Basic Electrical Circuit: Pre-start Alarm

As a follow on from the post on Basic Electrical Circuits, here is another circuit you may be able to find a good use for.

Consider an application where you need to start a machine where you have no full line of sight of the machine. This may be a very long conveyor and you have no way of seeing the entire length of it. This means you need to have a way of warning any person near the machine that it is about to start. In the old days you would walk the length and then come back to start it. This circuit will assist to remove some of the obvious dangers that this method of starting has

First thing to remember is the emergency stops. This need to be positioned along the conveyor and the best solution here would be a pull-cord along the entire length of the conveyor. You will note on the diagram that I have only shown one emergency stop but you can add as many as you like.

So what will this circuit do?

The operator proceeds to start the machine by pressing down on the start button. Assuming that the emergency stop is closed, he will have to hold down the start button for the duration of the timer setting. What this will do is that the timer is activated and starts timing, power goes via the normal closed contact of the timer and activates relay R1. Once this relay is activated, power will go via the normally open contact of R1, which has now closed because R1 is energised, and the audible alarm will sound.

This alarm stays on for the duration of the time set on the timer providing the operator holds down the start button. Once the timer times out, the normally open contact of the timer closes and the contactor K1 is activated. This also means that the normal open contact of K1 closes and retains the circuit. The operator can now let go of the start button.

At the same time, the normal closed contact of the timer changes state which means relay R1 is de-energised and as such the alarm stops. So how is this circuit helpful?

Well it serves to warn any person that the machine is about to start which allows them to make sure they are in the safety zone. The other thing they can do is to activate the pull-cord which will immediately prevent the unit from starting. The operator will know something has gone wrong and follow the prescribed procedure before attempting to start again. One thing of importance here: This circuit does not take into account the safety circuit which has to be designed based on the risk category of the machine. Should there be a safety relay in the circuit, it will provide the control circuit with and output which gets wired to the control circuit in place of the emergency stop shown on the drawing


Please Click on the Drawing for it to open

Before using this circuit, check with your local Legislative Requirements as the author cannot be held responsible for any issues arising from the use of this circuit. Always look at all the relevant risks on the machine, treat them and then do a further assessment to ensure that the methods you installed to control the risk have not created new risks

And as always, till next time, be safe and always be proud of your trade!

5 thoughts on “Basic Electrical Circuit: Pre-start Alarm”

  1. Where can i buy devices like this in 24volt systems for machines that are 24volt operated

    1. Hi David. Not sure where you are located, but your electrical wholesaler will definitely be able to source the required equipment. You build your safety circuit with the 24V relay(s), timer and any other switches you may have as you would any 230V circuit. Just ensure the relevant devices have coils with a 24V operating voltage. Let me know if you need assistance. Regards, Frans

    1. Hi David
      You will note there is a link available to open the circuit diagram labelled Pre_Start. Let me know if this helps.
      Kind Regards, Frans

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