Safety Control Two Buttons

In industry there are machines type presses that allow their operation only if the operator's hands are positioned above two buttons that give the consent only if pressed both.

Unfortunately, in some cases there were workers to have a free hand have decided to keep pressing one of two buttons to button pressed inserting a toothpick holding it so stuck simulating the " crushed " the hand.

So happened an accident and from there it became mandatory to provide that these machines in addition to having the two buttons had also a circuit that sees whether a button pressed for more than one / two seconds only while the other is still open, doff his consent also if the second button was pressed .

Safety Control Two Buttons Circuit Diagram:

Buttons Circuit Diagram

The circuit providing for three diode bridges , PD1 and PD2 and the group formed by four 1N4007 diodes , can run on a system is a 24V AC or 24V continuous .

When the circuit is subjected to voltage 24V and the buttons are open the photo - couplers FT1 and FT2 are disabled and that the relative transistors are blocked and do not affect the rest of the circuit. It follows that the BC237 which control the relays RL1 and RL2 can polarize through the network 3k3 , 1N4148 diode , 10K resistor and zener diode 6V8 . The relays are normally energized and will therefore closed contacts in series with the two buttons P1 and P2.

Under these conditions, if you tap P1 and P2 together energizes the relay machine RLM giving your consent to the normal operation.

If however , for any reason , one of the two buttons is pressed or remains alone for more than a couple of seconds , one of the two relay de-energizes and opens the series of links P1 , P2 and RLM inhibiting the operation of the machine even if you tap the button remained open .

In fact, suppose that P1 and P2 is pressed alone no: in the bridge of diodes PD1 current will flow in the diode as well as the photo - coupler FT1. The transitional FT1 saturated leading to mass through the positive 33K of capacity by 100 uF that after a few seconds’ drops below the 6V8 and the BC237 is cut off by de-energizing the relay RL1. Pressing P2 also now the series is interrupted and RLM can never excite. The same is true of course also redoing the same reasoning relative to P2.

Circuit Diagram

Circuit Diagram Image

If you want to further increase security, you can add a relay in parallel with each relay and put in series with the contact of RLM with the two contacts so I added a double safety switch it off even when it was only one relay to open his contact while the other to a fault remains with the contact closed. See additional relay

Procedures for Entry :

Electronic Circuit Diagram

I remind everyone that the devices are certified and can be used for security apparatuses, my project are supposed to be an example to the qualification and amateur.

The machines are subject to the certified UNI EN ISO 13849-1 (formerly EN 954-1) and UNI EN ISO 13851 (ex UNI EN 574) - Safety device with both hands.

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