Two button control by logic gates

Two button control systems are often used on industrial presses to keep the operator’s hands away from the dangerous parts of these machines. Just for hobby purpose I designed a logic gates circuit that activates an output if two buttons are pressed simultaneously (with a small tolerance).

Transistors and capacitors are used for delay function. The input pin of the gate number 7 takes the high level one seconds after the output number 5 is activated; the input pin of the gate number 8 gets high five seconds after the activation of the output number 4.

If a button is pressed and the other one is not pressed within a short time, the gate number 7 disables the output signal through the gate number 2. If the buttons are pressed almost simultaneously the gate number 7 is disabled by the gate number 6. After five seconds the gate number 8 disables the output through the gate number 3. So the operator has to release the buttons and press them again.

For safety reason each button features a second contact, which works directly on the relay coil. As I said, this is just an hobby project and should not be used on real equipments. To be in compliance with the law use certified devices only.

two button control

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