Help needed - Two momentary switches used as a safety

Thread Starter

Tcranford

Joined Mar 14, 2019
2
Greetings,
I’ve got a project that I would like to use two momentary switches (SW1 and SW2) to control a motor.
1) Pressing both SW1 and SW2 will be required to start the motor
2) Motor continues to run as long as either SW1 or SW2 is pressed. I’m ok if it would be necessary that either SW1 or SW2 needed to be dedicated to keep the motor running. For example, after the motor is started SW2 needed to be held to continue running.
3) When both SW1 and SW2 are released the motor stops.

The idea here is to create a safety so that both hands are required to start the motor and at least one to keep it running.

TIA!
 
I think you cannot use a momentary switch here.
momentary switch is of 2 types. Normally on and normally off.
They are push button just change there state of on or off when pressed .
Here you can simply use an OR gate to drive a relay or something.
A B Y
0 0 0
0 1 1
1 0 1
1 1 1
connecting the inputs of an OR gate to the swiches
 

dendad

Joined Feb 20, 2016
3,611
You may get away with the correct value of resistors by putting resistors in line with a relay coil. Both buttons feed power to the coil via a resistor each. Select the resistor value so one only will hold the relay operated but not allow enough current to pull it in from at rest. Both buttons pushed have 2 resistors feeding the relay so more current flows to operate the relay.
Relays hold in on less current that is needed to operate them.
This way is a bit of a hack and I would not really recommend it, but it could be interesting to see if it would work.
 

Yaakov

Joined Jan 27, 2019
1,612
The idea here is to create a safety so that both hands are required to start the motor and at least one to keep it running.
I am extremely uncomfortable with "safety" switches that aren't designed and tested by professional engineers.

Since you intend to start a motor, I assume this is to prevent injury to the operator. I would be looking for a commercial, UL/CE/Tüv rated product, not kludging something myself.

Just my opinion, but also the advice I would give a friend.
 

dendad

Joined Feb 20, 2016
3,611
Actually, I agree fully. Safety is not something to take lightly. Ans it has been my experiance in industrial control, that you need to make them tough and foolproof.
 

Yaakov

Joined Jan 27, 2019
1,612
This actually doesn't work for the TS's purpose because it specifically stops if one hand is removed, and will not reset until both hands are removed for a time, but boy, it's neat: https://www.ebay.com/itm/Two-Hand-O...Engineering-OTBVN6-Control-Relay/113606209501

The functionality is in that relay which is very serious safety stuff, and the optical buttons are really cool kit too. Makes me want to wire up some industrial stuff for fun.
 

MaxHeadRoom

Joined Jul 18, 2013
21,612
The idea here is to create a safety so that both hands are required to start the motor and at least one to keep it running.

TIA!
There are commercial switches that are aimed at this application, called Palm Safety Switches, I have used these on machines where a operator is required to place both hands on them for the machine to continue.
Mystically it is almost impossible to fool them by placing a foreign object on one to allow the use of a hand!
Trust me, they have tried it!:p
Looks like Yakov mentioned something similar to them in #4.
Max.
 
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MisterBill2

Joined Jan 23, 2018
7,260
For operating industrial machines where there was a need to assure that both operators hands were out of harm's way, the specification was always "two hand, anti-tiedown, anti-repeat" That meant that both buttons had to be pressed within typically 2 seconds, and held for the cycle until it was safe to release them, and that they had to be released to reset for the next cycle. Most of the push buttons that we used were Allen Bradley 800T series, normally open, spring return. Other companies make buttons just as suitable, and a whole lot cheaper. They were single pole buttons and I am trying to recall the circuit that made it work.
 

Yaakov

Joined Jan 27, 2019
1,612
For operating industrial machines where there was a need to assure that both operators hands were out of harm's way, the specification was always "two hand, anti-tiedown, anti-repeat" That meant that both buttons had to be pressed within typically 2 seconds, and held for the cycle until it was safe to release them, and that they had to be released to reset for the next cycle. Most of the push buttons that we used were Allen Bradley 800T series, normally open, spring return. Other companies make buttons just as suitable, and a whole lot cheaper. They were single pole buttons and I am trying to recall the circuit that made it work.
If you look at the nice kit I linked to in #6, the safety relay is of the rigorous source. The switches are optical.
 

MaxHeadRoom

Joined Jul 18, 2013
21,612
The problem with ordinary P.B. is they can be defeated or fooled easily.
I used a version that was safety approved, made by Pepperl + Fuchs.
They had to be operated by a single machine operator using two hands, if you tried any other object they would not work.
They are not cheap, but conform.
Max.
 

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