Reed relay and Arduino question

Can Arduino safely handle a couple reed relay without transistors? The one I am looking at are simple SPST reed relay, coil is 5v and 500 ohms which seems to be about 10mA total while holding.

https://www.digikey.com/products/en?keywords=%201835-1116-ND is what I am looking at. And no, this relay does not have diode, a Chinese seller sent me 1,000 SMD diodes by mistake (got fully refunded, return shipping wasn't worth it) so I figured I'd use the free diodes and save 14 cents off over version with built in snubber diodes. :smiley:

My project is this: DIY intervalometer based on Arduino | Rastroludico but with minor change to work with Canon camera. The remote focus and shutter switching works differently, 2 separate switch to camera's ground. I figured relay would work just fine since it'd be physical switch between 2 different contact to ground.

If you confirm the relays are indeed 5V with a 500 ohm coil, you are okay to power it by an Arduino output.
Use a kickback diode across the coil, something similar to 1N4148 would work.

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1N4148

Wouldn't any diode be fine? I have a bunch of SMD diodes I want to use, they are rated 50v 1A and is of 1n4001

Thanks!

Wouldn't any diode be fine?

Yes a lot better than no diode. But those rectifyer diodes have a slower turn on time so are not as good as a signal diode.

I would use a Schottky diode. MCU outputs are.more sensitive than NPN transistors.

jendalinda:
I would use a Schottky diode. MCU outputs are.more sensitive than NPN transistors.

Absolutely, otherwise the tiny input protection diode on the mcu gets all the current anyway as it will conduct before a hulking great 100mA external signal diode!

To the OP:

While the comments above correctly tell you should use a Schottky diode, they don't tell you why in obvious terms.

Using a generic silicon signal diode, a IN4148 or a 1N400x rectifier series part, will not provide any protection of the Arduino output pin. This is because the Arduino pin protection diodes begin to conduct at approximately 0.3 volts and the diodes mentioned conduct at 0.7 volts.

When you turn off an inductive load controlled directly by an I/O pin, the voltage seen at the pin will rise above the normal 5 volt level due to the energy being released by the inductor (relay coil). When the voltage rises above 5.3 volts, the internal diode conducts the excess voltage to the power supply, clamping it to 5.3 volts. The only problem is that diode can only withstand a few milliamps before failing and that current is uncontrolled, other than for the internal resistance of the I/O pin.

Putting a diode that conducts at 0.3 volts (a Schottky diode) directly across the relay coil as suggested should limit the maximum voltage on the I/O pin to a value that should not have the internal protection diodes conducting in a manner than may harm the processor.

Suggest you always drive an inductive load with a more proper solution, use a 10 cent transistor.

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avr_fred:
This is because the Arduino pin protection diodes begin to conduct at approximately 0.3 volts...

Where did you get that info from.
If the datasheet says that you should stay under that limit, that doesn't mean it does already conduct at that point.

I measured those clamping points some time ago by by sending a 1mA current into a pin.
From memory, clamping voltages were something like 0.65-0.7volt.

So yes, an external schottky is wise, a common diode is not.
Leo..