seriesswitches
Hi, I would like to know the best way of turning on an LED for each switch that is opened. The switches are normally closed. The load is a motor. When any switch opens the motor turns off. But it is not always easy to see which switch it is. A nice LED display would work good.
The function you're looking for is very common in, for example, a sump-pump controller. If the sump starts to overflow (because the pump has failed), an alarm contact closes, and it usually sounds a very loud buzzer. There's a momentary-action switch called "Alarm Silence" that will shut off the buzzer (so that you can fix the problem in peace), but then it will re-arm itself once the original alarm contact opens.
It is easily implemented with a single SPDT relay, as shown below. S1 is the alarm contact, S2 is the "silence" pushbutton, and the relay K1 keeps the load (buzzer) switched off until S1 opens again and the relay drops out. "Power Source" and "Load" can be pretty much anything, as long as the coil of K1 is compatible with the power source.
As John1024 mentioned in a comment, the standard way to implement something like this is to use a 555 circuit, something like this:
Whenever the switch is closed, it will temporarily ground pin 2 of the 555, and the output on pin 3 will go high for the delay set by the components surrounding the 555. If the delay needs to start on a positive going pulse instead, a transistor inverter can be added to the input side of the circuit.
The values for the 452K resistor connected to pins 6 and 7, and the 10 µF capacitor connected to pin 6, have been chosen to generate a time delay of 4.97 seconds which is the closest to 5 seconds you can get with standard components.
However the timing will actually only be as good as the tolerance of the capacitor, which can usually be no better than 5%. So that means your timeout go range from about 4.7 seconds to 5.3 seconds, depending on the components. (Of course for a given set of components, it will always be the same.)
The values for the two timing components were chosen using this online 555 monostable calculator.
Best Answer
You could put a resistor and a LED wired in series and together in parallel with each switch. This will mean that if a switch opens, a small current will flow through the resistor, the LED, the closed switches and then thru the motor. The problem might be that the small current (say 20mA) might still cause the motor to move a little especially if it's a small 48 volt type.
If you can live with this and the supply is DC and always polarized the same way (i.e. not an output from a H bridge) then it should work.
If it's an AC motor then an extra diode is needed in parallel with each LED to prevent reverse bias.