Two buttons provide safe start

Posted on Feb 25, 2013

The circuit in Figure 1 provides a safety interlock that checks the actuation of two pushbuttons before enabling a relay. When you push both buttons, the circuit actuates the relay. At that point, you can release one of the switches without the relay`s switching off. The circuit was intended to lock out the engine of an underwater propulsion unit. When handling such units on the surface, a person might accidentally press the actuator switch, which is mounted inside the handles of the propulsion unit. ANDing two switches makes the unit safer but requires two hands, a situation that is sometimes unsuitable.

Using the circuit in Figure 1, you need two hands to start the engine; once the engine is running, one hand is sufficient to keep it running. When you press no switch, the engine shuts off. The circuit uses a simple 555 timer, IC1 in a unique way. In thi

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