Inductive proximity sensor from Automation Direct.
At my current job site, we received a very simple job to do: install a simple circuit which would activate strobes upon opening a pedestrian gate for a walkway which crosses an area with forklift traffic. The engineering company gave us a box of parts: A multi-function timer with on-delay, off-delay, and recycle modes with one normally open contact; inductive proximity sensors with one normally open contact; a fuse holder; and, strobe lights. Everything was 120V, so there was no transformer.
In short, the proximity sensor would sense when the gate was opened, and energize the timer which would in turn immediately energize the strobes. When the gate was closed, the timer would begin timing (off-delay) and when it timed-out, the strobes would de-energize. We had not yet received the drawings, but the circuit was very simple, so I drew-out what it would most likely be with the parts we were given:
Original circuit, shown with gate open.
There was a very obvious problem with this circuit. Since the proximity sensor had the gate in its sensing field when the gate was closed, the strobe would always be on unless someone held the gate open for more than the set time on the timer. Upon sharing my findings with the engineering firm, they produced the drawings which showed the same error. I came-up with three solutions, two of which were the correct thing to do, and one which isn’t the best, but allows it to work in normal situations.
Solution 1: Purchase proximity sensors with a normally closed contact. When the gate is closed, the proximity sensor will change the state of the contact to normally open. When the gate opens, the proximity sensor will revert to it’s normally closed state, and the timer will activate and thus the strobe. When the gate shuts, the timer will start and the strobe will turn-off when the timer times-out:
Normally closed proximity sensor, shown with gate open.
Solution 2: Purchase a relay with a normally closed contact that we can interpose between the proximity sensor and the timer which behaves the same as if we had a proximity sensor with a normally closed contact:
Relay, shown with gate open.
Solution 3: Move the sensor away from the gate’s closed position, but within the path of the gate’s swing so as to sense the gate as it is opened and closed. This would use the original circuit and no additional parts would be purchased. The downside is that if the gate is held open, the timer will time-out and the strobe will not be on. Possible scenarios include people propping-open the gate while moving something heavy, or a longer line of people moving through the area, with one person holding open the gate. As long as the gate is under normal use, meaning a person or a few persons are moving through the gate and allowing it to shut, it will work just fine.
As of now, solution 3 was chosen by the engineering firm.
Update: It turns out that there is enough leakage current going across the proximity sensor to keep the timer constantly energized so that it never turns off. The timer also appears to not allow enough current to flow across the proximity sensor to allow it to change states. Therefore, we are going with solution 2 by adding a relay, which means I can move the proximity sensor back to where it is constantly sensing the closed gate. A much better solution overall and it produces the outcome that the customer desires.
