (maintained) foot switch. Most foot switch manufacturers offer both styles for roughly the same cost... Just change the part number and you're done! This may be cheaper than adding multiple relays to accomplish the same task. It also makes it a LOT less complicated and therefore more reliable. If you are an OEM, do you really want the customer (or you in the field) to have to troubleshoot this system rather than simply replacing ONE worn out foot switch?

IOW, always remember the K.I.S.S. principle!

Just my 2¢,

-Eric