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I have an application which requires me to break a 24v signal running with low current (60ma being the absolute max). The signal stays on allowing a large hydraulic motor to close a mold with great force. If an obstruction is in the way, it will damage the mold when the hydraulics are activated. The goal is to use a PIC advanced 8 bit microcontroller running at 5v to sense a condition when there is an obstruction and signal the relay to open. My problem is finding the proper normally closed relay and determine how much response time is needed. It would be helpful if the relay could also work of 5 volt power rail and not exceed 0.5 amps of current to open.

Sadly the relay will be need to be placed inside the injection machine and will need to work in a normally closed manner because the object sensing device is only used on one job and will not always be set up to due to space constraints. The object detection system is a vision system to sense plastic parts that get stuck once in a while. It uses a Webcam hooked to a pc that sends a uart packet to the pic to trigger the relay. Currently the injection molding job operates without any protection so fail safe isn't the primary concern. I want to use the pic at 5 volts because I can easily use one DC power rail for the both energizing the relay and powering the pic. I don't have microcontroller experience aside from pic advanced 8 bit running xc8.

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  • \$\begingroup\$ This seems like a product recommendation question. \$\endgroup\$ – Jashaszun Aug 10 '17 at 21:20
  • \$\begingroup\$ look at reed relays intended for telepone applications. \$\endgroup\$ – Jasen Aug 10 '17 at 21:50
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I have an application which requires me to break a 24v signal running with low current (60 mA being the absolute max).

That bit shouldn't present any problems to relay or transistor switching.

The signal stays on allowing a large hydraulic motor to close a mold with great force. If an obstruction is in the way, it will damage the mold when the hydraulics are activated.

So this is a machine safety feature to protect an expensive asset. This will be worth doing well ...

The goal is to use a PIC advanced 8 bit microcontroller running at 5 V to sense a condition when there is an obstruction and signal the relay to open.

This is where your solution starts to look a bit dodgy. For an expensive asset I would recommend that robust, industrial devices are used. To rely on a home-brew PIC-based safety system may be asking for trouble. You have to sort out power-supplies, interference suppression, voltage level-shifting and isolation, etc.

My problem is finding the proper normally closed relay and determine how much response time is needed.

Oh-oh! If you rely on energisation of a circuit in the event of a fault your system will not fail-safe. i.e. If the relay fails to energise for any reason you have no protection.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Fail-safe circuit.

Standard practice is to have the relay energised when conditions are safe. In the unsafe condition the relay drops. It will also drop if the power or wiring or switch fail thus alerting you to a safety monitor fault.

It would be helpful if the relay could also work of 5 volt power rail and not exceed 0.5 amps of current to open.

Edit your question to explain why you are intent on running on 5 V and what condition you are detecting and how that works.

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  • \$\begingroup\$ See revised summary \$\endgroup\$ – piman Aug 10 '17 at 21:48

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